GCSE Chemistry (AQA)

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Isotopes

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Atomic structure and the periodic table A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

Isotopes

4:18 Isotopes and Relative Atomic Mass of Elements
Spec 4.1.1.5
Previous Atom Size Next Relative Atomic Mass
  • Atoms of the same element can have different numbers of neutrons.
  • These atoms are called isotopes of that element.
  • Example: Carbon 12, which has a mass number of 12.
  • Carbon 12 makes up about 98-99% of naturally occurring carbon on Earth.
  • The rest is mostly carbon 13.
  • Isotopes have the same number of protons but different numbers of neutrons.
  • Carbon 12 has six protons and six neutrons.
  • Carbon 13 has six protons and seven neutrons.
  • Isotopes have the same atomic number but different mass numbers.
  • Protons and neutrons both have a mass of one.
  • Carbon 13's mass number is one greater due to an extra neutron.
  • In exams, you may need to identify the number of protons, neutrons, and electrons in isotopes.
  • Example calculation:
  • Carbon 12: Atomic number 6, so 6 protons, 6 electrons, and 12 - 6 = 6 neutrons.
  • Carbon 14: Atomic number 6, so 6 protons, 6 electrons, and 14 - 6 = 8 neutrons.
  • The number of protons is equal to the atomic number.
  • The number of electrons in a neutral atom is also equal to the atomic number.
  • To find the number of neutrons, subtract the atomic number from the mass number.

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