Ionisation Energy

Question 1

Why does the first ionisation energy increase across a period?

Answer 1

Because the number of shells stay the same but the nuclear charge increases meaning the electrons are more attracted to the nucleus and require more energy to be removed.

Question 2

Why does the first ionisation energy decrease between group 2 and group 3 elements?

Answer 2

Because the highest energy electron is in the p-sub shell for a group three element and in a s-sub shell for a group 2 element. Since the p sub shell is at slightly higher energy than the s-subshell it is slightly easier to remove an electron from a group 3 element.

Question 3

Why does the ionisation energy decrease between group 5 and group 6 elements?

Answer 3

This is because the highest energy electron is a paired p-electron for the group 6 element but an impaired p electron for the group 5 element. Repulsion raises the energy of electrons so it is easier to remove a paired electron.