Patterns In Ionisation Energies Across A Period

Question 1

Why is there a general increase in 1st ionization energies

Answer 1

Nuclear charge increases

Electron removed from same shell

Force of attraction between positive nucleus and outer negative electrons increase

Distance between nucleus and outer electrons remains reasonably constant

Shielding by inner shells remains reasonably constant

Question 2

Why is there a rapid decrease in the last element in one period and the first element in the next period

Answer 2

Helium has 2e in first quantum shell

Helium has 3e, third goes to next quantum shell further away from the nucleus

Force of attraction decreases

Nuclear charge increases

Distance, shielding increases
This outweighs increased nuclear charge

Question 3

Why is there a slight decrease between beryllium and boron

Answer 3

Boron has one more proton than beryllium

Beryllium 1s2 2s2
Boron 1s2 2s2 2p1

5th e in broom must be in 2p sub shell
It’s slightly further than 2s

Less attraction between 5th in boron
Distance increased
Shielding increased slightly
Outweighs increased nuclear charge

Question 4

Why is there a slight decrease between nitrogen and oxygen

Answer 4

Oxygen has one more proton
Electron removed in the same 2p sub shell

Electron removed in nitrogen from orbital with unpaired electron
Electron removed in oxygen from orbital with pair of electrons

Extra repulsion, less energy needed to remove it, lower ionization for Oxygen