Chapter 8: Periodicity

Question 1

Explain why the value of the first ionisation energy of sulfur is less than the value of the first ionisation energy of phosphorous. (2)

Answer 1

Sulfur has 4 electrons in 3p whereas phosphorous has 3 electrons in 3p. The repulsion between the pairs of electrons in sulfur means it is easier to remove an electron.

Question 2

State the trend in first ionisation energies in Group 2 from beryllium to barium. Explain your answer in terms of a suitable model of atomic structure. (2)

Answer 2

Trend: decreases
Explanation: the atomic radius increases, and there is more shielding as the group descends.

Question 3

Explain why the second ionisation energy of magnesium is greater than the first ionisation energy of magnesium. (1)

Answer 3

The electron being removed is closer to the nucleus.

Question 4

State and explain the general trend in first ionisation energies of the Period 3 elements sodium to chlorine. (3)

Answer 4

Trend: increases
Explanation: The nuclear charge increases from Na to Cl, and there is similar shielding.

Question 5

State how the element sulfur deviates from the general trend in first ionisation energies across Period 3. Explain your answer. (3)

Answer 5

How sulfur deviates from the trend: lower

Explanation: pair of electrons in 3p orbital repel each other.

Question 6

Explain why the second ionisation energy of sodium is greater than the second ionisation energy of magnesium. (3)

Answer 6

Na2+ needs to lose an electron from the 2p orbital whereas Mg2+ needs to lose an electron from the 3s orbital. There is less shielding in Na, meaning that the electron is closer to the nucleus so the electron is more attracted to the nucleus in Na.

Question 7

Explain why sodium has a lower melting point than magnesium. (3)

Answer 7

Na is a bigger atom and therefore has a smaller nuclear charge, so there is less attraction between the nucleus and the delocalised electrons.

Question 8

Explain why the melting point of sulfur, S8, is higher than that of phosphorous, P4. (2)

Answer 8

There are van der Waals forces between the molecules. Sulfur has more atoms, meaning that the intermolecular forces are stronger.

Question 9

State and explain the trend in melting point of the Group II elements Ca-Ba. (3)

Answer 9

Trend: decreases
Explanation: There are more shells, so there is weaker attraction for the delocalised electrons.