The elements in alkali metal and halogen groups of the periodic table are the most reactive since they only need to gain or lose one electron to become stable by filling their valence orbital. what happens to reactivity, moving down the column of a group?
a) reactivity stays the same because they are in the same group.
b) reactivity increases because the valence level is further from the nucleus of the atom.
c) reactivity decreases because the increased number of protons and electrons provides more stability.
d) reactivity stays the same because the number of electrons needed to fill the valence orbital remains the same.

B) Reactivity increases because the valence level is further from the nucleus of the atom.

Explanation:

In both halogens and alkali metals, the reactivity decreases down the group. Both periodic groups required a change of one electron to complete their octet and be like the noble gases.

As we go down a periodic group, the number of shells of electrons increases. This makes the outermost valence electrons less protected from the shielding effect of the nucleus. Therefore, elements further down will have a low grip on their outermost shell electrons and the upper ones will have a better protection.

As we can see, the most reactive metal is francium and it is located further down in group I. The most electronegative atom which would not readily lose an electron is fluorine and it is up the group of halogens.

B) Reactivity increases because the valence level is further from the nucleus of the atom.

Explanation:

Moving down a group reactivity increases. The number of shell increases as we move down a group from one element to the other in a group. So the distance from the nucleus to the outer shell electron increases. As a result attraction between the nucleus (positive) to the outer shell electron (negative) increases.

The inner shell electrons experience more shielding effect. Thus the outer shell electrons are easily gets lost or gained.

is more reactive than

Thats why we see here Br is replaced from KBr by

cannot replace Cl from KCl since it is less reactive than