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Chemistry Question 64 – NEET-UG 2023

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Amongst the following, the total number of species NOT having eight electrons around central atom in its outer most shell, is NH3, AlCl3, BeCl2, CCl4, PCl5 :

The octet rule states that atoms tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.

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Ninja Strategy•Recognize Common Exceptions

Quickly identify common octet rule exceptions like electron-deficient Be and Al compounds, and expanded octets for P, S, etc., to narrow down the possibilities and quickly find the count.

Step 1: Understand the Octet Rule and Exceptions✦ Active

The octet rule states that atoms in a molecule tend to achieve eight electrons in their outermost shell. Exceptions include electron-deficient molecules (less than 8 electrons) and expanded octets (more than 8 electrons). To count electrons around the central atom, sum its own valence electrons and one electron from each bond it forms with surrounding atoms, plus any lone pair electrons.

💡 Teacher's Secret Hint

Remember that for covalent bonds, each shared pair contributes two electrons to the octet count of the central atom. Lone pairs also contribute two electrons.

Step 2: Analyze each species for octet compliance○ Expand

- NH3: Central atom N (Group 15). Valence electrons of N = 5. Forms 3 single bonds with H (3 electrons). Has 1 lone pair (2 electrons). Total electrons = 3×2 (bond pairs)+2 (lone pair)=8. **Satisfies octet.** - AlCl3: Central atom Al (Group 13). Valence electrons of Al = 3. Forms 3 single bonds with Cl (3 electrons). Total electrons = 3×2 (bond pairs)=6. **Does NOT satisfy octet (electron deficient).** - BeCl2: Central atom Be (Group 2). Valence electrons of Be = 2. Forms 2 single bonds with Cl (2 electrons). Total electrons = 2×2 (bond pairs)=4. **Does NOT satisfy octet (electron deficient).** - CCl4: Central atom C (Group 14). Valence electrons of C = 4. Forms 4 single bonds with Cl (4 electrons). Total electrons = 4×2 (bond pairs)=8. **Satisfies octet.** - PCl5: Central atom P (Group 15). Valence electrons of P = 5. Forms 5 single bonds with Cl (5 electrons). Total electrons = 5×2 (bond pairs)=10. **Does NOT satisfy octet (expanded octet).**

💡 Teacher's Secret Hint

For main group elements, the number of valence electrons is equal to their group number (for groups 13-18, subtract 10). Elements in period 3 and beyond can often expand their octet due to the availability of d-orbitals.

Step 3: Count the species that do NOT have eight electrons○ Expand

The species not having eight electrons around the central atom are AlCl3, BeCl2, and PCl5. The total number of such species is 3. This corresponds to option (1).

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