2.1.3 Trends of Period 3 Elements : First Ionisation Energy

Question 1

What is the first ionisation energy?

Answer 1

Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions.

Question 2

What is GENERAL the trend in ionisation energy across a period?

Answer 2

• Increase across period
• Nuclear charge increases
• Atomic radius decreases
• Less shielding because shells remain fairly constant
• Stronger forces of attraction between nucleus and outer electrons
• So harder to remove outer electrons
• So high ionisation energy

Question 3

Why is there a small drop in ionisation energy at Al and S?

Answer 3

Al :
- Valence electron in Mg fills its 3S subshell with 2 electrons
- But in Al, new subshell 3p contains 1 electron which is in a higher energy state (3p is higher than 2s), so easier to lose the electron, so Al has a lower ionisation energy than Mg

S :
- S outer electron is paired with another electron in 3p subshell, and when this 2nd electron in that orbital is added, there is slight repulsion between the 2 negatively charged electrons, making it easier to lose that outer electron, so its ionisation energy is lower