Step 1: Understanding the Concept:
This question asks us to identify all elements from Z=1 to Z=20 that have exactly one unpaired electron in their ground state electron configuration. This requires writing out the electron configuration for each element and examining the orbital diagrams, especially for the outermost shell.
Step 2: Key Formula or Approach:
We will list the elements and their ground state electron configurations, then count the number of unpaired electrons. An unpaired electron is an electron that occupies an orbital by itself.
Step 3: Detailed Explanation:
Let's go through the elements from Z=1 to 20 and check for one unpaired electron:
1. H (Z=1): $1s^1$. (1 unpaired electron).
2. He (Z=2): $1s^2$. (0 unpaired electrons).
3. Li (Z=3): $[He] 2s^1$. (1 unpaired electron).
4. Be (Z=4): $[He] 2s^2$. (0 unpaired electrons).
5. B (Z=5): $[He] 2s^2 2p^1$. (1 unpaired electron).
6. C (Z=6): $[He] 2s^2 2p^2$. (2 unpaired electrons - Hund's rule).
7. N (Z=7): $[He] 2s^2 2p^3$. (3 unpaired electrons).
8. O (Z=8): $[He] 2s^2 2p^4$. (2 unpaired electrons).
9. F (Z=9): $[He] 2s^2 2p^5$. (1 unpaired electron).
10. Ne (Z=10): $[He] 2s^2 2p^6$. (0 unpaired electrons).
11. Na (Z=11): $[Ne] 3s^1$. (1 unpaired electron).
12. Mg (Z=12): $[Ne] 3s^2$. (0 unpaired electrons).
13. Al (Z=13): $[Ne] 3s^2 3p^1$. (1 unpaired electron).
14. Si (Z=14): $[Ne] 3s^2 3p^2$. (2 unpaired electrons).
15. P (Z=15): $[Ne] 3s^2 3p^3$. (3 unpaired electrons).
16. S (Z=16): $[Ne] 3s^2 3p^4$. (2 unpaired electrons).
17. Cl (Z=17): $[Ne] 3s^2 3p^5$. (1 unpaired electron).
18. Ar (Z=18): $[Ne] 3s^2 3p^6$. (0 unpaired electrons).
19. K (Z=19): $[Ar] 4s^1$. (1 unpaired electron).
20. Ca (Z=20): $[Ar] 4s^2$. (0 unpaired electrons).
Now, let's count the elements we identified:
H, Li, B, F, Na, Al, Cl, K.
There are a total of 8 elements.
Step 4: Final Answer:
There are 8 elements from Z=1 to Z=20 with only one unpaired electron in their ground state. Therefore, option (C) is correct.