Question

Consider the elements with atomic numbers Z = 1 to Z=20. The number of elements with only one unpaired electron in their ground state is

• A. 10
• B. 6
• C. 8
• D. 12

Hint:

The elements with one unpaired electron are typically in Group 1 (alkali metals), Group 13 (Boron group), and Group 17 (halogens). Quickly identifying these groups can help you count the elements faster.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to examine the ground state electron configurations of the first 20 elements (from Hydrogen to Calcium) and count how many of them have exactly one unpaired electron.

Step 2: Key Formula or Approach:
We will list the elements and their electron configurations, then inspect the outermost orbital to count the unpaired electrons. An unpaired electron is one that occupies an orbital by itself.

Step 3: Detailed Explanation:
Let's list the elements whose configurations result in one unpaired electron:
1.

Z=1, Hydrogen (H): \(1s^1\). The single electron in the 1s orbital is unpaired.

(1)
2.

Z=3, Lithium (Li): \([He] 2s^1\). The single electron in the 2s orbital is unpaired.

(2)
3.

Z=5, Boron (B): \([He] 2s^2 2p^1\). The single electron in the 2p subshell is unpaired.

(3)
4.

Z=9, Fluorine (F): \([He] 2s^2 2p^5\). The 2p subshell has three orbitals. The configuration is \(\uparrow\downarrow, \uparrow\downarrow, \uparrow\). There is one unpaired electron.

(4)
5.

Z=11, Sodium (Na): \([Ne] 3s^1\). The single electron in the 3s orbital is unpaired.

(5)
6.

Z=13, Aluminum (Al): \([Ne] 3s^2 3p^1\). The single electron in the 3p subshell is unpaired.

(6)
7.

Z=17, Chlorine (Cl): \([Ne] 3s^2 3p^5\). The 3p subshell has the configuration \(\uparrow\downarrow, \uparrow\downarrow, \uparrow\). There is one unpaired electron.

(7)
8.

Z=19, Potassium (K): \([Ar] 4s^1\). The single electron in the 4s orbital is unpaired.

(8)
Other elements:
- Noble gases (He, Ne, Ar) have 0 unpaired electrons.
- Alkaline earth metals (Be, Mg, Ca) have 0 unpaired electrons (\(ns^2\)).
- Carbon (Z=6, 2p²) has 2 unpaired electrons.
- Nitrogen (Z=7, 2p³) has 3 unpaired electrons.
- Oxygen (Z=8, 2p⁴) has 2 unpaired electrons.
- Silicon (Z=14, 3p²) has 2 unpaired electrons.
- Phosphorus (Z=15, 3p³) has 3 unpaired electrons.
- Sulfur (Z=16, 3p⁴) has 2 unpaired electrons.
- Scandium (Z=21) is outside the range.
Counting the elements with exactly one unpaired electron, we have a total of 8.

Step 4: Final Answer:
There are 8 elements with only one unpaired electron in their ground state from Z=1 to Z=20.