The periodic table arranges all of the known elements in order of increasing atomic number. Order generally coincides with increasing atomic mass. The different rows of elements are called periods. The period number of an element signifies the highest energy level an electron in that element occupies (in the unexcited state). The number of electrons in a period increases as one moves down the periodic table; therefore, as the energy level of the atom increases, the number of energy sub-levels per energy level increases.
Elements that lie in the same column on the periodic table (called a "group") have identical valance electron configurations and consequently behave in a similar fashion chemically. For instance, all the group 18 elements are inert, or noble gases.
Element groups are either nonmetals or various subsets of metals, but there is no distinct line between the two types of elements. Metal elements are usually good conductors of both electricity and heat. Subsets are based on similar characteristics and chemical properties. Our version of the periodic table uses the most commonly accepted demarcations between the elements.
Alkali metals: The alkali metals make up Group 1 of the table, and comprise lithium (Li) through francium (Fr). These elements have very similar behavior and characteristics. Hydrogen is Group 1, but it exhibits few characteristics of a metal and is often categorized with the nonmetals.
Alkaline earth metals: The alkaline earth metals make up Group 2 of the periodic table, from beryllium (Be) through radium (Ra). The alkaline earth metals have very high melting points and oxides that have basic alkaline solutions.
Lanthanides: The lanthanides comprise elements 57 — lanthanum (La), hence the name of the set — through 71, lutetium (Lu). They, along with the actinides, are often called "the f-elements" because they have valence electrons in the f shell.
Actinides: The actinides comprise elements 89, actinium (Ac), through 103, lawrencium (Lr). They, along with the lanthanides, are often called "the f-elements" because they have valence electrons in the f shell. Only thorium (Th) and uranium (U) occur naturally with significant abundance. They are all radioactive.
Transition metals: The transition elements are metals that have a partially filled d subshell and comprise Groups 3 through 12 and the lanthanides and actinides.
Post-transition metals: The post-transition elements are aluminum (Al), gallium (Ga), indium (In), thallium (Tl), Tin (Sn), lead (Pb) and bismuth (Bi). As the name implies, these elements have some of the characteristics of the transition metals, but they tend to be softer and conduct more poorly than the transition metals.
Metalloid: The metalloids are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po). They sometimes behave as semiconductors (B, Si, Ge) rather than as conductors. Metalloids are also called "semi-metals" or "poor metals."
Nonmetals: The term "nonmetals" is used to classify hydrogen (H), carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulfur (S) and selenium (Se).
Halogens: The halogen elements are a subset of the nonmetals. They comprise Group 17 of the periodic table, from fluorine (F) through astatine (At). They are generally very chemically reactive and are present in the environment as compounds rather than as pure elements.
Noble gases: The inert, or noble, gases comprise Group 18. They are generally very stable chemically and exhibit similar properties of being colorless and odorless.
(Source: Los Alamos National Laboratory)