Ionization energy is the energy required to remove an electron from an atom in its gaseous state. Across a period, ionization energy increases because the atoms become smaller and the effective nuclear charge is stronger, making it harder to remove an electron. Down a group, ionization energy decreases as the atomic radius increases, making it easier to remove an electron due to the greater distance from the nucleus and increased electron shielding.Electronegativity is a measure of an atom's ability to attract and bond with electrons. It increases across a period as the effective nuclear charge increases, making atoms more attractive to electrons. Down a group, electronegativity decreases because the increased atomic radius and electron shielding reduce the nucleus's ability to attract electrons.