Potassium has an atomic number of 19. what is its electron configuration?
shells 1 and 2 are full, and shell 3 has nine electrons
shells 1, 2 and 3 are full and shell 4 has three electrons
shells 1, 2 and 3 are full and shell 4 has one electron
shells 1, 2 and 3 are full and no other electrons are present

shells 1, 2 and 3 are full and shell 4 has one electron

Explanation:

The distribution fo the electrons possessed by an atom in the shells surrounding it is termed as its electronic configuration.

According to the rules of writing the electronic configuration:

1. The first shell can be occupied by a maximum of two electrons.

2. the subsequent shells can be occupied by a maximum of 8 electrons.

3. the subsequent shells are filled only once the lower shells have been filled.

Based on these rules, the electronic configuration of potassium should be 2, 8, 8, 1. (2+8+8+1 = 19).

Thus, we can see that the first three shells are filled, and the last one has one electron left.