Question

Looking across period 4 of the periodic table, potassium (atomic number 19) is followed by calcium (atomic number 20), which is followed by scandium (atomic number 21). Which element should have the largest atomic radius?

Answer 1

Potassium should have the largest atomic radius because the atomic radius decreases in size from left to right on the periodic table.

Answer 2

atomic radius decreases from left to right across a period because of increasing Zeff (pull of protons) so potassium should have the largest atomic radius.