Periodic Table Key Concepts: Groups, Periods, and Element Properties

Group / Family

A vertical column on the periodic table. Elements in the same group have similar chemical properties because they have the same number of valence electrons.

Period / Series

A horizontal row on the periodic table. Atomic number increases from left to right.

Atomic Number

The number of protons in the nucleus of an atom.

Atomic Mass

The mass of a single atom, measured in atomic mass units (amu).

Valence Electrons

Electrons found in the outermost energy level (shell) of an atom.

Oxidation Number

The charge an atom has after gaining or losing electrons when forming compounds.

Representative Elements

Elements found in groups 1-2 and 13-18 (the main group elements).

Metals

Found on the left and center of the periodic table; shiny, malleable, and good conductors of heat and electricity.

Nonmetals

Found on the right side of the periodic table; poor conductors and often brittle.

Metalloids

Have properties of both metals and nonmetals; semiconductors that can conduct electricity under certain conditions.

Diatomic Elements

Elements that naturally exist as molecules made of two atoms bonded together (e.g., H₂, O₂, N₂).

Sublevel Blocks

Sections of the periodic table that show how electrons are arranged in atoms — s-block, p-block, d-block, and f-block.

Alkali Metals

Found in Group 1; very reactive metals with low density (e.g., lithium, sodium).

Alkaline Earth Metals

Found in Group 2; reactive but less so than alkali metals (e.g., beryllium, magnesium).

Transition Metals

Found in the middle of the periodic table; have partially filled d orbitals and often form colored compounds (e.g., iron, copper, nickel).

Chalcogens

Found in Group 16; have six valence electrons and include oxygen and sulfur.

Pnictogens

Found in Group 15; have five valence electrons and include nitrogen and phosphorus.

Halogens

Found in Group 17; very reactive nonmetals that form salts (e.g., fluorine, chlorine).

Noble Gases

Found in Group 18; have full outer electron shells, making them stable and colorless gases (e.g., helium, neon).

Lanthanide & Actinide Series

Two rows of elements placed below the main table to keep it compact; include rare earth and radioactive elements.

Synthetic Elements

Man-made elements that do not occur naturally on Earth.

Naturally Radioactive Elements

Elements with unstable nuclei that emit radiation as they decay into more stable forms (e.g., uranium, radium, polonium.