Final answer:
Chlorine tends to gain an electron to create an ion with a net negative charge of -1, known as a chloride ion.
Step-by-step explanation:
The chlorine atom has seven electrons in its outer shell.
It is more energy efficient for the chlorine atom to gain one electron than to lose seven.
Therefore, chlorine tends to gain an electron to create an ion with 17 protons and 18 electrons, giving it a net negative (-1) charge. It is now called a chloride ion.
This movement of electrons from one element to another is referred to as electron transfer.