Final answer:
The element described is chlorine, a halogen gas from group 17 that bonds easily with other elements and typically forms a -1 ion.
Step-by-step explanation:
The element described as a gas that easily bonds with other elements, often forms a -1 ion, and is the lightest element in its group is chlorine.
Chlorine is in group 17 of the periodic table, which is known as the halogen group. It is the lightest halogen because it has the smallest atomic number within that group. Halogens are known for forming -1 ions when they bond with other elements. Therefore, the correct answer is chlorine.