Note: the following is excerpted from Ca STATE science FRAMEWORK, The complete document is available on-line at http://www.cde.ca.gov/be/st/ss/index.asp

Focus on Physical Sciences

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STANDARD SET 3. Structure of Matter

3. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure. As a basis for understanding this concept:

a. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons.

b. Students know that compounds are formed by combining two or more different elements, and that compounds have properties that are different from their constituent elements.

c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers.

d. Students know the states of matter (solid, liquid, gas) depend on molecular motion.

e. Students know that in solids the atoms are closely locked in position and can only vibrate; in liquids the atoms and molecules are more loosely connected and can collide with and move past one another; and in gases the atoms and molecules are free to move independently, colliding frequently.

f. Students know how to use the periodic table to identify elements in simple compounds.

Teachers should assess students’ knowledge prior to instruction of this topic, as the atomic theory of matter may be very challenging to them. Students are expected to recall terms and definitions from earlier introductions to the concepts of atoms, molecules, and elements. Instruction should provide empirical evidence for the atomic theory, which will be useful for understanding science and crucial to the study of chemistry.

When students learn about the structure of matter, teachers should emphasize that the historical evidence for atoms was based largely on indirect measurements and inferences far removed from direct experience. Recently, instruments have been built that produce images of individual atoms, confirming what was inferred earlier as a result of overwhelming evidence from many scientific experiments. Most scientists come to know the atomic theory is true by repeatedly using the concepts and principles presented in the theory to explain observed properties and predict changes in matter.

3.a    Shortly after British physicist Ernest Rutherford inferred the existence of atomic nuclei, the general idea emerged that atoms are mostly empty space with a tiny, massive nucleus at the center containing positively charged protons and neutral neutrons. This nucleus is surrounded by tiny, negatively charged electrons, each with about 1/2,000 the mass of a proton or neutron. Danish physicist Niels Bohr developed a model of the hydrogen atom to explain its visible spectrum. At the high school level, the chemistry standards require students to know the historical importance of this model. Bohr’s model succeeded in predicting the spectrum of light emitted by hydrogen atoms and is therefore the acknowledged starting point for understanding atomic structure. However, Bohr’s “solar” model of the atom, diagrammed in most textbooks as showing electrons in circular orbits about the nucleus, is oversimplified. Rather than try to describe how the electrons in an atom are moving, teachers are better advised to help students develop a model of the atom in which each electron has definite energy. Students should know that the energy of each electron in an atom keeps it in motion around the positive nucleus to which it is attracted. The structure of multielectron atoms is understood in terms of electrons filling energy levels that define orbitals.

3.b    The word combining implies bonding. Understanding the concepts of ionic and covalent bonding helps explain why some elements combine to form compounds and some do not. Atoms of different elements combine to form compounds; a compound may, and usually does, have chemical characteristics and physical properties that are different from those of its constituent elements. Examples and generalizations may be drawn from ionic compounds formed of metals and nonmetals and covalently bonded, organic compounds formed from carbon and other elements.

Students often learn to manipulate chemical equations without having a picture in their minds of physical reality at the atomic level. The ability to create such a picture is a useful skill that helps students keep track of all the atoms in the process. For example, the reaction of methane and oxygen to form carbon dioxide and water can be visualized by using models or drawing pictures of the atoms and molecules in the reactants. These molecules can then be rearranged into new products. (Make sure that all the atoms in the starting reactants are accounted for in the new products.) Instruction in this standard will help students understand that compounds are collections of two or more different kinds of atoms that are bonded together. Knowing exactly how the atoms are organized to form a molecule is not essential.

3.c    Crystals of table salt, the compound NaCl, have a regular, cubic structure in which sodium (Na⁺) ions alternate with chlorine (Cl⁻) ions in three-dimensional array with the atoms at the corner of cubes forming the lattice. In organic polymers, the carbon, hydrogen, sometimes oxygen, and nitrogen atoms combine to form long, repetitive, stringlike molecules.

Inexpensive models of molecules may be made by using colored gumdrops (held together by toothpicks) to represent molecules. Students identify the atoms that constitute the molecules by using a color-coded key relating the color of the gumdrop to an atom of an element. They learn that the shape of a molecule is important to its chemical and physical properties. At the high school level, students will be introduced to the idea that shape is determined mainly by the electron con-figuration that provides the most energy-stable system.

