Nuclear Notation (OCR GCSE Physics A (Gateway))

Nuclear Notation

Properties of Atoms

• The atom is made up of:

  • Protons - positively charged particles with a relative atomic mass of one unit, found in the nucleus

  • Neutrons – no charge, and also with a relative atomic mass of one unit, found in the nucleus

  • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)

• The properties of each of the particles are shown in the table below:

• Although atoms contain particles of different charge, the total charge within an atom is zero

  • This is because the number of electrons is equal to the number of protons

• For example, the following table sets out the calculation of the total charge in the Lithium atom:

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 atoms have 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

• The mass number is the 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 Neutron = 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

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: