First ionization energy
The nucleus is positive and the electrons are negative, so pulling an electron out of an atom means overcoming that attraction. Overcoming it takes energy.
Remove an electron from an atom and what is left is a positive ion.
First ionization energy
The minimum energy that must be supplied to remove an electron from an atom of an element in the gaseous state, forming a gaseous unipositive ion.
Na(g) → Na+(g) + e
Why the values are given per mole
For a single atom this energy is a very small quantity. So the value is quoted for 6.022 × 1023 atoms — one mole — and the unit is kJ mol−1.
On that scale, the first ionization energy of sodium is 495 kJ mol−1.
| Period | I | II | III | IV | V | VI | VII | VIII / 0 |
|---|---|---|---|---|---|---|---|---|
| 1 | H 1310 | He 2372 | ||||||
| 2 | Li 519 | Be 897 | B 799 | C 1085 | N 1406 | O 1314 | F 1682 | Ne 2080 |
| 3 | Na 495 | Mg 738 | Al 577 | Si 786 | P 1018 | S 1000 | Cl 1255 | Ar 1521 |
| 4 | K 418 | Ca 590 |
Figure 3.7 — first ionization energies of the elements with atomic numbers 1 to 20, in kJ mol−1.