Question

Select the molecule in which the central atom has incomplete octet.

• A. Beryllium
• B. Phosphorous
• C. Silicon
• D. Chlorine

Answer 1

The correct option is A

Beryllium

Explanation for correct answer:-

Option (A) Beryllium

• Beryllium$\left(\mathrm{Be}\right)$ has an atomic number of 4, and its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^2$.
• As we can see from this electronic configuration, we can see that it does not need 8 electrons in its valence shell to achieve the octet state.
• This is because the outermost shell is s-orbital, which can only accommodate 2 electrons, not 8.
• Thus Beryllium$\left(\mathrm{Be}\right)$ along with Lithium$\left(\mathrm{Li}\right)$, Hydrogen$\left(\mathrm{H}\right)$, and Helium$\left(\mathrm{He}\right)$ does not obey this octet rule and hence are incomplete octet.
• Hence it is the correct option.

Explanation for incorrect answer:-

Option (B) Phosphorus

• Phosphorus has an atomic number of 15, and its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^3$.
• As we can see It needs 3 more electrons to complete its octet.
• As a result it reacts with another Phosphorus atom and they both share the 3 electrons present in the p-orbital and thus complete its octet.
• Hence it is a complete octet.
• Hene it is the incorrect option.

Option (C) Silicon

• Silicon has an atomic number of 14, and its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^2$.
• As we can see It needs 4 more electrons to complete its octet.
• As a result it reacts with another Silicon atom and they both share the 4 electrons present in their valence shell and thus complete its octet.
• Hence it is a complete octet.
• Hence it is the incorrect option.

Option (D) Chlorine

• Chlorine has an atomic number of 17, and its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^5$.
• As we can see It needs 1 more electron to complete its octet.
• As a result it reacts with another Chlorine atom and they both share 1 electron from their valence shell and thus complete its octet.
• Hence it is a complete octet.
• Hence it is the incorrect option.