C2.5 Explaining Trends

As you go down a group in the periodic table, the number of shells occupied by the electrons increases, by one extra electron shell per period. This means that the atoms become larger going down any group.

This has two effects:

1. Larger atoms lose electrons more easily going down a group.

2. Larger atoms gain electrons less easily going down a group.

This occurs because the outer electron, which are negatively charged, are further away from the central nucleus, which is positively charged due to the protons. Also, the inner shell electrons sheild the outer shell electrons from positive charge of the nucleus. 

Reactivity increases going down Group 1 because the atoms lets larger so the single electron in the outer most shellis attracted less strongly to the positive nucelus.

The electrostatic attraction with the nucleus gets weaker because the distance between the outer electrons and nucleus increases.

Also the outer electron experiences a sheilding effect from inner shells of electrons. This reduces the attraction between the oppositely charged outer electrons in the nucleus.

The size of the positive charge on the nucleus does become larger as you go down a group, as more protons are present inside the nucleus. This suggests tee attraction for the outer electron should get stronger. 

However, the greater distance and sheilding effect of inner electrons outweigh the increasing nuclear charge.

Therefore, in Group 1, the outer electron gets easier to remove going down the group and the elements et more and more reactive. 

Reactivity decreases going down Group 7. To explain this, you consider the same factor you saw in Group 1: the size of the atom, the shielding effect of inner electrons and the nuclear charge.

When Group 7 elements react, their atoms gain an electron in their outermost shell.Going dowm their outermost shell's electrons get further away from attractive force of the nucleus and so it is harder to attract and gain an extra incoming electron. 

The outer shell will also be shielded by more inner shell electrons, again reduces tehe electromagnetic attraction of the nucleus for an incoming electron.

The effect of the increased nuclear charge going down the group is outweighed by the effect of increased distance and shielding by more inner electrons.