Help Me with this chemistry question

Question

1. which element/ion has the largest atomic/ionic size in each of the following pairs?

1. Na or Na+1 f) I-1 or Cl-1 k) Na+1 or Al+3

1. S or S-2 g) Se-2 or S-2 l) Br -1 or Br

1. O or O-2 h) K+1 or Li+1 m) O-2 or F-1

1. Al or Al+3 i) N-3 or P-3 n) K+1 or Ca+2

Ca+2 or Ba+2 j) Sr+2 or Mg+2 o) Rb+1 or Cs+2

Answer 1

All fifteen pairs come down to three rules. Sort each pair into the right category first and the answer is immediate — there is nothing to memorise pair by pair.

Rule 1 — an atom versus its own ion

Cations are smaller than the parent atom: electrons are removed, often emptying an entire outer shell, and the remaining electrons feel a stronger pull per electron. Anions are larger: added electrons increase electron–electron repulsion while the nuclear charge stays the same.

Rule 2 — same group, different period

Size increases going down a group, because each period adds a whole electron shell. This holds for ions just as it does for atoms, as long as the charges match.

Rule 3 — isoelectronic species (the same electron count)

When two ions have identical electron configurations, the one with fewer protons is larger — less nuclear charge pulling on the same number of electrons. This is the rule the question is really testing, and it is the one students miss.

The answers

a) Na or Na⁺ → Na (rule 1)
b) S or S²⁻ → S²⁻ (rule 1)
c) O or O²⁻ → O²⁻ (rule 1)
d) Al or Al³⁺ → Al (rule 1)
e) Ca²⁺ or Ba²⁺ → Ba²⁺ (rule 2)
f) I⁻ or Cl⁻ → I⁻ (rule 2)
g) Se²⁻ or S²⁻ → Se²⁻ (rule 2)
h) K⁺ or Li⁺ → K⁺ (rule 2)
i) N³⁻ or P³⁻ → P³⁻ (rule 2)
j) Sr²⁺ or Mg²⁺ → Sr²⁺ (rule 2)
k) Na⁺ or Al³⁺ → Na⁺ (rule 3)
l) Br⁻ or Br → Br⁻ (rule 1)
m) O²⁻ or F⁻ → O²⁻ (rule 3)
n) K⁺ or Ca²⁺ → K⁺ (rule 3)
o) Rb⁺ or Cs⁺ → Cs⁺ (rule 2)

Working the three isoelectronic ones explicitly

k) Na⁺ has 11 protons and 10 electrons; Al³⁺ has 13 protons and 10 electrons. Both are [Ne]. Aluminium's two extra protons pull those ten electrons in tighter, so Na⁺ is larger.

m) O²⁻ has 8 protons, F⁻ has 9 — both with 10 electrons. O²⁻ is larger.

n) K⁺ has 19 protons, Ca²⁺ has 20 — both with 18 electrons. K⁺ is larger.

A useful shortcut for isoelectronic sets: the more positive the charge, the smaller the ion. That single line handles k, m, and n at once.

One correction: item (o) is written as "Cs⁺²". Caesium is an alkali metal and forms only Cs⁺, so that is a typo in your problem set. The comparison is Rb⁺ versus Cs⁺, and caesium is one period lower, so Cs⁺ is larger either way.