Draw the Lewis structure for HCN.
a. What is the electron domain geometry of C?
b. What is the molecular geometry of C?
c. What is the approximate bond angle of the H-C-N?
d. Is this molecule polar?
The Correct Answer and Explanation is:
To determine the Lewis structure and answer related questions about HCN (hydrogen cyanide), we need to follow proper steps and reasoning based on valence electrons and molecular geometry.
Lewis Structure of HCN:
- Count the valence electrons:
- Hydrogen (H) = 1 electron
- Carbon (C) = 4 electrons
- Nitrogen (N) = 5 electrons
Total = 1 + 4 + 5 = 10 valence electrons
- Determine the central atom:
Carbon (C) is less electronegative than nitrogen, so it is the central atom. - Place atoms and form bonds:
Connect H—C—N with single bonds initially. Then adjust bonds to satisfy octets. - Distribute electrons:
After placing single bonds (2 electrons each), we have 6 electrons left. Nitrogen requires a triple bond with carbon to satisfy the octet rule. Final Lewis structure:
H–C≡N- H has 2 electrons (full shell)
- C has 8 electrons (4 from triple bond + 2 from H)
- N has 8 electrons (from triple bond and a lone pair)
Answers:
a. What is the electron domain geometry of C?
→ Linear.
Carbon has two regions of electron density (a single bond to H and a triple bond to N), leading to a linear geometry.
b. What is the molecular geometry of C?
→ Linear.
With two bonding regions and no lone pairs, the molecular geometry is also linear.
c. What is the approximate bond angle of H–C≡N?
→ 180°.
Linear molecules have bond angles of 180°.
d. Is this molecule polar?
→ Yes, HCN is a polar molecule.
Explanation (300 words):
The Lewis structure of hydrogen cyanide (HCN) reveals a simple linear molecule with 10 valence electrons. The hydrogen atom bonds to carbon via a single bond, and the carbon atom forms a triple bond with nitrogen. Carbon, being the central atom, shares its electrons with both atoms to achieve a stable octet, while nitrogen also reaches an octet with a lone pair and a triple bond.
From this structure, the electron domain geometry around carbon is linear because it involves two bonding domains (one to hydrogen and one to nitrogen). According to VSEPR (Valence Shell Electron Pair Repulsion) theory, when there are two electron domains, the repulsion between them pushes the atoms as far apart as possible, resulting in a bond angle of 180°. Thus, both the electron domain geometry and molecular geometry around carbon are linear.
The polarity of the molecule is determined by the difference in electronegativity and the shape. Nitrogen is significantly more electronegative than both hydrogen and carbon. This causes the shared electrons in the C≡N bond to be pulled closer to nitrogen, creating a partial negative charge on nitrogen and a partial positive charge on the hydrogen end. Although the molecule is linear, the difference in electronegativity is not canceled out because of the asymmetric electron distribution. Therefore, HCN is a polar molecule, with a dipole pointing from H toward N.
This polarity makes HCN soluble in polar solvents like water and gives it distinct intermolecular forces. In summary, HCN is linear, has a bond angle of 180°, and is polar due to the electronegativity difference and shape.