Chlorine has an atomic number of 17 - Edexcel - GCSE Chemistry - Question 2 - 2020 - Paper 1

The atomic number of chlorine is 17, which means it has 17 electrons. The electronic configuration describes how these electrons are arranged in the energy levels or shells around the nucleus of the atom.

To deduce the electronic configuration, we follow these rules:

1. Electrons fill the lowest energy levels first.
2. The maximum number of electrons in each shell can be predicted by the formula: 2n², where n is the shell level.

For chlorine:

• The 1st shell (n=1) can hold 2 electrons.
• The 2nd shell (n=2) can hold up to 8 electrons.
• The 3rd shell (n=3) can hold up to 18 electrons, but for chlorine, it only needs to hold 7 more electrons because it has 17 total.

Following these rules, the electronic configuration of chlorine is:

1. 2 electrons in the 1st shell.
2. 8 electrons in the 2nd shell.
3. 7 electrons in the 3rd shell.

Thus, the electronic configuration is:

$1s^2 2s^2 2p^6 3s^2 3p^5$.