I know the trend in group 1 is that ionisation energy decreases down the group due to an increase in atomic radius and more energy levels are added so more shielding, but I'm not sure why there's such a big jump between sodium and potassium.
My textbook doesn't explain it very clearly for me but says the d orbital doesn't shield the outer electrons very well explaining why K to Cs has similar values, but potassium's 3d orbital is not yet filled, so I'm not sure why it's value is so much lower.
I'll put the values from my textbook in to help explain what I mean. Li=513, Na=496, K=419, Rb=403, Cs=376. (in kJ/mol)
The gap is much more obvious when you look at a graph.
Sorry if I haven't made it very clear but any help would be appreciated. Thanks