Question

draw the following lewis structures:
$ph_3$
$cn^-$
$h_2s$
$hf$
$cs_2$
$cocl_2$

for each molecule, state the molecular geometry.
select two of the molecules above that have different molecular geometries. then,
write a paragraph explaining:

• at least three similarities and three differences between the two molecules.
• why the molecules have different molecular geometries/structures.

Explanation:

Step1: Draw Lewis Structure for \(PH_3\)

Phosphorus (P) has 5 valence electrons, each Hydrogen (H) has 1. Total valence electrons: \(5 + 3\times1 = 8\). P is the central atom, bonded to 3 H atoms (single bonds, 2 electrons each), leaving 1 lone pair on P. Lewis structure: \( \text{H}-\text{P}(\text{lone pair})-\text{H} \) (with H on three sides).

Step2: Determine Molecular Geometry of \(PH_3\)

Using VSEPR theory, central P has 3 bonding pairs and 1 lone pair (\(AX_3E\) notation). Molecular geometry is trigonal pyramidal.

Step3: Draw Lewis Structure for \(CS_2\)

Carbon (C) has 4 valence electrons, each Sulfur (S) has 6. Total valence electrons: \(4 + 2\times6 = 16\). C is central, double - bonded to each S (each double bond has 4 electrons), so each S has 2 lone pairs. Lewis structure: \( \text{S}=\text{C}=\text{S} \).

Step4: Determine Molecular Geometry of \(CS_2\)

Central C has 2 bonding pairs (double bonds count as one region of electron density) and 0 lone pairs (\(AX_2\) notation). Molecular geometry is linear.

Step5: Identify Similarities between \(PH_3\) and \(CS_2\)

1. Both are covalent molecules (atoms share electrons).
2. Both have a central atom bonded to other atoms.
3. Both follow the octet rule (or duet for H in \(PH_3\), octet for P, C, S).

Step6: Identify Differences between \(PH_3\) and \(CS_2\)

1. \(PH_3\) has a central atom (P) with a lone pair, \(CS_2\) central C has no lone pairs.
2. \(PH_3\) has single bonds, \(CS_2\) has double bonds.
3. \(PH_3\) has 4 regions of electron density (3 bonding, 1 lone), \(CS_2\) has 2 regions (2 bonding).

Step7: Explain Different Molecular Geometries

In \(PH_3\), the lone pair on P exerts greater repulsion than bonding pairs, pushing the H atoms down from a tetrahedral electron - pair geometry to trigonal pyramidal molecular geometry. In \(CS_2\), with 2 bonding regions (double bonds) and no lone pairs, the electron - pair geometry (linear) is the same as molecular geometry, as the bonding pairs are arranged to minimize repulsion in a straight line.

Answer:

Lewis Structures:

• \(PH_3\): P as central, 3 single bonds to H, 1 lone pair on P.
• \(CN^-\): C and N triple - bonded, C has 1 lone pair, overall - 1 charge (total valence electrons: \(4 + 5+1 = 10\), triple bond (6 electrons), 2 lone pairs on C and N? Wait, correction: C (4) + N (5)+1 (charge) = 10. Triple bond (6 electrons), so C has 1 lone pair (2 electrons), N has 1 lone pair (2 electrons)? Wait, no: \([:\text{C}\equiv\text{N}:]^-\), C has 1 lone pair (2 e⁻), N has 1 lone pair (2 e⁻), total 2 + 6+2 = 10.
• \(H_2S\): S (6) + 2×H (1) = 8. S central, single bonds to 2 H, 2 lone pairs on S. Lewis: \( \text{H}-\text{S}(\text{2 lone pairs})-\text{H} \).
• \(HF\): H (1) + F (7) = 8. Single bond, F has 3 lone pairs. Lewis: \( \text{H}-\text{F}(\text{3 lone pairs}) \).
• \(CS_2\): As above, \( \text{S}=\text{C}=\text{S} \).
• \(COCl_2\): C (4) + O (6)+2×Cl (7) = 4 + 6+14 = 24. C central, double - bonded to O, single - bonded to each Cl. O has 2 lone pairs, each Cl has 3 lone pairs. Lewis: \( \text{Cl}-\text{C}(=\text{O})-\text{Cl} \).

Molecular Geometries:

• \(PH_3\): Trigonal Pyramidal
• \(CN^-\): Linear (as it is a diatomic ion with triple bond, \(AX_2\) with no lone pairs on central? Wait, \(CN^-\) is linear, like \(CO_2\) - type, but with a charge.
• \(H_2S\): Bent (central S has 2 bonding, 2 lone pairs, \(AX_2E_2\))
• \(HF\): Linear (diatomic)
• \(CS_2\): Linear
• \(COCl_2\): Trigonal Planar (central C has 3 bonding regions: 1 double to O, 2 single to Cl, no lone pairs, \(AX_3\))

Comparison (Taking \(PH_3\) and \(CS_2\)):

Similarities:

1. Covalent bonding (electron sharing).
2. Central atom bonded to other atoms.
3. Obey octet/duet rule.

Differences:

1. \(PH_3\) has a lone pair on central atom, \(CS_2\) does not.
2. \(PH_3\) has single bonds, \(CS_2\) has double bonds.
3. \(PH_3\) has 4 electron - density regions, \(CS_2\) has 2.

Reason for Different Geometries:

\(PH_3\) has a lone pair on P, which causes greater repulsion, leading to trigonal pyramidal geometry (from tetrahedral electron - pair geometry). \(CS_2\) has 2 bonding regions (double bonds) and no lone pairs, so electron - pair and molecular geometry are linear, as bonding pairs arrange in a straight line to minimize repulsion.