How many lone pairs of electrons are on the central atom of HCN and what is the molecular shape? Group of answer choices: a. one, linear b. zero, bent c. one, trigonal planar d. one, bent e. zero, linear
The Correct Answer and Explanation is:
Correct answer: e. zero, linear
Explanation:
To determine the number of lone pairs on the central atom and the molecular shape of HCN (hydrogen cyanide), we first analyze its Lewis structure and electron geometry.
Step 1: Determine the total number of valence electrons.
- Hydrogen (H) contributes 1 valence electron.
- Carbon (C) contributes 4 valence electrons.
- Nitrogen (N) contributes 5 valence electrons.
Total = 1 + 4 + 5 = 10 valence electrons
Step 2: Draw the skeletal structure.
Hydrogen can only form one bond, so it must be terminal. The most logical skeletal structure is:
H — C — N
Step 3: Distribute electrons and form bonds.
Carbon makes a single bond with hydrogen and a triple bond with nitrogen to satisfy the octet rule. The structure becomes:
H — C ≡ N
In this structure:
- Carbon has four bonding electrons from the triple bond with nitrogen and two bonding electrons from the single bond with hydrogen.
- All electrons around carbon are involved in bonding, so carbon has no lone pairs.
- Nitrogen has one lone pair of electrons after forming three bonds with carbon.
- Hydrogen, with one bond, is also satisfied.
Step 4: Identify the central atom and molecular geometry.
Carbon is the central atom. It has two regions of electron density (one single bond to hydrogen and one triple bond to nitrogen). According to VSEPR (Valence Shell Electron Pair Repulsion) theory, two regions of electron density will orient themselves 180 degrees apart to minimize repulsion.
This results in a linear molecular shape.
Conclusion:
- Central atom (carbon) has zero lone pairs.
- The molecular geometry is linear.
Thus, the correct answer is: e. zero, linear.
