Question

re-order each list in the table below, if necessary, so that the atoms or ions in it are listed in order of decreasing size.

atoms or ions	atoms or ions in order of decreasing size
i, i⁻, br	□, □, □
xe, i⁻, i	□, □, □

Explanation:

Step1: Analyze period trend (Be, O, B)

Atomic size decreases across a period (left to right). Be (Group 2, Period 2) > B (Group 13, Period 2) > O (Group 16, Period 2).

Step2: Analyze group/ion trend (I, I⁻, Br)

Anions are larger than parent atoms; size increases down a group. I⁻ > I (Period 5) > Br (Period 4).

Step3: Analyze isoelectronic/group trend (Xe, I⁻, I)

I⁻ is isoelectronic with Xe; Xe (Period 5, noble gas) has larger radius than neutral I; anion I⁻ is larger than Xe. I⁻ > Xe > I.

Answer:

1. Be, B, O
2. I⁻, I, Br
3. I⁻, Xe, I