Periodicity

• An element is classified as s, p, d or f block according to its position in the Periodic Table, which is determined by its proton number
• blocks of periodic table

  
• atomic radius decreases across a period as more electrons are added to the outer shell pulling it closer to the nucleus
• across group 3, the melting points increase from sodium to silicon and then generally decrease from phosphorus to argon

  • sodium, magnesium and aluminium are metals - their melting point increases across the group as they form increasing positive ions and have more delocalised electrons
  • silicon is a tetrahedral macromolecular structure with strong covalent bonds
  • phosphorus, sulphur, chlorine and argon are all molecular substances with weak Van der Waals forces between them

  
• across group 3, first ionisation energy generally increases due to the increase in nuclear charge however there is a small drop between:

  • groups 2 and 3 due to the outer electron being in the 3p orbital which has higher energy
  • groups 5 and 6 due to electron repulsion