In which group do elements become less reactive as you go down the group?

Elements in Group 7, also known as the halogens, become less reactive as you go down the group. This trend can be attributed to several factors related to atomic structure and the nature of chemical bonding.

As you move down Group 7, the size of the atoms increases due to the addition of electron shells. This increase in atomic size means that the outermost electrons are further from the nucleus, resulting in weaker attractive forces between the nucleus and these outer electrons. Consequently, it becomes more challenging for these larger atoms to gain electrons during chemical reactions, which is what defines their reactivity.

Additionally, the increased shielding effect from the inner electron shells further reduces the nucleus's ability to attract additional electrons. Therefore, while fluorine, being at the top of the group, is very reactive as it can readily gain an electron to achieve a stable octet, iodine, which is further down, is significantly less reactive.

In contrast, in Groups 1 and 2, reactivity actually increases down the group, while Group 0 consists of noble gases that are generally inert and do not react under standard conditions.