Atoms and ions

Find proton, neutron and electron numbers from atomic number, mass number and ion charge.

The chemistry

The atomic number tells you how many protons an atom has. An element keeps this proton number when it forms an ion.

Mass number counts protons and neutrons. Subtract the atomic number to find the neutron count.

Charge compares protons with electrons. Losing electrons makes a positive ion; gaining electrons makes a negative ion.

A sodium-23 ion has atomic number 11 and charge 1+.

It has 11 protons, 23 − 11 = 12 neutrons and 11 − 1 = 10 electrons.

Specification references

AQA 4.1.1.4 and 4.2.1.2; Edexcel 1.3–1.6 and 1.22

Particle counts and ion formation.

Course

Element Cl; atomic number 17; mass number 35; charge 1−.

Find the numbers of protons, neutrons and electrons in this chlorine ion.

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