3.2.1 Periodicity

Question 1

What happens to atomic radius across a period?

Why?

Answer 1

Atomic radius decreases

1. As the number of protons increases, the positive charge of the nucleus increase. Therefore electrons are pullled closer to the nucleus and the atomic radius decreases
2. The extra electrons that elements gain across the period are added to the outer energy level so they don’t contrubute any extra electron shielding

Question 2

What happens to melting points of metals across a period?

Why?

E.g. Soduim, magnesium and aluminium

Answer 2

Melting/ boiling points increase as metallic bonds get stronger, due to metal ions having a greater positive charge, increasing number of delocalised electrons, and a decreasing atomic radius.

Question 3

What happens to the melting point of macromolecular elements across a period?

Why?

E.g. Silicon

Answer 3

Silicon is macromolecular, with a tetrahedral structure and strong covalent bonds link it’s atoms. Therefore a lot of energy is required to overcome the bonds, therefore sillicon has the highest melting point across the period.

Question 4

What happens to the melting points of molecular substances across a period?

Why?

E.g. Phosphorus, sulfer, chlorine

Answer 4

Melting points depend on the strength of van der Waals forces. VDW forces are easily overcome so molecular substances have low melting points. Elements with more atoms e.g. S₈, will have stonger VDW forces.

Question 5

What happens to the melting points of monotomic elements across a period?

Why?

E.g. Argon

Answer 5

Monotomic elements only exist as individual atoms therefore have very weak VDW forces. Therefore have a very low melting point.

Question 6

What is the trend in ionisation energies across a period?

Why?

Answer 6

Ionistaion energies generally increase across a period, due to increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing.