Atomic & Mass Number (Edexcel GCSE Physics)

Atomic & Mass Number

Atomic Number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)

  • Elements in the periodic table are ordered by their atomic number

  • Therefore, the number of protons determines which element an atom is

• The atomic number of a particular element is always the same

• For example:

  • Hydrogen has an atomic number of 1. It always has just one proton

  • Sodium has an atomic number of 11. It has 11 protons

  • Uranium has an atomic number of 92. It has 92 protons

• The atomic number is also equal to the number of electrons in an atom

  • This is because a neutral atom has the same number of electrons and protons in order to have no overall charge

Mass Number

• The total number of particles in the nucleus of an atom is called its mass number (it is also called the nucleon number)

  • The mass number is the total number of protons and neutrons in the atom

• The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutrons = Mass Number - Atomic Number

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

• The mass number of an element can change, which means they are are isotopes

Nuclear Notation

• The mass number and atomic number of an atom are shown by writing them with the atomic symbol

  • This is called nuclear notation

• Here are three examples:

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

• The top number is the mass number

  • This is equal to the total number of particles (protons and neutrons) in the nucleus

• The lower number is the atomic number

  • This is equal to the total number of protons in the nucleus

• The atomic and mass number of each type of atom in the examples above is shown in this table:

Protons, Neutrons & Electrons Table