Students can also grow crystals from a solution and should understand that this process leads to the building up of atoms into a lattice. Students may begin the process by dissolving an excess of sodium chloride, sugar, or Epsom salts in water. Then they hang a string in the water and store the container in a place where it will be undisturbed while the water evaporates. Crystals will form on the string. Putting a small (seed) crystal tied to a piece of thread in the solution will accelerate the growth process. Books and kits (including chemicals, glassware, and instructions) on crystal growing are available commercially. Students can watch crystals grow on slides under a microscope. Some crystals display vivid colors when viewed between crossed sheets of polarizing material.

3.d    All atoms, and subsequently all molecules, are in constant motion. For any given substance the relative freedom of motion of its atoms or molecules increases from solids to liquids to gases. When a thermometer is inserted into a substance and the temperature is measured, the average atomic or molecular energy of motion is being measured. The state of matter of a given substance therefore depends on the balance between the internal forces that would restrain the motion of the atoms or molecules and the random motions that are in opposition to those restraints.

The change in phases is evidence of various degrees of atomic and molecular motion. The conditions of temperature and pressure under which most materials change from solid to liquid or liquid to vapor (gas) or gas to plasma have been measured. Those properties are difficult to predict but are highly reproducible for different samples of the same material and can be used to identify substances. Some substances will go from solid to gas directly at one atmosphere pressure. Dry ice, which is frozen carbon dioxide, is an example. Chemistry handbooks contain the melting points (or freezing points) and boiling points (or condensation temperatures) of most materials usually under one atmosphere pressure. If the pressure is not one atmosphere, those temperatures change. Some substances have more than one stable solid phase at room temperature. Graphite, with its soft black texture and its hard, clear crystalline diamond atomic structure, represents the two solid phases of elemental carbon.

Water is another example of a substance that undergoes a change in atomic and molecular motion under extreme conditions of temperature and pressure. At one atmosphere pressure, ice forms when water is cooled below zero degrees Celsius (or 32 degrees Fahrenheit). Above the freezing point the average molecular energy of motion of the water molecules is just enough to overcome the attractive forces be-tween the molecules. The water molecules thereby avoid being locked in place and remain liquid. At and below the freezing point, the water molecules become the solid, crystalline material called ice. When liquid water is heated to temperatures of 100 degrees Celsius, molecular motion increases until large groups of water molecules overcome the attractive forces between the molecules. At this point those energetic molecules form bubbles of steam, which are bubbles of gas made not of air but of water. The process in which bubbles of water vapor escape from liquid water is called boiling. Continued heating will change the liquid water entirely into vapor instead of raising the temperature of the water above 100 degrees Celsius.

3.e The atoms or molecules of a solid form a pattern that minimizes the structural energy of the solid consistent with the way in which the atoms or molecules attract at long distances but repel at short distances. The atoms or molecules vibrate about their equilibrium positions in this pattern. When raised above the melting temperature, the atoms or molecules acquire enough energy to slide past one another so that the material, now a liquid, can flow; the density of the liquid remains very close to that of the solid, demonstrating that in a solid or a liquid the atoms stay at about the same average distance.

If a single atom or molecule acquires enough energy, however, it can pull away from its neighbors and escape to become a molecule of a gas. Gas molecules move about freely and collide randomly with the walls of a container and with each other. The distance between molecules in a gas is much larger than that in a solid or a liquid, and this point may be emphasized when students study density.

3.f     The periodic table of elements is arranged horizontally in order of increasing atomic number (number of protons) and vertically in columns of elements with similar chemical properties. Students should learn to use the periodic table as a quick reference for associating the name and symbol of an element in compounds and ions. They should be able to find the atomic number and atomic weight of the element listed on the table. The periodic table is both a tool and an organized arrangement of the elements that reveals the underlying atomic structure of the at-oms. This standard focuses on the table as a tool.

Every field of science uses the periodic table, and various forms of it exist. Astrophysicists may have a table that includes elemental abundances in the solar system. Physicists and engineers may use tables that include boiling and melting points or thermal and electrical conductivity of the elements. Chemists have tables that show the electron structures of the element. Students should be encouraged to refer to the periodic table as they study the properties of matter and learn about the atomic model.  

