The first ionisation energy is the least endothermic as energy is needed to remove an electron from a neutral atom. In a +1 ion, the proton:electron ratio is increased. The same number of protons are attracting fewer electrons and the effective nuclear charge is increased.
The second ionisation energy is therefore more endothermic than the first as more energy is needed to remove an electron from a +1 ion. The third electron needs even more energy than the second, and so on.A large jump in successive ionisation energies occurs when an electron is being removed from a shell closer to the nucleus. For example,The first five ionisation energies of magnesium are: 737, 1450, 7731, 10545 and 13627 kJmol-1.There is a large jump between the second and third ionisation energies. Magnesium has two electrons in the outermost shell. The third electron is being removed from a shell closer to the nucleus.This is evidence that electrons are arranged in shells.
Successive ionisation energies of period 2 elements in kJmol-1
The second ionisation energy is therefore more endothermic than the first as more energy is needed to remove an electron from a +1 ion. The third electron needs even more energy than the second, and so on.A large jump in successive ionisation energies occurs when an electron is being removed from a shell closer to the nucleus. For example,The first five ionisation energies of magnesium are: 737, 1450, 7731, 10545 and 13627 kJmol-1.There is a large jump between the second and third ionisation energies. Magnesium has two electrons in the outermost shell. The third electron is being removed from a shell closer to the nucleus.This is evidence that electrons are arranged in shells.
Successive ionisation energies of period 2 elements in kJmol-1
| outer e- | 1st I.E. | 2nd I.E. | 3rd I.E. | 4th I.E. | 5th I.E. | 6th I.E. | 7th I.E. | 8th I.E. | 9th I.E. | |
| Li | 1 | 520 | 7300 | |||||||
| Be | 2 | 900 | 1760 | 14850 | ||||||
| B | 3 | 800 | 2430 | 3660 | 25020 | |||||
| C | 4 | 1090 | 2350 | 4620 | 6220 | 37830 | ||||
| N | 5 | 1400 | 2860 | 4580 | 7480 | 9440 | 53270 | |||
| O | 6 | 1310 | 3390 | 5300 | 7470 | 10980 | 13330 | 71330 | ||
| F | 7 | 1680 | 3370 | 6050 | 8410 | 11020 | 15160 | 17870 | 92040 | |
| Ne | 8 | 2080 | 3950 | 6120 | 9370 | 12180 | 15240 | 20000 | 23070 | 115380 |