Considering periodic trends, valence electrons in which of the following atoms experience the greatest effective nuclear charge (Zeff)?
A) CI
B) C
C) F
D) Ne
E) B
The Correct Answer and Explanation is:
The correct answer is: D) Ne (Neon)
Explanation
Effective nuclear charge (Z_eff) is the net positive charge experienced by valence electrons in an atom. It accounts for the attraction from the nucleus and the repulsion from inner (core) electrons. The formula is: Zeff=Z−SZ_{\text{eff}} = Z – S
Where:
- ZZ = atomic number (number of protons)
- SS = shielding constant (core electron repulsion)
As we move left to right across a period in the periodic table:
- The atomic number increases, adding more protons to the nucleus.
- Electrons are added to the same principal energy level, not increasing shielding significantly.
- As a result, Z_eff increases across a period.
Now, let’s compare the options:
| Atom | Atomic Number (Z) | Electron Configuration | Z_eff Trend |
|---|---|---|---|
| B | 5 | 1s² 2s² 2p¹ | Low |
| C | 6 | 1s² 2s² 2p² | Higher than B |
| F | 9 | 1s² 2s² 2p⁵ | Higher than C |
| Ne | 10 | 1s² 2s² 2p⁶ | Highest |
| Cl | 17 | 1s² 2s² 2p⁶ 3s² 3p⁵ | Lower than Ne due to added shell (3rd period) which increases shielding |
Even though Cl has more protons, its valence electrons are in the third energy level, which is further from the nucleus and more shielded, so they experience a lower Z_eff compared to Ne.
Neon, being at the end of Period 2, has:
- 10 protons pulling on its valence electrons.
- Minimal shielding by inner electrons (only 1s² core).
- Electrons in the second shell, close to the nucleus.
Thus, Neon’s valence electrons experience the greatest effective nuclear charge, making it the correct answer.
Answer: D) Ne