Students in grade eight are ready to tackle the larger picture of galaxis and astronoimical distances. They are ready to study stars compared with and contrasted tp the Sun and to learn in greater detail about the planets and other objects in the solar system. High school studies of earth sciences will include the dimension of time along with three-dimensional space in the study of astronomy.

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STANDARD SET 4.  The structure and composition of the universe

The structure and composition of the universe can be learned from studying stars and galaxies and their evolution. As a basis for understanding this concept:

a. Students know galaxies are clusters of billions of stars and may have different shapes.

b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color.

c. Students know how to use astronomical units and light years as measures of distances between the Sun, stars, and Earth.

d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light.

e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids originally thought to be stars are now known to be distant galaxies.

Galaxies them-selves appear to form clusters that are separated by vast expanses of empty space. As galaxies are discovered they are classified by their differing sizes and shapes. The most common shapes are spiral, elliptical, and irregular. Beautiful, full-color photo-graphs of astronomical objects are available on the Internet, in library books, and in popular and professional journals. It may also interest students to know that astronomers have inferred the existence of planets orbiting some stars.

4.b The Sun is a star located on the rim of a typical spiral galaxy called the Milky Way and orbits the galactic center. In similar spiral galaxies this galactic center appears as a bulge of stars in the heart of the disk. The bright band of stars cutting across the night sky is the edge of the Milky Way as seen from the perspective of Earth, which lies within the disk of the galaxy. Stars vary greatly in size, temperature, and color. For the most part those variations are related to the stars’ life cycles. Light from the Sun and other stars indicates that the Sun is a fairly typical star. It has a mass of about 2 × 10³⁰ kg and an energy output, or luminosity, of about 4 × 10²⁶ joules/sec. The surface temperature of the Sun is approximately 5,500 degrees Celsius, and the radius of the Sun is about 700 million meters. The surface temperature determines the yellow color of the light shining from the Sun. Red stars have cooler surface temperatures, and blue stars have hotter surface temperatures. To connect the surface temperature to the color of the Sun or of other stars, teachers should obtain a “black-body” temperature spectrum chart, which is typically found in high school and college textbooks.

4.c Distances between astronomical objects are enormous. Measurement units such as centimeters, meters, and kilometers used in the laboratory or on field trips are not useful for expressing those distances. Consequently, astronomers use other units to describe large distances. The astronomical unit (AU) is defined to be equal to the average distance from Earth to the Sun: 1 AU = 1.496 × 10¹¹ meters. Distances between planets of the solar system are usually expressed in AU. For distances between stars and galaxies, even that large unit of length is not sufficient. Interstellar and intergalactic distances are expressed in terms of how far light travels in one year, the light year (ly): 1 ly = 9.462 × 10¹⁵ meters, or approximately 6 trillion miles. The most distant objects observed in the universe are estimated to be 10 to 15 billion light years from the solar system. Teachers need to help students become familiar with AUs by expressing the distance from the Sun to the planets in AUs instead of meters or miles. A good way to become familiar with the relative distances of the planets from the Sun is to lay out the solar system to scale on a length of cash register tape.  

4.d The energy from the Sun and other stars, seen as visible light, is caused by nuclear fusion reactions that occur deep inside the stars’ cores. By carefully analyzing the spectrum of light from stars, scientists know that most stars are composed primarily of hydrogen, a smaller amount of helium, and much smaller amounts of all the other chemical elements. Most stars are born from the gravitational compression and heating of hydrogen gas. A fusion reaction results when hydrogen nuclei combine to form helium nuclei. This event releases energy and establishes a balance between the inward pull of gravity and the outward pressure of the fusion reaction products.

Ancient peoples observed that some objects in the night sky wandered about while other objects maintained fixed positions in relation to one another (i.e., the constellations). Those “wanderers” are the planets. Through careful observations of the planets’ movements, scientists found that planets travel in nearly circular (slightly elliptical) orbits about the Sun.

Planets (and the Moon) do not generate the light that makes them visible, a fact that is demonstrated during eclipses of the Moon or by observation of the phases of the Moon and planets when a portion is shaded from the direct light of the Sun.

