Answer
The relative numbers represent how much more energy is required to remove the second electron than the first one.
Work Step by Step
The second one always takes more energy, as shielding and electron repulsion are decreased due to the removal of the first electron, resulting in greater attraction to nucleus. Usually, this number is around 2. However, for some elements, the number is greater. For example, Na has a ratio of around 9 due to the fact that the second electron removed would come from a lower energy level than the first and so would require much more energy to remove due to the lower energy level's lower radius and higher attraction. Similarly, Al's number is around 3 because the second electron comes from the 2s orbital, while the first comes from the 2p orbital.
