Ionization Energy, Electronegativity, Relative Sizes

The amount of energy required to pull an electron off a neutral atom is called the ionization energy. Since each successive electron shell is larger than the previous one, the electrons in the shells further from the nucleus require less energy to be pulled off. In other words, the larger the shell number or the further down in the Periodic Table of the Elements, the lower the ionization energy.

As we go from left to right on the Periodic Table of the Elements, there are more and more protons in the nucleus. The greater number of protons pulls stronger at the valence electrons (the electrons in the outermost shell). This causes an increase in the ionization energy.