Element story

Reading the periodic table

Atomic structure is part of the shared Mendeleev scientific core. This page is intentionally conservative until more native-language terminology is verified.

Start with the nucleus

Atomic structure is part of the shared Mendeleev scientific core. This page is intentionally conservative until more native-language terminology is verified.

Carbon-12 has 6 protons and 6 neutrons; changing the proton count would create a different element.

The periodic table is not just a list: recurring electronic structure creates trends in atomic size, ionization energy and other properties. Trends are patterns, not magic rules, and exceptions are scientifically interesting.

Keep exploring

Atomic Structure · Periodic Trends

Start with the atomic number

The atomic number Z is the number of protons in the nucleus. It uniquely identifies the element. Sodium has Z = 11, so every sodium nucleus contains 11 protons.

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.

Blocks link the table to electron configuration

The s, p, d and f blocks reflect the type of subshell associated with electron filling. Helium is a useful exception: it is in group 18 because of its chemistry, but its 1s² configuration places it in the s block.

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⁻.

How to read the periodic table