Atomic Structure

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The Periodic Table


Groups and Periods

The horizontal rows are called periods and the vertical columns are called groups. Periods refer to how many shells the atom has and groups refer to how many electrons there are in the outer shell of the element.


  • atomic mass increases as you go down each group
  • melting points and boiling points decrease as you go down group I and II
  • melting points and boiling points increase as you go down group VII and VIII
  • alkaline metals become more reactive whereas halogens become less reactive

Atomic Structure

  • centre of an atom is called the nucleus
  • rings around nucleus is shells
  • protons, neutrons and electrons are known as the three sub-atomic particles


Protons and neutrons have the same mass. That is why it is described as 'relative mass'. Electrons are considerably lighter so the mass number is basically the mass of the nucleus as it contains the protons and neutrons. Electrons do not contribute to the mass. Protons have a positive charge, neutrons have a neutral charge and electrons have a negative charge. All atoms of an element are neutral because they contain equal numbers of protons and electrons (+1-1=0 charge)

Mass Number and Atomic Number

Atomic Number (Z) - This is the number of protons that are contained within the nucleus of an atom. It also equals the number of electrons but it is known as the proton number.

Mass Number - The mass number of an element is the sum of the number of protons plus the number of neutons in one atom of the element.

eg. If the mass number was 27 and the atomic number was 13,

  • Protons = 13 (the atomic number)
  • Electrons = 13 (same as protons)
  • Neutrons =  27-13 = 14 (Mass number - Proton Number)


Isotopes are atoms of the same element with the same atomic number but a different mass number. This means that they have the same number of protons and electrons but different numbers of neutrons.

Electron Arrangement

Electrons occupy energy levels at set distances from the nucleus. Each energy level (shell) can hold a maximum number of electrons. The first shell can hold a max. of 2, the second can hold 8 and the third can hold 8. 

eg. Nitrogen has an atomic number of 7 which means there are 7 electrons.

      Electron Arrangement : 2 , 5

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The Alkali Metals

Group I metals…


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