The first ionization energy is the energy it takes to remove the outermost valence electron. As you move down a column, it becomes easier to remove a valence electron because each step down a column is an extra electron shell and there is an effect called shielding. What shielding is, is electrons in lower energy levels shield outer energy levels from the attractive forces of the nucleus. This makes it easier to remove an electron because the nucleus has a weaker hold on it = a lower ionization energy. As you move right across a row on the periodic table the first ionization energy increases. This is because each move to the right is the addition of a proton, resulting in a higher effective nuclear charge (which may be represented as Zeff. Because there are more protons, which are positively charged the nucleus has a stronger pull on the negatively charged electrons.