Which is larger: Fe 2+ or Fe 3+ ?

1. Which is larger: Fe²⁺ or Fe³⁺? +

Correct Answer

Fe²⁺ is larger than Fe³⁺.

Explanation

Both ions are formed from the same element, iron (Fe), so they have the same nuclear charge:

Fe: Z = 26

Their electronic configurations are:

Fe = [Ar] 3d⁶ 4s²
Fe²⁺ = [Ar] 3d⁶
Fe³⁺ = [Ar] 3d⁵

Fe³⁺ has lost one additional electron compared with Fe²⁺. Therefore, the same nuclear charge attracts fewer electrons in Fe³⁺. This produces a stronger effective attraction per electron and a more contracted electron cloud.

Fe²⁺ 24 electrons
Less contraction
Larger radius

Fe³⁺ 23 electrons
Greater contraction
Smaller radius

For the same element:
Higher positive charge → fewer electrons → stronger attraction → smaller radius

Therefore:

Fe³⁺ < Fe²⁺

JEE-Level Concept

Cations become smaller as their positive charge increases. The reason is that removal of electrons decreases electron–electron repulsion while the nuclear charge remains unchanged. For iron:

Fe²⁺ → 26 protons, 24 electrons
Fe³⁺ → 26 protons, 23 electrons

Since Fe³⁺ has one fewer electron but the same 26 protons, its remaining electrons are pulled more strongly toward the nucleus. Thus the electron cloud contracts.

Fe²⁺ > Fe³⁺ in size

Quick Trick

For ions of the same element:

M⁺ > M²⁺ > M³⁺ > M⁴⁺

So, increasing positive charge means decreasing ionic radius. Higher positive charge = smaller cation.

30 Related JEE-Level MCQs with Solutions

1. Which is larger?

A. Fe²⁺

B. Fe³⁺

C. Both are equal

D. Cannot be determined

Answer: A. Fe²⁺

2. Number of electrons in Fe²⁺ is:

A. 23

B. 24

C. 25

D. 26

Answer: B. 24

3. Number of electrons in Fe³⁺ is:

A. 23

B. 24

C. 25

D. 26

Answer: A. 23

4. Atomic number of Fe is:

A. 24

B. 25

C. 26

D. 27

Answer: C. 26

5. Which ion has the greater effective attraction on its remaining electrons?

A. Fe²⁺

B. Fe³⁺

C. Fe atom

D. All equal

Answer: B. Fe³⁺

6. Which has greater electron–electron repulsion?

A. Fe²⁺

B. Fe³⁺

C. Both exactly equal

D. Neither

Answer: A. Fe²⁺

7. The correct size order is:

A. Fe³⁺ > Fe²⁺

B. Fe²⁺ > Fe³⁺

C. Fe²⁺ = Fe³⁺

D. Fe < Fe³⁺

Answer: B

8. Removal of one electron from Fe²⁺ gives:

A. Fe⁺

B. Fe³⁺

C. Fe⁴⁺

D. Fe

Answer: B. Fe³⁺

9. Which ion has fewer electrons?

A. Fe²⁺

B. Fe³⁺

C. Both equal

D. Fe atom

Answer: B. Fe³⁺

10. Increasing positive charge generally causes ionic radius to:

A. Increase

B. Decrease

C. Remain constant

D. Become infinite

Answer: B. Decrease

11. Fe²⁺ has which configuration?

A. [Ar]3d⁶

B. [Ar]3d⁵

C. [Ar]4s²

D. [Ar]3d⁷

Answer: A

12. Fe³⁺ has which configuration?

A. [Ar]3d⁶

B. [Ar]3d⁵

C. [Ar]4s¹

D. [Ar]3d⁴

Answer: B

13. Fe³⁺ is smaller mainly because:

A. It has more electrons

B. It has fewer protons

C. It has fewer electrons for the same nuclear charge

D. Its nucleus disappears

Answer: C

14. Which has 26 protons?

A. Fe only

B. Fe²⁺ only

C. Fe³⁺ only

D. All three

Answer: D

15. Which statement is correct?

A. Cations become larger with increasing positive charge

B. Cations become smaller with increasing positive charge

C. Charge has no effect

D. Fe³⁺ has more electrons than Fe²⁺

Answer: B

16. Which has the more compact electron cloud?

A. Fe²⁺

B. Fe³⁺

C. Fe atom

D. None

Answer: B. Fe³⁺

17. Fe³⁺ has a half-filled 3d subshell containing:

A. 3 electrons

B. 4 electrons

C. 5 electrons

D. 6 electrons

Answer: C. 5 electrons

18. Fe²⁺ contains how many 3d electrons?

A. 4

B. 5

C. 6

D. 7

Answer: C. 6

19. Which ion has greater charge density?

A. Fe²⁺

B. Fe³⁺

C. Both equal

D. Neutral Fe

Answer: B. Fe³⁺

20. Which species has a smaller radius than Fe²⁺?

A. Fe³⁺

B. Fe atom

C. Both A and B

D. Neither

Answer: A. Fe³⁺

21. The number of electrons removed from Fe to form Fe³⁺ is:

A. 1

B. 2

C. 3

D. 4

Answer: C. 3

22. Which ion has the stronger attraction for its 3d electrons?

A. Fe²⁺

B. Fe³⁺

C. Both equal

D. Neither

Answer: B. Fe³⁺

23. Which is the general trend for the same metal?

A. M⁺ < M²⁺ < M³⁺

B. M⁺ > M²⁺ > M³⁺

C. All equal

D. No trend

Answer: B

24. Fe³⁺ has a more contracted radius because electron–electron repulsion is:

A. Greater

B. Lower

C. Infinite

D. Unchanged in all respects

Answer: B. Lower

25. Which ion is expected to be more strongly polarising?

A. Fe²⁺

B. Fe³⁺

C. Both equal

D. Neither

Answer: B. Fe³⁺

26. Which statement about Fe²⁺ and Fe³⁺ is false?

A. Both have 26 protons

B. Fe³⁺ has fewer electrons

C. Fe³⁺ is smaller

D. Fe³⁺ has 24 electrons

Answer: D — Fe³⁺ has 23 electrons.

27. If an electron is removed from Fe²⁺, the ionic radius will generally:

A. Increase

B. Decrease

C. Remain unchanged

D. Become neutral

Answer: B. Decrease

28. The size relationship between Fe²⁺ and Fe³⁺ illustrates:

A. Increase of radius with positive charge

B. Contraction of cation size with increasing charge

C. Increase of shielding down a group

D. Periodic law only

Answer: B

29. Which has the greater electron/proton ratio?

A. Fe²⁺

B. Fe³⁺

C. Both equal

D. Neutral Fe

Answer: A. Fe²⁺

30. The correct final conclusion is:

A. Fe³⁺ > Fe²⁺

B. Fe²⁺ > Fe³⁺

C. Fe²⁺ = Fe³⁺

D. Size is independent of charge

Answer: B. Fe²⁺ > Fe³⁺

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