Why same-group elements are a family

For main-group elements, the group number in the Periodic Table equals the number of outer-shell (valence) electrons in each atom.

Sodium's electron arrangement is 2, 8, 1. The last number, 1, tells us there is 1 electron in the outermost shell.

Because sodium has 1 outer-shell electron, it belongs in Group 1. This is exactly why elements in Group 1 all behave similarly — they all have 1 outer electron.

The correct answer is: Sodium has 1 outer-shell electron, which determines its group number.

The question asks why sodium is in Group 1 — so we need the answer that correctly links atomic structure to group placement.

The option 'Sodium is a metal, and all metals are placed in Group 1' is wrong. Many metals (iron, copper, gold) are in completely different groups — the transition metals, for example.

'Sodium has 3 electron shells, so it belongs in the first group' is wrong. The number of shells tells you the period (horizontal row), not the group.

'Sodium has 11 protons in total, and 1 + 1 = 2, so it is in Period 2' is nonsensical arithmetic with no chemical basis.

The remaining option — Sodium has 1 outer-shell electron, which determines its group number — is correct and reflects the key principle of group chemistry.

In Group 7, reactivity decreases going down the group. Fluorine is most reactive, then chlorine, then bromine, and iodine is least reactive of the four.

Halogens react by gaining one electron to complete their outer shell. Going down Group 7, each element has more electron shells, so the outer shell is further from the nucleus. This means the nucleus attracts an incoming electron less strongly, making it harder to gain that electron — so reactivity decreases.

Bromine is below chlorine in Group 7, so its outer shell is further from the nucleus. It is less reactive than chlorine.

The correct answer is: Bromine will be less reactive than chlorine because its outer electron shell is further from the nucleus and holds electrons more loosely.

We need the most accurate prediction about bromine's reactivity compared to chlorine, using knowledge of Group 7 trends.

'More reactive because it has more electrons overall' is wrong. Total electron count is irrelevant — it's the outer-shell electron behaviour that determines reactivity.

'Less reactive because it has a full outer shell' is wrong. Halogens have 7 outer electrons — one short of a full shell. It's the noble gases that have a full outer shell.

'Identical reactivity because they are in the same group' is wrong. Same-group elements share the type of reaction, but there are gradual trends in reactivity within a group.

The correct answer is that bromine is less reactive than chlorine, with the correct explanation that the outer shell is further from the nucleus — matching the Group 7 trend.