Group 7A elements, also known as the halogens, have seven valence electrons. This configuration is crucial for understanding their chemical behavior.
The presence of seven electrons in their outermost shell means these elements are just one electron short of achieving a stable octet configuration. As a result, halogens are highly reactive as they tend to gain that one additional electron. This tendency to react makes them more unstable and prone to forming compounds with other elements, especially alkali and alkaline-earth metals, which readily donate their electrons.
In summary, group 7A elements have seven valence electrons, and their high reactivity is due to their desire to complete their outer electron shell.