Atomic radii is the distance between a nuclei and the outermost electron.Atomic radius decreases across a period because the outermost electron is being added to the same energy level at the same time the nucleus is increasing in protons. The increase in nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus.The atomic radius usually increases while going down a group due to the addition of a new energy level (shell).The first ionisation energy of an element is the energy to lose or gain the first electron. There is a greater attraction between the nucleus and the outmost element ,which means that more energy is needed to remove an electron as you go across the period.The first ionisation energy decreases on going down a group.
This is because the electron to be removed from the outer energy level is increasingly distant from the nucleus, as a result of the atoms getting bigger. The attraction of the nucleus for the electron becomes less, and it becomes easier to pull it away.Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electron.Trends in electronegativity across the period is that as we move from left to right in a period, the electronegativity increases.Trends in electronegativity in groups decrease as we move down the group.