The number of electrons in the outermost principal energy level of a chlorine atom is

seven electrons

Explanation:

Chlorine is present in group seventeen of periodic table. It is halogen element. All halogens have seven electrons in outer most valance shell.

The require only one electron to gain the stable electronic configuration or to complete the octet.

Electronic configuration of chlorine:

Cl₁₇ = 1s² 2s² 2p⁶ 3s² 3p⁵

Abbreviated electronic configuration:

Cl₁₇ = [Ne] 3s² 3p⁵

Properties of chlorine:

1. it is greenish-yellow irritating gas.

2. its melting point is 172.2 K

3. its boiling point is 238.6 K

4. it is disinfectant and can kill the bacteria.

5. it is also used in manufacturing of paper, paints and textile industries.