Question

In the elements with atomic number $Z=1$ to $Z=20$, how many of them have no unpaired electrons in their ground state?

• A. 8
• B. 4
• C. 10
• D. 6

Hint:

To solve this quickly, remember that elements with an even atomic number might have no unpaired electrons, but elements with an odd atomic number always have at least one unpaired electron.

Answer 1

The Correct Option is D

1. Criteria for No Unpaired Electrons: This occurs in elements belonging to Group 2 (Alkaline Earth Metals) and Group 18 (Noble Gases) because their valence shells ($s^2$ or $s^2p^6$) are full.

2. Identification of Elements:

• Helium ($Z=2$): $1s^2$ (Noble Gas)

• Beryllium ($Z=4$): $1s^2 2s^2$ (Group 2)

• Neon ($Z=10$): $1s^2 2s^2 2p^6$ (Noble Gas)

• Magnesium ($Z=12$): $[Ne] 3s^2$ (Group 2)

• Argon ($Z=18$): $[Ne] 3s^2 3p^6$ (Noble Gas)

• Calcium ($Z=20$): $[Ar] 4s^2$ (Group 2)

3. Counting: There are exactly

6 such elements in the range $Z=1$ to $Z=20$. Elements in other groups (like Group 14 Carbon or Group 16 Oxygen) have partially filled $p$ subshells, leading to unpaired electrons.