To understand the reactions of sodium and potassium in water, it is also necessary to look at the ionization energy of these elements. Recall that ionization energy tells how difficult it is to remove the most loosely held electron from an atom.
Look at this example
Bohr model of sodium showing that it has two electrons in the first energy level, eight electrons in the second energy level, and one electron in the third energy level.
This diagram shows an atomic model of sodium. The most loosely held electron for sodium would be the valence electron (the one electron in the outer energy level).
The ionization energy for sodium is 496 kJ/mol.
Bohr model of potassium showing that it has two electrons in the first energy level, eight electrons in the second energy level, eight electrons in the third energy level, and one electron in the fourth energy level.
This diagram shows an atomic model of potassium. The most loosely held electron for potassium is also its valence electron or the electron in the outer energy level.
The ionization energy for potassium is 419 kJ/mol.
What is different between the valence electrons for these two elements?
They are in different energy levels. Sodium’s valence electron is in the third energy level and potassium’s valence electron is in the fourth energy level.
Keep in Mind
For elements that have more than one electron in the outer energy level, the first electron removed will be the electron that is the farthest away from the nucleus or the most loosely held electron. For example, the electron configuration for aluminum is 1s22s22p63s23p1. The electron in the p orbital is farther from the nucleus than the 3s, so it would be the electron that is removed. The ionization energy for aluminum is 577 kJ/mol.
Question
Which has a lower ionization energy: sodium or potassium?
Potassium has the lower ionization energy.
The ionization energy for sodium is 496 kg/mol and that of potassium is 419 kJ/mol.
Reflection
Why do you think that potassium has a lower ionization energy than sodium?
