Question

Which of the following ion has largest size?

• A. ${\mathrm{F}}^{-}$
• B. ${\mathrm{Cs}}^{+}$
• C. ${\mathrm{Al}}^{+3}$
• D. ${\mathrm{O}}^{-2}$

Answer 1

The correct option is B

${\mathrm{Cs}}^{+}$

The explanation for the correct answer:

(b) ${\mathrm{Cs}}^{+}$

1. The ion's size is determined by two factors, the charge of the ion and the relative position of an element in the periodic table.
2. The size of an element decreases as we move left to right across the periodic table because the addition of respective electrons to the same shell and the outermost electron experience more nuclear attraction, resulting in an increase in the effective nuclear charge, which makes the atom more compact and reduces its size.
3. Also, when we move down the group in the periodic table, the size of the element increases because there is an increase in atomic and number of orbitals which increases the atomic radius.
4. Since, ${\mathrm{Cs}}^{+}$belongs to group 1 which is the right-most corner of the periodic table, it will be the largest in size.

The explanation for the incorrect answer:

(a) ${\mathrm{F}}^{-}$

1. It belongs to group 17
2. Moving left to right across the period, the size of the element decreases.
3. So it will be smaller than ${\mathrm{Cs}}^{+}$

(c) ${\mathrm{Al}}^{+3}$

1. But it belongs to group 13
2. Moving left to right across the period, the size of the element decreases.
3. So it will be smaller than ${\mathrm{Cs}}^{+}$

(d) ${\mathrm{O}}^{-2}$

1. It belongs to group16
2. Moving left to right across the period, the size of the element decreases.
3. So it will be smaller than ${\mathrm{Cs}}^{+}$

Therefore, Option (b) is correct. ${\mathrm{Cs}}^{+}$ has the largest size.