Potassium, Bromine and Potassium Bromide

Here is some notes about Group 1, Group 7 and Ionic Bonding, using K, Br and KBr as examples:

Potassium

  • Potassium is in group 1 – the alkali metals – meaning it has one electron in its outer shell, and in the fourth period, so it has four shells.
  • This makes potassium very reactive because the first ionisation energy is only 418.8 kJ/mol as the outer electron is far from the nucleus so potassium easily loses this electron in reactions to form a K+1 ion.
  • The electron arrangement is 2, 8, 8, 1.
  •  The electron configuration for potassium is: 1s2 2s2 2p6 3s2 3p6 4s1. 
  • As potassium isn’t very dense, at room temperature it is a soft silvery solid that can easily be cut with a knife.
  • Potassium has a low melting point – 63°C and a boiling point of 759°C.
  • In a flame test potassium emits a lilac colour flame.
  • Potassium reacts with water in an exothermic reaction to produce potassium hydroxide and hydrogen gas. The metal floats of the water’s surface and ignites the escaping hydrogen.

 

Bromine

  • Bromine is in group 7 – the halogens – so it has 7 electrons in its outer shell, and is also in the fourth period so has four shells.
  • Elemental bromine exists as a diatomic molecule, Br2 and it is a dense, red-brown liquid at room temperature, and gives an orange vapour when heated.
  • It can form ionic bonds with metals as it becomes a -1 ion, but it can also form covalent bonds with non-metals. It is very reactive with metals forming a salt (metal bromide).
  • Reactivity decreases down the group as the outer electrons are further away from the nucleus so are less attracted to it so it becomes harder to attract an extra electron. This property means that Fluorine and Chlorine will both displace bromine, but bromine can displace iodine.
  • The electron arrangement is 2, 8, 18, 7.
  • The electron configuration is 1s2 2s2 2p6 3s2 3p6 4s3d10  4p5.
  • Bromine has a low melting point of -7°C and a low boiling point of 59°C.
  • Bromine can be used to test for saturation in hydrocarbons as the bromine solution will turn colourless if the other solution is unsaturated.
  • Bromine does dissolve in water but it is highly soluble in organic solvents such as carbon disulfide as it is a non-polar molecule.

 

Potassium Bromide

  • It is a white crystalline powder and easily dissolves in water as it is an ionic molecule and so dissociates, with potassium attracting the slightly negative oxygen atoms while bromine attracts the slightly positive hydrogen atoms.
  • It has a high melting point of 734 °C and a boiling point of 1435 °C due to its strong ionic bonds.
  • It can be reacted with silver nitrate to form silver bromide which is used in photographic film.
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