Draw the Lewis structure for hydrogen sulfide, H2S. What is the electron pair geometry? What is the molecular geometry? Estimate the bond angle. Is this molecule polar? If so, draw the dipole arrow. Draw the Lewis structure for hydrogen cyanide, HCN. What is the electron pair geometry? What is the molecular geometry? Estimate the bond angle. Is this molecule polar? If so, draw the dipole arrow.
The Correct Answer and Explanation is:
Hydrogen Sulfide (H₂S)
Lewis Structure:
markdownCopyEdit ..
H – S – H
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Sulfur has 6 valence electrons and needs 2 more to complete its octet. Each hydrogen atom contributes 1 electron, forming two single bonds with sulfur. The sulfur atom also holds two lone pairs.
Electron Pair Geometry:
Tetrahedral (based on four electron domains: two bonding pairs and two lone pairs)
Molecular Geometry:
Bent (or V-shaped)
Bond Angle:
Approximately 104.5°, similar to water, due to lone pair repulsion compressing the angle.
Polarity:
Yes, H₂S is polar. The bent shape and the difference in electronegativity between hydrogen (2.1) and sulfur (2.58) cause an uneven distribution of charge.
Dipole Arrows:
nginxCopyEditH ← δ⁺ S δ⁻ → H
Pointing from hydrogen (partial positive) toward sulfur (partial negative)
Hydrogen Cyanide (HCN)
Lewis Structure:
mathematicaCopyEditH – C ≡ N:
Carbon has 4 valence electrons, forming a single bond with hydrogen and a triple bond with nitrogen. Nitrogen has 5 valence electrons and completes its octet with one lone pair.
Electron Pair Geometry:
Linear (only two electron domains around carbon)
Molecular Geometry:
Linear
Bond Angle:
Exactly 180°
Polarity:
Yes, HCN is polar. Nitrogen is much more electronegative than hydrogen and carbon, pulling electron density toward itself.
Dipole Arrow:
mathematicaCopyEditH → C ≡ N
The dipole moment points toward nitrogen (more negative) from the hydrogen (more positive)
Explanation
Lewis structures are diagrams that represent the bonding between atoms and the lone pairs of electrons. For hydrogen sulfide (H₂S), sulfur is the central atom with two single bonds to hydrogen and two lone pairs. The tetrahedral arrangement of electron pairs gives a bent molecular geometry because only the positions of atoms (not lone pairs) determine the molecular shape. The bond angle is around 104.5°, slightly less than the ideal tetrahedral angle due to lone pair repulsion.
Because of the bent shape and the difference in electronegativity, H₂S has a net dipole moment, making it polar. The dipole arrows show the direction of electron density shifting toward sulfur, the more electronegative atom.
For hydrogen cyanide (HCN), the structure has hydrogen bonded to carbon, which in turn is triple bonded to nitrogen. Carbon and nitrogen complete their octets with these bonds and lone pairs. The molecular geometry of HCN is linear, as there are two regions of electron density around the central carbon atom. The bond angle is exactly 180°.
HCN is also polar due to the electronegativity difference, especially between nitrogen and hydrogen. The molecule has a large dipole moment directed from the hydrogen end to the nitrogen end.
Both molecules exhibit polarity, but for different reasons. H₂S is polar due to its bent shape and lone pairs, while HCN is polar due to its linear shape and significant electronegativity difference across the molecule.
