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Isotopes

The atomic number is the number of protons, and it decides which element the atom is. The mass number is protons plus neutrons. Isotopes of the same element have the same number of protons but different numbers of neutrons, and therefore different masses. In a neutral atom there are as many electrons as protons. The neutrons affect the mass and the stability of the nucleus, but not the chemistry. Carbon-12 and carbon-14 are therefore both carbon.

A=Z+NA = Z + Nmass number

Symbols

AAmass number
ZZatomic number, number of protons
NNnumber of neutrons

Example

Carbon-14 has atomic number 6.

Neutrons: 14−6=814 - 6 = 8. A neutral atom also has 6 electrons.

The mass number is written at the upper left, the atomic number at the lower left. The neutrons are the difference.
Practise atoms and the periodic table for free →

Electron shells →

Part of Chemistry 1: Atoms and the periodic table.