Various types of exploratory missions have yielded much information about the reflectivity, structure, and composition of the Moon and the planets. Those missions have included spacecraft flying by and orbiting those bodies, the soft landing of spacecraft fitted with instruments, and, of course, the visits of astronauts to the Moon.

4.e Nine planets are currently known in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. They vary greatly in size and appearance. For example, the mass of Earth is 6 × 10²⁴ kg and the radius is 6.4 × 10⁶ m. Jupiter has more than 300 times the mass of Earth, and the radius is ten times larger. The planets also drastically vary in their distance from the Sun, period of revolution about the Sun, period of rotation about their own axis, tilt of their axis, composition, and appearance. The inner planets (Mercury, Venus, Earth, and Mars) tend to be relatively small and are composed primarily of rock. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are generally much larger and are composed primarily of gas. Pluto is composed primarily of rock and is the smallest planet in the solar system.

All the planets are much smaller than the Sun. All objects are attracted toward one another gravitationally, and the strength of the gravitational force between them depends on their masses and the distance that separates them from one an-other and from the Sun. Before Newton formulated his laws of motion and the law of universal gravitational attraction, German astronomer Johannes Kepler deduced from astronomical observations three laws (Kepler’s laws) that describe the motions of the planets.

Planets have smaller objects orbiting them called satellites or moons. Earth has one moon that completes an orbit once every 28 days (approximately). Mercury and Venus have no moons, but Jupiter and Saturn have many moons. Very small objects composed mostly of rock (asteroids) or the ice from condensed gases (comets) or both also orbit the Sun. The orbits of many asteroids are relatively circular and lie between the orbital paths of Mars and Jupiter (the asteroid belt). Some asteroids and all comets have highly elliptical orbits, causing them to range great distances from very close to the Sun to well beyond the orbit of Pluto.

Teachers should look for field trip opportunities for students to observe the night sky from an astronomical observatory or with the aid of a local astronomical society. A visit to a planetarium would be another way of observing the sky. If feasible, teachers should have students observe the motion of Jupiter’s inner moons as well as the phases of Venus. Using resources in the library-media center, students can research related topics of interest.

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STANDARD SET 5. Reactions

5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:

a. Students know reactant atoms and molecules interact to form products with different chemical properties.

b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same.

c. Students know chemical reactions usually liberate heat or absorb heat.

d. Students know physical processes include freezing and boiling, in which a material changes form with no chemical reaction.

e. Students know how to determine whether a solution is acidic, basic, or neutral.

When substances react, the atoms involved in the reactants are rearranged, forming other products. Students have learned that the physical and chemical properties of the newly formed substances (products) are different from the physical and chemical properties of the original substances (reactants).

Students in grade eight will learn that it is the underlying arrangement of the atoms in the reactants and products and the energy needed or released during the rearrangement process that explain chemical reactions. Understanding chemical reactions is essential because they constitute, directly or indirectly, a large portion of the discipline of chemistry.

Students need to be able to distinguish a chemical change from a physical change. In a physical change one or more physical properties of the material are altered, but the chemical composition (i.e., the arrangement of the atoms in molecules) remains the same. In a chemical change the atoms are rearranged to form new substances with different chemical and physical properties. Students must be familiar with the periodic table and the names and symbols of the chemical elements.

In grade one students are prepared for the idea of chemical reactions when they learn that the properties of substances can change when they are mixed, cooled, or heated. In grade three they learn that when two or more substances are combined, a new substance may be formed with properties that are different from those of the original materials. In grade five they learn that during chemical reactions, the atoms in the reactants rearrange to form products with different properties.

The study on reactions begun in grade eight will support future studies about conservation of matter and stoichiometry as well as work on acids, bases, and solutions. Students will go beyond studying reactions and their reactant/product relationships to work with the rates of reaction and chemical equilibrium. Students should be able to envision a chemical equation at the atomic and molecular levels. They should “see” the number of reactant atoms and molecules in the equation coming together and by some process rearranging into the correct number of atoms and molecules that form the products. This important conceptual skill helps students to keep track of all the atoms.

