Periodicity

• What happens to the atomic radius across a period?

  Decreases as the number of protons increases, the positive charge of the nucleus increases. This pulls electrons closer to the nucleus making the atomic radius smaller
• What is the trend of melting point across period 3?

  Increases from sodium to silicon, but then gradually decrease from silicon to argon
• Why is the melting point of sodium, magnesium and aluminium high?

  They have metallic bonding, thus across the period the metallic bonds are stronger and their melting points increase as across the period, the metal ions have an increasing positive charge, an increasing number of delocalised electrons and a decreasing radius.
• Why is the melting point of silicon the largest?

  Silicon is macromolecular, with a tetrahedral structure and strong covalent bonds which require a lot of energy to break
• Why is the melting point of phosphorous, sulfur, chlorine and argon decreasing?

  They are all molecular substances and they have weak Van der Waals forces which are easily overcome, so these elements have low melting points
• What is the trend of ionisation energy across period 3?

  A general increase, as there’s an increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing