Unit 2: Section 1 - Periodicity

Question 1

What is periodicity?

Answer 1

The trends that occur (in physical and chemical properties) as you move across the periods

Question 2

How does atomic radius change across a period?

Answer 2

Decreases
1. As the number of protons increases, the positive charge of the nucleus increases
2. This means electrons are pulled closer to the nucleus, making the atomic radius smaller
3. The extra electrons that the elements gain across a period are added to the outer energy level so they don’t provide any extra shielding effect (shielding is mainly provided by the electrons in the inner shells)

Question 3

How does melting point change across a period?

Answer 3

Varies depending on the structure of the elements and the bonding within them

Question 4

How does melting point change across period 3?

Answer 4

1. Na, Mg and Al are metals so their boiling points increase across the period because the metal-metal bond gets stronger - increased positive charge, increasing number of electrons and radius
2. Si is macromolecular with a tetrahedral structure - strong covalent bonds which require a lot of energy to break
3. P4, S8 and Cl2 are all molecular so it depends on the strength of their Van der Waals forces between the molecules - weak forces that are easily overcome so have low melting points
4. S8 is the biggest molecule so has stronger Van der Waals forces so has a higher melting point than P4 and Cl2
5. Ar has a very low melting point because it exists as individual atoms (monoatomic) so has very weak Van der Waals forces

Question 5

What is the trend in ionisation energy across a period?

Answer 5

Generally increases
The increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing