This atom REALLY wants to bond with another atom!
Lets think of the octet rule.
The octet rule is a scientific rule about atoms that states that the valence shell, (the outer most shell, the shell closest to the outside), wants to have 8 electrons in it. This outer orbital or valence shell, wants to have 8 electrons so bad because it wants to fulfill its octet rule.
Now, this atom of chlorine has 7 electrons in the outer shell. This is why it REALLY wants to bond. It is so close to fulfilling its octet rule, it is only one away.
Hope this helped!