This standard focuses on changes that occur when atoms and molecules as reactants form product compounds with different chemical properties. Teachers may have students perform simple reactions or demonstrate the reactions for students. All students should be able to learn the more important chemical reactions and the elements involved in them, especially if common compounds such as vinegar (acetic acid), baking soda (sodium bicarbonate), table salt (sodium chloride), carbonated water, and nutritional minerals and foods are used in activities or demonstrations. An example might involve adding calcium chloride and baking soda to water. Such reactions demonstrate clearly the differences in properties between reactants (solids and liquid) and products (solid, liquid, and gas).

5.b     The conservation of matter is a classical concept, reinforcing the idea that atoms are the fundamental building blocks of matter. Atoms do not appear or disappear in traditional chemical reactions in which the constituent atoms and/or polyatomic ions are simply rearranged into new and different compounds. Conservation of atoms is fundamental to the idea of balancing chemical equations. The total number of atoms of each element in the reactants must equal the total number of atoms of each element in the products. The total number of atoms, hence the total mass, stays the same before and after the reaction.

There are several ways to teach and assess students’ understanding of the concept of conservation of mass in chemical reactions. Weighing reactants before and products after a reaction shows that mass is neither gained nor lost. However, experimental errors are possible; the most common one is not sufficiently drying the products before weighing. One simple demonstration of the concept that atoms (or matter) are conserved in chemical reactions in which mass might appear to be lost is to determine the combined mass of a small, sealed container filled one-third with water, the screw-on cap, and one-quarter of an effervescent tablet. After the piece of tablet is dropped in the water, the container is immediately sealed. When the fizzing has stopped, the combined mass of the sealed container and the tablet should remain the same. After the seal is broken, much of the carbon dioxide gas formed by the reaction escapes, and the mass of the container and its contents decrease.

Students should also be taught to balance simple chemical equations. This step reinforces the idea that atoms do not appear or disappear in chemical reactions and, therefore, that matter is conserved.

5.c     In chemical reactions the atoms in the reactants rearrange to form products, and there is usually a net change in energy. Breaking bonds between atoms requires energy; making a bond releases energy. If the total making and breaking of all bonds for a particular chemical reaction results in a net release of energy, the reaction is said to be exothermic. The energy is typically released as heat into nearby matter. If the total making and breaking of bonds results in a net absorption of energy, the reaction is called endothermic. The energy is typically absorbed as heat from nearby matter, which therefore cools. A convenient way to demonstrate that chemical reactions release or absorb heat is the application of the hot packs or cold packs used for athletic injuries. The change in temperature produced by those packs may be the result of a chemical reaction, or it may be caused by a “heat of solution” and not by a chemical reaction. For example, dissolving is considered a physical and not a chemical change because the compound may be recovered, unchanged chemically, by evaporation.

5.d     When heated, many solid materials undergo a reversible change of state into a liquid (melting). Under the standard condition of one atmosphere of pressure, the temperature at which such a solid material melts is the same as the temperature at which the liquid material freezes; this temperature, called the melting point, is characteristic of the material. Many liquid materials when heated also undergo a reversible change of state into a gas. Under one atmosphere of pressure, such a liquid material may boil; the temperature at which this occurs is also characteristic of the material and is called the boiling point. Such reversible changes—back and forth from solid to liquid or from liquid to gas—are called physical changes because no chemical change (a permanent reordering of the atoms into new molecules) occurs. Similarly, the dissolving of one substance into another, such as a solid or gas into a liquid, is often reversible (by evaporating the liquid to leave the solid or heating the liquid to drive out the gas) and is also called a physical rather than a chemical change. Physical changes can usually be undone to recover the original materials unchanged. Activities such as mixing iron filings with sand demonstrate a physical change. In this case a magnet can recover the iron filings from the mixture.

5.f      Indicators that change color are routinely used to determine whether a solution is acidic, basic, or neutral. A pH scale indicates with numbers the concentration of hydrogen ions in a solution and characterizes a solution as acidic (lower than 7), basic (higher than 7), or neutral (near 7). There are electrodes and electronic instruments that can measure directly the pH of a solution. Some acids and bases are defined other than by their hydrogen ion concentration, but they will be addressed in high school chemistry. Teachers may give students the opportunity to test solutions, including foods such as fruits and vegetables, with pH paper, litmus paper, indicator solutions, or pH meters to determine whether a solution or food is acidic, basic, or neutral. Students should be familiar with the pH scale to know what a given pH value indicates.

