Groups and periods
A group is a vertical column; a period is a horizontal row. Sodium is in group 1 and period 3. Elements in the same group often share features of their outer electron structure.
Group and period locate the element
A group is a vertical column; a period is a horizontal row. Sodium is in group 1 and period 3. Elements in the same group often share features of their outer electron structure.
Na
Z = 11 · 1 · 3
Cl
Z = 17 · 17 · 3
Worked example: sodium
Sodium has Z = 11, is in period 3, group 1 and the s block. Its ground-state electron configuration is 1s² 2s² 2p⁶ 3s¹.
What can be inferred for chlorine?
Chlorine has Z = 17, lies in period 3 and group 17, and its configuration ends in 3s² 3p⁵. It therefore has seven valence electrons in its ground state and commonly forms Cl⁻.
Position helps identify chemical families, electron patterns and broad trends. Precise reactivity, toxicity, melting point or the behaviour of a compound require additional data and models.