Mass Number

Specification Point 1.7:
  • Recall the meaning of the term mass number of an atom.
  • Mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom. The symbol for this number is A.
  • The nucleon number minus the proton number gives you the number of neutrons of an atom.
  • Note that protons and neutrons can collectively be called nucleons.
  • The atomic number and mass number for every element is on the Periodic Table.

Atomic Number

Specification Point 1.8:
  • Describe atoms of a given element as having the same number of protons in the nucleus and that this number is unique to that element.
  • The atomic number (or proton number) is the number of protons in the nucleus of an atom. The symbol for this number is Z.
  • It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table.
  • The proton number is unique to each element and no two elements have the same number of protons.
  • Electrons come and go during chemical processes but the proton number doesn’t change.

Atomic Structure Notation, Edexcel GCSE Chemistry

Diagram showing the notation used on the Periodic Table


Specification Point 1.9:
  • Describe isotopes as different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei.
  • Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
  • The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
  • So C-14 is the isotope of carbon which contains 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
  • Since isotopes of the same element have the same number of electrons, their chemical properties are identical.

The atomic structure and symbols of the three isotopes of hydrogen

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Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.