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STANDARD SET 6. Chemistry of Living Systems

6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept:

6.a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms.

6.b. Students know that living organisms are made of molecules consisting largely of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

6. c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.

Because all living organisms are made up of atoms, chemical reactions take place continually in plants and animals , including humans. The uniqueness of organic chemistry stems from chain polymers. Life could not exist without the ability of some chemicals to join together, repetitively, to form large, complex molecules. Concepts learned in this standard set are critical for understanding fully the chemistry of the cells of organisms, genetics, ecology, and physiology that will be taught in the high school biology/life sciences standard sets.

Carbon is unique among the elements because it can bond to itself and to many other elements. This attribute makes possible many different kinds of large, carbon-based molecules. Typically, carbon will make four separate covalent bonds (to other carbon atoms), but double and triple bonds are also possible. The variety of bonds allows carbon-based molecules to have a wide range of shapes and chemical properties. Key shapes include tetrahedral (e.g., methane and carbon tetrachloride), planar (e.g., formaldehyde and ethylene), and linear (e.g., acetylene and carbon dioxide). Students can research the nomenclature, composition, and structure of organic molecules by using textbooks and supplemental instructional mate-rials. They can also construct models of carbon-based molecules by using commercial modeling kits or inexpensive alternatives (e.g., gumdrops and tooth-picks).

Living organisms are made up of a great variety of molecules consisting of many atoms (with carbon atoms playing the main roles), but the number of different elements involved is quite small. Carbon and only five other elements make up most of Earth’s biomass. Those six elements, however, can combine in many different ways to make large, organic molecules and compounds. To demonstrate this idea, teachers may burn organic material, such as bone, leaves, wood, or a variety of candles. They may hold a cold glass or plate above the flame to condense droplets of water, one of the combustion products. They may also hold a heat-treated glass in the flames to collect carbon deposits in the form of soot. Students can discuss what elements were in the organic material. Teachers may draw students’ attention to the black material that forms when meat is roasted or grilled or when toast is charred.

6. c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA. /p>

Living organisms require a variety of molecules; some molecules contain carbon and some do not. The molecules that make up organisms and control the biochemical reactions that take place within them are usually large molecules, such as DNA, proteins, carbohydrates, and fats. Organisms also require simple substances, such as water and salt, to support their functioning. Teachers may encourage students to research why plants and animals need simple molecules such as water. Other activities for teachers may include squeezing the water from celery or turnips to demonstrate the presence of water. Or they may ask students how they can demonstrate that water is in fruits and vegetables (e.g., dried fruit). Teachers may also ask students how they know that there is salt in their bodies. Most students know that their perspiration tastes salty.

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STANDARD SET 7. Periodic Table

7. b. Students know each element has a specific number of protons in the nucleus (the atomic number) and each isotope of the element has a different but specific number of neutrons in the nucleus.

7. c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity.

study of this standard set, they should be familiar with the periodic table and should know the names and chemical symbols of most of the common elements. In this standard set they must now look in greater detail at and learn the significance of atomic numbers and isotopes and how they relate to the classification of elements. Students need to go more deeply into the elemental properties that serve as the basis for the periodic arrangement. Meeting the standards in this set will serve as a strong foundation for the study of atomic and molecular structures and of the relationship between these structures and the arrangement of elements on the periodic table that will take place in high school chemistry.

A common form of the periodic table has 18 columns (groups of elements) in the main body. This form shows the periodicity, or repeating pattern, of chemical and some physical properties of the elements. What varies most in published periodic tables is the information provided in the box that represents each element. The most useful tables are those that show the physical properties of the most common form of the element in addition to the atomic number and the atomic weight. A table that color-codes metals and nonmetals is also useful.

Elements shown toward the top of the periodic table are lighter, and those to-ward the bottom are heavier. Elements shown to the left are generally metallic, and those toward the right are nonmetallic. The word metallic refers to the collective properties of common metals: luster, malleability, high electrical and thermal (heat) conductivity. Although the majority of elements in the periodic table are metals, a few are classified as semimetals and may be found bordering the transition between the metals and nonmetals. When atoms from the left side of the table combine with atoms from the right side, they tend to form ionic salts, which are brittle crystalline compounds with high melting temperatures.

