| A | B |
|---|
| Alkali metals | Elements of Group 1A (1) except hydrogen. These are soft, shiny metals with one outer shell electron. |
| Alkaline earth metals | Group 2A (2) elements, which have 2 electrons in their outer shell |
| Atom | The smallest particle of an element that retains the characteristics of the element |
| Atomic mass | The weighted average mass of all the naturally occurring isotopes of an element |
| Atomic Mass Unit (amu) | A small mass unit used to describe the mass of very small particles, such as atoms and subatomic particles. 1 amu is equal to 1/12 the mass of a carbon-12 atom. |
| Atomic number | A number that is equal to the number of protons in an atom |
| Atomic Symbol | An abbreviation used to indicate the mass number and atomic number of an isotope |
| Chemical symbol | An abbreviation that represents the name of an element |
| Compound | A pure substance consisting of two ore more elements, with a definite composition, that can be broken down into simpler substances by chemical methods |
| Electron | A negatively charged subatomic particle that has a very small mass and is usually ignored in calculations; its symbol is e- |
| Electron-dot symbol | The representation of an atom that shows valence electrons as dots around the symbol or the element |
| Element | A pure substance that cannot be separated into any simpler substances by chemical methods |
| Energy level | Level within the atom that contains electrons or similar energy |
| Group | A vertical column in the periodic table that contains elements having similar physical and chemical properties |
| Group number | A number that appears at the top of each vertical column (group) in the periodic table and indicates the number of valence electrons |
| Halogen | Group 7A (17) elements of fluorine, chlorine, bromine, iodine, and astatine |
| Heterogeneous mixture | A mixture of two or more substances that are not mixed uniformly |
| Homogeneous mixture | A mixture of two or more substances that are mixed uniformly |
| Ionization energy | The energy needed to remove the least tightly bound electron from the outermost energy level of an atom |
| Isotope | An atom that differs only in mass number from another atom of the same element. Isotopes have the same atomic number (number of protons) but different numbers of neutrons. |
| Mass number | The total neutrons and protons in the nucleus of an atom |
| Metal | An element that is shiny, malleable, ductile and a good conductor of heat and electricity. The metals are located to the left of the zigzag line in the periodic table. |
| Metalloid | Elements with properties of both metals and nonmetals located along the heavy zigzag line in the periodic table |
| Mixture | A neutral subatomic particle having a mass of 1 amu and found in the nucleus of an atom; its symbol is n or n0 |
| Neutron | A neutral subatomic particle that has a mass of 1 amu and is found in the nucleus of an atom; its symbol is n or n zero |
| Noble gas | An element in Group 8A (18) of the periodic table, generally non-reactive and rarely found in combination with other elements |
| Nonmetal | An element with little or no luster that is a poor conductor of heat and electricity. The nonmetals are located to the right of the zigzag line in the periodic table. |
| Nucleus | The compact, very dense center of an atom that contains the protons and neutrons of an atom |
| Orbital | The region around the nucleus where electrons of a certain energy are more likely to be found. The s orbitals are spherical; the p orbitals have two lobes. |
| Period | A horizontal row of elements in the periodic table |
| Periodic table | An arrangement of elements by increasing the atomic number such that elements that have similar chemical behavior are grouped in vertical columns |
| Physical property | A characteristic that can be observed or measured without affecting the identity of an element, including shape, color, odor, taste, density, hardness, melting point, and boiling point |
| Proton | A positively charged subatomic particle that has a mass of 1 amu and is found in the nucleus of an atom; its symbol is p or p+ |
| Pure substance | A type of matter with a fixed composition: elements and compounds |
| Valence electrons | Electrons in the outermost energy level of an atom |
