a. Students know that during chemical reactions the atoms in the reactants rearrange to form products with different properties.
When iron reacts with O2 and forms rust, the rust has different properties than iron, for example, it isn’t magnetic.
Atoms and Molecules
b. Students know all matter is made of atoms, which may combine to form molecules.
atom: The smallest unit of an element that has all the properties of that element.
molecule: A grouping of two or more atoms joined together.
- A molecule may be made up of one type of atom, like O2, two oxygen atoms bonded together, or H2O, a molecule combining hydrogen and oxygen atoms.
c. Students know metals have properties in common, such as high electrical and thermal conductivity. Some metals, such as aluminum (Al), iron (Fe), nickel (Ni), copper (Cu), silver (Ag), and gold (Au), are pure elements; others, such as steel and brass, are composed of a combination of elemental metals.
- Metals are good conductors of 1) electricity and 2) heat. Copper (Cu), for example, is a good conductor of electricity. Many cooks like cast iron skillets because of their good conductivity of heat (thermal energy).
- Some metals are made up of just one type of atom; these are pure metals. They can be found on the periodic table of elements.
- Some metals are alloys; they are mixtures of metals or metals and other elements. Bronze is an alloy of copper (C) and tin (Sn), while brass is an alloy of copper (C) and zinc (Zn). Steel is an alloy of iron (Fe) and carbon (C); nickel (Ni) is added to steel to make it stainless.
Elements and the Periodic Table
d. Students know that each element is made of one kind of atom and that the elements are organized in the periodic table by their chemical properties.
element: A substance made up of only one kind of atom.
- Elements are organized in the periodic table according to their chemical properties. About one-fourth of elements are nonmetals; they are on the right hand side of the periodic table, except for hydrogen. The majority of elements are classified as metals. In between the metals and nonmetals are the metalloids or semimetals; there are only eight metalloids.
- The periodic table of elements has rows (periods) and columns (groups). All elements in a period have the same number of electron shells or energy levels. The first period has one shell, the second period has two shells, and so on up to seven shells.
- All elements in a group have the same number of electrons in their outer shell. There are 18 groups in the periodic table, and the right-most group is often called the noble gases; they are usually inert, not reacting with other elements, because their outer electron shell is full.
e. Students know scientists have developed instruments that can create discrete images of atoms and molecules that show that the atoms and molecules often occur in well-ordered arrays.
Electron microscopes can be used to produce images of individual atoms in a crystalline array. Those images show atoms as “fuzzy balls” aligned in orderly and repeating patterns. From those images it is possible to infer that atoms are discrete objects of finite size and nearly spherical shape. Those images confirm that atoms in metals and crystals are arranged in orderly array.
Differences to Separate Substances
f. Students know differences in chemical and physical properties of substances are used to separate mixtures and identify compounds.
Physical properties, such as changes in temperature, can be used to separate mixtures, such as salt water. The water will evaporate and leave the salt behind. Similarly, magnetism can be used to separate some items from others which are nonmagnetic. Also, combustibility and melting can be used to separate materials. If a metal has a lower melting point than another, it will turn to liquid before the other and can be easily separated.
Properties of Common Elements
g. Students know properties of solid, liquid, and gaseous substances, such as sugar (C6H12O6), water (H2O), helium (He), oxygen (O2), nitrogen (N2), and carbon dioxide (CO2).
- Sugar is typically a solid, i.e., it has a definite shape and a definite volume.
- Water is typically a liquid, i.e., it has a definite volume but not a definite shape. Water can be frozen to become a solid, and it can be heated, so that it becomes water vapor, a gas.
- Helium, oxygen, nitrogen, and carbon dioxide are typically gases, i.e., they have no definite shape or volume. Except for carbon dioxide, these gases can be cooled and compressed into liquids. Solid carbon dioxide, dry ice, sublimes, i.e. it goes directly from solid to gas.
Organisms and Materials
h. Students know living organisms and most materials are composed of just a few elements.
- By weight 98.59 percent of Earth’s entire crust consists of eight elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.
- Similarly, living organisms are mostly composed of the elements carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus; also, calcium is in the bones and iron in the blood.
i. Students know the common properties of salts, such as sodium chloride (NaCl).
Salts are compounds typically made from a metal and a nonmetal. Many salts are hard and brittle and have high melting temperatures. Most salts are soluble in water. When dissolved, they become conductors of electricity. Salts are made when strong acids react with strong bases.