Why does the reactivity of halogens decreases down group 7.

With group 7 elements, the outer shell has 7 electrons. This means that an electron must be gained. Electrons are attracted to the nucleus, therefore if there is a short distance between the outer shell and the nucleus, it will react more. As you go down the group, there are more electron shells which causes shielding as electrons repel. The more repulsion between electrons, the further away the outer shell will be from the nucleus. This would make it harder for the element with many shells to gain an electron so reactivity will be lower. Hope this helps

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Increase in Reactivity of Group 1 Elements
The reactivity of Group 1 elements, also known as alkali metals, increases as you move down the group. This is due to the decrease in ionization energy and the increase in atomic size from lithium to francium.
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Ionization Energy
Ionization energy is the energy required to remove an electron from an atom. As you move down Group 1, the ionization energy decreases. This is because the outermost electron is further away from the nucleus due to the increase in atomic size. Therefore, it is easier to remove the outermost electron, leading to increased reactivity.
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Increase in Atomic Size
The atomic size of Group 1 elements increases as you move down the group. This is because each successive element has an additional energy level, leading to a larger atomic radius. The larger atomic size results in weaker attraction between the nucleus and the outermost electron, making it easier for the outermost electron to be removed, thus increasing reactivity.
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Decrease in Reactivity of Group 17 Elements
On the other hand, the reactivity of Group 17 elements, also known as halogens, decreases as you move down the group. This is due to the increase in atomic size and the decrease in electron affinity from fluorine to iodine.
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Increase in Atomic Size
Similar to Group 1 elements, the atomic size of Group 17 elements increases as you move down the group. The larger atomic size results in weaker attraction for an additional electron, making it more difficult to gain an electron and decreasing reactivity.
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Decrease in Electron Affinity
Electron affinity is the energy released when an atom gains an electron. As you move down Group 17, the electron affinity decreases. This is because the additional electron is further away from the nucleus due to the increase in atomic size, resulting in decreased reactivity.