Periodic Table Glossary

Short, plain-language definitions of the periodic table's vocabulary. Use this as a quick reference while studying or answering homework questions. Free, public, and ad-free.

Period: A horizontal row of the periodic table. The seven periods correspond to electron shells: electrons fill the n=1 shell during period 1, the n=2 shell during period 2, and so on. Each new period starts when a new shell is opened.
Group: A vertical column of the periodic table. The 18 groups contain elements that share the same number of outer-shell electrons and therefore have similar chemical behavior. Group 1 (alkali metals) and Group 17 (halogens) are the most reactive families; Group 18 (noble gases) is the least reactive.
Alkali metals: Group 1, excluding hydrogen. Soft, shiny, low-density metals that react vigorously with water. They have one electron in their outer shell, which they easily give up to form +1 ions. Examples: lithium, sodium, potassium.
Alkaline earth metals: Group 2. Slightly less reactive than alkali metals but still form basic (alkaline) compounds with oxygen. They have two outer-shell electrons and form +2 ions. Examples: magnesium, calcium.
Transition metals: The large block in the middle of the table (Groups 3–12). Hard, dense, high-melting-point metals that form colorful compounds and can take on multiple oxidation states. Examples: iron, copper, gold, platinum.
Post-transition metals: Metals to the right of the transition block. Softer and lower-melting than transition metals. Examples: aluminum, tin, lead, bismuth.
Metalloids: Elements that share properties of both metals and nonmetals — they conduct electricity, but not as well as true metals. They sit on the staircase line between metals and nonmetals. Examples: boron, silicon, germanium, arsenic.
Nonmetals: The elements in the upper right of the table (excluding the noble gases). Poor conductors, brittle when solid, and tend to gain electrons to form negative ions. Examples: carbon, nitrogen, oxygen, sulfur.
Halogens: Group 17. Highly reactive nonmetals that form salts when combined with metals (the word 'halogen' means 'salt-former' in Greek). They have seven outer-shell electrons and only need one more to fill the shell. Examples: fluorine, chlorine, bromine, iodine.
Noble gases: Group 18. Colorless, odorless, almost completely nonreactive gases. Their outer electron shells are full, which makes them very stable. Examples: helium, neon, argon, krypton, xenon, radon.
Lanthanides: The 15 metallic elements from lanthanum (57) to lutetium (71). Shiny, soft, reactive metals with similar chemistry — their 4f electron shell is being filled. They are usually shown in the two rows below the main table.
Actinides: The 15 elements from actinium (89) to lawrencium (103). All are radioactive. The first few (thorium, uranium) occur naturally; the rest are synthesized. They sit just below the lanthanides in the table.
Atomic number: The number of protons in an atom's nucleus. Each element has a unique atomic number — that's how the table is ordered. Hydrogen is 1, helium is 2, lithium is 3, and so on up to oganesson at 118.
Atomic mass: The weighted average mass of an element's atoms, measured in atomic mass units (u). For most elements, the atomic mass is close to the number of protons plus neutrons in the most common isotope.
Electron configuration: A shorthand for how electrons are distributed across an atom's shells and subshells. For example, carbon's configuration is 1s² 2s² 2p² — two electrons in the first shell, four in the second. The shape of the periodic table directly reflects these configurations.
Phase at STP: Whether the element is a solid, liquid, or gas at standard temperature and pressure (0 °C, 1 atm). Most elements are solid; only two (bromine and mercury) are liquid at STP; eleven (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon, radon) are gas.
Periodicity: The recurring pattern of properties as you move across a period or down a group. Atomic radius shrinks across a period (more protons pull the electrons closer) and grows down a group (more shells). Ionization energy, electronegativity, and metallic character all follow predictable trends.
Electronegativity: How strongly an atom attracts electrons in a chemical bond. Fluorine is the most electronegative element (4.0 on the Pauling scale); the alkali metals are the least. Electronegativity increases across a period and decreases down a group.