GCSE Combined Science (AQA)

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Atomic Number & Mass Number

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

Atomic Number & Mass Number

6:38 Atomic and Mass Number: Subatomic Particles and Size
Spec 4.1.1.4 4.1.1.5
Previous Protons, Electrons & Neutrons Next Atom Size
  • Atoms of an element always have the same number of protons.
  • Oxygen atoms have exactly eight protons.
  • The atomic number of an atom is its number of protons, also referred to as the proton number.
  • Different elements have different numbers of protons.
  • Carbon atoms have 12 protons, nitrogen atoms have 14 protons.
  • Atomic nuclei contain protons and neutrons.
  • Lithium atom example: 3 protons (atomic number) and 4 neutrons.
  • Mass number is the sum of protons and neutrons.
  • Lithium has a mass number of seven.
  • Periodic table displays elements with their atomic number.
  • Atomic number is the number of protons in the nucleus.
  • Mass number is usually larger than the atomic number.
  • Not all periodic tables display the mass number.
  • Atomic number and mass number help calculate subatomic particles.
  • Magnesium example: 12 protons, 12 electrons, 12 neutrons.
  • Sodium example: 11 protons, 11 electrons, 12 neutrons.
  • Calculation of neutrons: mass number minus atomic number.

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