At the high school level, students will learn that the arrangement of the elements in the columns of the periodic table reflects the electron structure of the atoms of each element. This pattern explains the similarity in the chemical properties of the elements in each column of the periodic table.

Students should be readily able to use the periodic table to find the atomic number of an element and should know that there is a pattern of increasing atomic numbers as the table is read from left to right and down one row at a time. The lanthanides and actinides are placed off the table to save space; however, if they were placed in the table they would still be read in the same manner—from left to right and then down. Students should also know that the atomic number is the number of protons in the nucleus.

7.a. The periodic table of elements is structured so that metals are shown on the left, with the most reactive metals on the far left. Nonmetals are located on the right, with the most reactive next to the “inert” gases on the far right. Despite the name, inert gases are not truly inert. Although no naturally occurring inert gas compounds are known, some have been synthesized in the laboratory. Therefore most scientists use the term noble gas instead of inert gas. Semimetals, found between the metals and nonmetals in the periodic table, are elements, such as silicon, that have some properties of metals but also have properties that are typical of nonmetals. Although only a few elements fit this category, the unique electrical property of semimetal elements is that they are semiconductors, an essential property for computer chips. The rare earth elements can be used to produce very strong magnets.

Students should know that scientists have the right to name their discoveries and that some elements have been named after famous men and women scientists, such as curium, einsteinium, and seaborgium.

7.b. A rigorous definition of the term element is based on the number of protons in the atom’s nucleus (the atomic number). All atoms of a given element have the same number of protons in the nucleus. Atoms with different atomic numbers are atoms of different elements. Although the number of protons is fixed for a particular element, the same is not true for the number of neutrons in the nucleus. An element that has different numbers of neutrons in its atoms is called an isotope of the element. For example, all hydrogen atoms have one proton in the nucleus, but there are two additional isotopes of hydrogen with different numbers of neutrons. One is called deuterium (one proton and one neutron), and the other is called tritium (one proton and two neutrons). The common isotope of hydrogen has one proton and no neutrons in its nucleus.

Some isotopes are radioactive, meaning that the nucleus is unstable and can spontaneously emit particles or trap an electron to become the nucleus of a different element with a different atomic number. All the isotopes of some elements are radioactive, such as element 43, technetium, or element 86, radon. No stable samples of those elements exist. Element 92, uranium, is another example of an element in which no stable isotopes exist. However, uranium (atomic weight 238) is found in nature because it decays so slowly that it is still present in Earth’s crust. The atomic number of each element represents the number of protons in the nucleus. There-fore, as the atomic number increases, the mass of the atoms of succeeding elements generally increases although exceptions exist because of the varying numbers of neutrons in some isotopes. Typically, however, the atoms of the elements in the periodic table increase from left to right, and those elements listed in the lower rows are more massive than those in the upper rows.

7.c. The physical properties of substances reflect their chemical composition and atomic structure. The melting temperature or hardness of the common forms of the elements is related to the forces that hold the atoms and molecules together. One can compare the boiling points of carbon and nitrogen. Carbon is solid up to very high temperatures (3,600 degrees Celsius); nitrogen, the element next to it, is a gas until it is cooled to below negative 196 degrees Celsius. This dramatic difference between two adjacent elements on the periodic table shows there must be very different intermolecular forces acting as a result of a slight change in atomic structure.

Density is the mass per unit volume and is a function of both the masses of individual atoms and the closeness with which the atoms are packed.

Electrical conductivity and thermal conductivity are strongly dependent on how tightly electrons are held to individual atoms. Metals and nonmetals may be found in portions of the periodic table. Metal atoms combine in regular patterns in which some electrons are free to move from atom to atom, a condition that accounts for both high electrical and high thermal conductivity.

1-Motion     2-Forces    3-Structure of Matter    4-Struct. of Universe     5-Reactions    6-Chem of Living Syst    7-Density/Buoyancy     8-Periodic Tbl.    9 - Investigate & Experiment    top

1-Motion     2-Forces    3-Structure of Matter    4-Struct. of Universe     5-Reactions    6-Chem of Living Syst    7-Density/Buoyancy     8-Periodic Tbl.    9 - Investigate & Experiment    top

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