Protons, neutrons and electrons in atoms

The nuclear symbol of an atom shows two numbers. The larger number is the mass number and the smaller number is the atomic number. For sodium the symbol is ²³₁₁Na, so the mass number is 23 and the atomic number is 11.

The atomic number is the number of protons. An atom has no overall charge, so the number of electrons is equal to the number of protons. The number of neutrons is found by taking the atomic number away from the mass number.

Worked example for ²³₁₁Na. Protons = 11. Electrons = 11. Neutrons = 23 − 11 = 12. Check: 11 protons + 12 neutrons = 23, which is the mass number. For ³⁵₁₇Cl, protons = 17, electrons = 17 and neutrons = 35 − 17 = 18. For potassium, ³⁹₁₉K, protons = 19, electrons = 19 and neutrons = 20. For fluorine, ¹⁹₉F, protons = 9, electrons = 9 and neutrons = 10.