IGCSE Physics / Atoms and radioactivity
Atomic number, mass number and isotopes
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.3, 7.2. Free lesson.
Read the revision notes · IGCSE Physics past papers
Atomic number tells you how many protons an atom has, while mass number counts its protons and neutrons. These numbers let you find the neutron count and explain why isotopes are atoms of the same element with different numbers of neutrons.
What atomic number and mass number count
The atomic number, Z, is the number of protons in the nucleus. It decides which element an atom is: an atom with 6 protons is always carbon. The mass number, A, is the total number of protons and neutrons in the nucleus. These particles are called nucleons. Atomic number (proton number), Z = number of protons. Mass number (nucleon number), A = protons + neutrons.
How to find the number of neutrons
Once you know A and Z, subtract the number of protons from the total number of nucleons. The neutron number, N, is a count, so it has no unit. N = A − Z
What makes atoms isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 each have 6 protons. Carbon-12 has 6 neutrons, while carbon-14 has 8. Both carbon isotopes have 6 protons, so both are carbon. A neutral atom of either isotope has 6 electrons, because a neutral atom has as many electrons as protons. Only the neutron count, and therefore the mass number, differs.
How to compare isotopes
When deciding whether two atoms are isotopes, compare their protons and neutrons. The proton number must match, while the neutron numbers must differ. Do not say that isotopes have different numbers of electrons: for neutral atoms of the same element, the electron numbers match too. Uranium-235 has 92 protons and 143 neutrons. Uranium-238 has 92 protons and 146 neutrons. They are isotopes because they have the same number of protons but different numbers of neutrons.