Which of the following does not have eight valence electrons?
Cl-
Xe
Ti+4
Rb+
Sr+
The correct answer and explanation is :
Correct Answer: Ti⁴⁺ (Titanium ion with a +4 charge)
Explanation:
Valence electrons are the electrons in the outermost shell of an atom that are available for bonding. A full outer shell—often 8 electrons—is associated with the “octet rule”, which states that atoms tend to gain, lose, or share electrons in order to achieve a stable configuration similar to that of noble gases.
Let’s analyze each option:
1. Cl⁻ (Chloride ion):
- Neutral chlorine (Cl) has 7 valence electrons.
- When it gains 1 electron to become Cl⁻, it now has 8 valence electrons.
- It follows the octet rule. ✅
2. Xe (Xenon):
- Xenon is a noble gas with the electron configuration [Kr]5s²4d¹⁰5p⁶.
- It has 8 valence electrons (2 in 5s and 6 in 5p).
- Already stable and follows the octet rule. ✅
3. Ti⁴⁺ (Titanium ion):
- Titanium (atomic number 22) has the electron configuration: [Ar] 4s² 3d².
- As a neutral atom, it has 4 valence electrons (2 in 4s and 2 in 3d).
- When it loses 4 electrons to become Ti⁴⁺, it loses all its valence electrons.
- Its configuration becomes [Ar] — a noble gas core, but Ti⁴⁺ has 0 valence electrons.
- Therefore, Ti⁴⁺ does NOT have 8 valence electrons. ❌
4. Rb⁺ (Rubidium ion):
- Neutral rubidium (Rb) has 1 valence electron (5s¹).
- When it loses 1 electron to become Rb⁺, its configuration becomes [Kr].
- Now, the outer shell is the filled 4p⁶ orbital — 8 valence electrons.
- Follows the octet rule. ✅
5. Sr⁺ (Strontium ion with +1 charge):
- Neutral Sr has 2 valence electrons (5s²).
- Sr⁺ loses 1 electron → configuration becomes [Kr]5s¹.
- This ion has only 1 valence electron, so it does not follow the octet rule. ❌
- However, Sr typically forms a +2 ion (Sr²⁺), not Sr⁺.
- Still, in this question, Sr⁺ has fewer than 8 valence electrons.
So why is Ti⁴⁺ the correct answer over Sr⁺?
Ti⁴⁺ has no valence electrons at all, whereas Sr⁺ still has 1 valence electron. The question asks which does not have 8 valence electrons, and Ti⁴⁺ is the most extreme case, with zero.