Groups and Periods on the Periodic Table
By the Periodixy Editorial Team · Last reviewed July 10, 2026
The periodic table is a grid with meaning in both directions. Columns (groups) collect elements with the same outer electron count and therefore similar chemistry. Rows (periods) collect elements filling the same electron shell. Read both and the table becomes a prediction machine.
Groups: the vertical families
The 18 groups are numbered left to right. Same group = same valence electrons = similar reactions. Several groups have family names worth knowing:
| Group | Family name | Signature behaviour |
|---|---|---|
| 1 | Alkali metals | Soft, explosive with water, +1 ions |
| 2 | Alkaline earth metals | Reactive metals, +2 ions |
| 3–12 | Transition metals | Hard, colourful compounds, variable charges |
| 17 | Halogens | Reactive nonmetals, −1 ions, salt-formers |
| 18 | Noble gases | Full shells, almost totally unreactive |
Family resemblance in action
Sodium reacts with chlorine to give NaCl. What do potassium and bromine give?
- Potassium is in sodium's family (group 1) → behaves like Na.
- Bromine is in chlorine's family (group 17) → behaves like Cl.
Answer: KBr — same 1:1 pattern, exactly as the group logic predicts.
Periods: the horizontal shells
The 7 periods are numbered top to bottom. The period number equals the number of occupied electron shells: period 1 elements (H, He) use one shell; period 4 elements use four. Crossing a period left to right, one proton and one electron are added at each step, transforming a violent metal (sodium) into an inert gas (argon) in just eight elements.
Using both together
Group + period pins an element down completely: “group 17, period 3” can only be chlorine. It also decodes electron structure instantly — chlorine must have 3 shells (period) with 7 electrons in the outer one (group): 2, 8, 7.
Explore the group and period filters on the interactive periodic table to see these patterns highlighted.
Summary
- Groups (columns, 1–18): same valence electrons, similar chemistry.
- Periods (rows, 1–7): same number of electron shells.
- Named families: alkali metals, alkaline earths, transition metals, halogens, noble gases.
- Group + period together identify an element and its electron arrangement.