What do the chemistry terms period, group (family), atomic radius, and ion mean?

What you are being asked to do

You just need clear definitions of four common periodic table and atomic-structure terms. The goal is to connect each term to what you can actually see in the periodic table or what happens to electrons.

Period (row)

• A period is a horizontal row on the periodic table.
• Key idea: elements in the same period have the same number of occupied electron shells (energy levels).
• Example: Period 2 elements (Li to Ne) have electrons in two shells.

Group or family (column)

• A group (family) is a vertical column on the periodic table.
• Key idea: elements in the same group tend to have the same number of valence electrons, so they often react in similar ways.
• Example: Group 1 (alkali metals) usually have 1 valence electron.

Atomic radius (atom size)

• Atomic radius describes the size of an atom.
• A common working definition is: half the distance between the nuclei of two identical atoms bonded together.
• Written as: if the nucleus-to-nucleus distance is $d$, then atomic radius $r = \frac{d}{2}$.

Ion (charged particle)

• An ion is an atom or molecule that has a net electric charge because the number of electrons and protons are not equal.
• If it loses electrons, it becomes a cation with a positive charge (for example, $Na \rightarrow Na^+ + e^-$).
• If it gains electrons, it becomes an anion with a negative charge (for example, $Cl + e^- \rightarrow Cl^-$).

Quick memory hooks

• Period = across (same shells).
• Group = down (similar valence electrons).
• Radius = size.
• Ion = charge from electron gain/loss.