Hello, Ely,
The first ionization energy is the amount of energy it takes to dislodge one electron from the outermost orbital of a neutral atom.in the gas phase. The second and third ionization energies are the values of energy to remove the second and third electron for the same atom. These rise since it becomes increasingly difficult to remove an electron from a positively charged atom.
While these values can be found in tables, the order of increasing first ionization energy can often be predicted from the periodic table. Elements in the groups to the left of the table have lower energies
The first ionization energy(FIE) varies in a predictable way across the periodic table. The FIE decreases as one moves down the table (e.g., Sodium's FIE is larger than Cesium's) and increases as you move from left to right across the table (e.g., Sodium is much lower than Chlorine).
The reason this pattern exists:
- As you go down a group, the outer electron most likely to leave when enough energy is applied is further from the positively-nucleus, so it takes less force to remove it
- As you move across a row, the FIE increases due to the increase of protons in the nucleus. The electrons are added to the outer p orbitals as the atomic number increases, so the attraction of the positive protons in the nucleus becomes significant.
Based on these observations, one might predict the following order of increasing FIE: Cs, Rb, Si, O.
These guidelines aren't always accurate, as an ionization energy graph would demonstrate. There are a few "teeth" in an otherwise predictable trend of the FIE, but the main trend predominates, particularly as one moves from Group 1 to Group 18.
Bob
