Identify the species that are isoelectronic: CO,CN − ,NO + ,C 2 2− ​ .

1. Identify the species that are isoelectronic: CO, CN⁻, NO⁺, C₂²⁻. +

Correct Answer

CO, CN⁻, NO⁺ and C₂²⁻ are all isoelectronic.

Solution

Isoelectronic species are atoms, ions or molecules having the same total number of electrons. Calculate the total number of electrons in each species:

CO = 6 + 8 = 14 e⁻

CN⁻ = 6 + 7 + 1 = 14 e⁻

NO⁺ = 7 + 8 − 1 = 14 e⁻

C₂²⁻ = 6 + 6 + 2 = 14 e⁻

Therefore:

CO = CN⁻ = NO⁺ = C₂²⁻ = 14 electrons

Hence, all four species are isoelectronic.

Quick JEE Trick

For a species: Electrons = Total atomic numbers − positive charge + negative charge For example:

CN⁻ → 6 + 7 + 1 = 14

NO⁺ → 7 + 8 − 1 = 14

If the final electron count is the same, the species are isoelectronic.

30 Related MCQs with Solutions

1. Which of the following is isoelectronic with CO?

A. N₂

B. O₂

C. NO

D. F₂

Answer: A. N₂
N₂ = 7 + 7 = 14 electrons.

2. Number of electrons in CN⁻ is:

A. 12

B. 13

C. 14

D. 15

Answer: C. 14

3. Number of electrons in NO⁺ is:

A. 12

B. 13

C. 14

D. 15

Answer: C. 14

4. Number of electrons in C₂²⁻ is:

A. 12

B. 13

C. 14

D. 16

Answer: C. 14

5. Isoelectronic species have the same:

A. Number of protons

B. Number of neutrons

C. Number of electrons

D. Atomic number

Answer: C. Number of electrons

6. Which species contains 10 electrons?

A. O²⁻

B. F⁻

C. Ne

D. All of these

Answer: D. All of these

7. Which pair is isoelectronic?

A. Na⁺ and Ne

B. Na and Ne

C. Na⁺ and F

D. Mg and Ne

Answer: A
Na⁺ = 10 e⁻, Ne = 10 e⁻.

8. Which species has 18 electrons?

A. Cl⁻

B. Ar

C. K⁺

D. All of these

Answer: D. All of these

9. Which is NOT isoelectronic with Ne?

A. Na⁺

B. F⁻

C. O²⁻

D. Na

Answer: D. Na
Na has 11 electrons.

10. N₂ and CO are:

A. Isotopes

B. Isobars

C. Isoelectronic species

D. Isotones

Answer: C

11. NO⁺ is isoelectronic with:

A. CO

B. N₂

C. CN⁻

D. All of these

Answer: D. All of these

12. Which species has 14 electrons?

A. CO

B. CN⁻

C. NO⁺

D. All of these

Answer: D

13. The electron count of a neutral molecule is equal to:

A. Sum of atomic numbers

B. Difference of atomic numbers

C. Sum of mass numbers

D. Number of neutrons

Answer: A

14. For a negative ion, electrons are:

A. Removed

B. Added

C. Unchanged

D. Converted into protons

Answer: B. Added

15. For a positive ion, electrons are:

A. Added

B. Removed

C. Doubled

D. Converted into neutrons

Answer: B. Removed

16. Which pair has the same electron count?

A. O²⁻ and Mg²⁺

B. O and Mg

C. O⁻ and Mg

D. O²⁺ and Mg²⁺

Answer: A
Both contain 10 electrons.

17. Which pair is isoelectronic?

A. S²⁻ and Ar

B. S and Ar

C. S⁺ and Ar

D. S²⁺ and Ar

Answer: A

18. Which species has 10 electrons?

A. Mg²⁺

B. Al³⁺

C. Na⁺

D. All of these

Answer: D

19. Which species has 18 electrons?

A. P³⁻

B. S²⁻

C. Cl⁻

D. All of these

Answer: D

20. Which isoelectronic species generally has the smallest radius among the following?

A. O²⁻

B. F⁻

C. Na⁺

D. Mg²⁺

Answer: D. Mg²⁺
For the same number of electrons, greater nuclear charge gives smaller radius.

21. Among N³⁻, O²⁻, F⁻, Na⁺, the largest radius is:

A. N³⁻

B. O²⁻

C. F⁻

D. Na⁺

Answer: A. N³⁻

22. For an isoelectronic series, ionic radius decreases with increasing:

A. Number of electrons

B. Nuclear charge

C. Neutron number only

D. Mass number only

Answer: B. Nuclear charge

23. Which species has 14 electrons?

A. N₂

B. CO

C. NO⁺

D. All of these

Answer: D

24. C₂²⁻ contains how many electrons?

A. 10

B. 12

C. 14

D. 16

Answer: C. 14

25. Which statement about isoelectronic species is correct?

A. They always have the same atomic number

B. They have the same number of electrons

C. They always have the same number of protons

D. They must be isotopes

Answer: B

26. Which pair is NOT isoelectronic?

A. Na⁺, Ne

B. Cl⁻, Ar

C. Mg²⁺, Ne

D. Na, Ne

Answer: D

27. CO and CN⁻ have the same:

A. Number of electrons

B. Number of protons

C. Atomic number

D. Nuclear charge

Answer: A

28. The electron count of NO⁺ is calculated as:

A. 7 + 8 + 1

B. 7 + 8 − 1

C. 7 − 8 + 1

D. 7 + 8

Answer: B. 14 electrons

29. Which is the correct electron-count sequence?

A. CO = 14, CN⁻ = 14, NO⁺ = 14

B. CO = 12, CN⁻ = 14, NO⁺ = 16

C. CO = 16, CN⁻ = 12, NO⁺ = 14

D. All are different

Answer: A

30. Which of the given species are isoelectronic?

A. Only CO and CN⁻

B. Only NO⁺ and C₂²⁻

C. CO, CN⁻, NO⁺ and C₂²⁻

D. None

Answer: C. All four species

Advertisement