ionisation energy patterns across a period and down a group

general increase across period

increased nuclear charge causes increased force of attraction…atomic radii and shielding effect remains constant

rapid decrease in IE between first and last element in two periods

decreased force of attraction caused by increased atomic radii and shielding effect…these two factors outweigh increased nuclear charge

slight decrease between beryllium and boron

less attraction between fifth electron in boron and nucleus due to slight increase in atomic radii and shielding effect…these two factors outweigh increased nuclear charge

decreased IE down a group

less attraction due to increased atomic radii and shielding by complete inner shells…these two factors outweigh increased nuclear charge