Atomic number

Definition

The number of protons in an atom’s nucleus, which defines the element’s identity and sets its electron structure.

Each element box on the periodic table lists the atomic number, and it equals the proton count exactly. Because it determines how many electrons fill the shells, it ultimately governs valence configuration and therefore chemical behavior. Increasing atomic number across a period is what drives the gradual change in properties.

Related concepts

Proton · Valence electrons · Atomic mass and atomic weight

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Part of The Blueprint of Water · 🧪 chemistry