Question

For each of the following molecules, give the steric number and sketch and name the approximate molecular geometry. In each case, the central atom is listed first and the other atoms are all bonded directly to it.

(a) $\mathit{C}\mathit{B}{\mathit{r}}_{\mathbf{4}}$ (b) $\mathit{S}{\mathit{O}}_{\mathbf{3}}$

(c) $\mathit{S}\mathit{e}{\mathit{F}}_{\mathbf{6}}$ (d)$\mathit{S}\mathit{O}\mathit{C}{\mathit{l}}_{\mathbf{2}}$

(e) $\mathit{I}\mathit{C}{\mathit{l}}_{\mathbf{3}}$

Short answer

a) In the molecule $CB{r}_4$ the steric number is 4, and the molecular geometry is tetrahedral.

b) In the molecule $S{O}_3$ the steric number is , and the molecular geometry is trigonal planar.

c) In the molecule $Se{F}_6$ the steric number is , and the molecular geometry is octahedral.

d) In the molecule $SOC{l}_2$ the steric number is , and the molecular geometry is trigonal pyramidal.

e) In the molecule $IC{l}_3$ the steric number is , and the molecular geometry is distorted T-shaped.

Step-by-step solution

Solving

In order to solve all the following parts, one has to calculate the steric numbers of the central atoms which is the sum of the ligands and lone pairs around it. From this, the overall geometry of the center can be determined. The name of the molecular geometry is derived from that, omitting the positions occupied by lone pairs.

Subpart (a)

$CB{r}_4$

In the molecule carbon tetrabromide, the steric number is 4.

This is because the carbon atom is strictly tetravalent and usually has no lone pairs.

The geometry of the center is tetrahedral.

The image below depicts the 3D structure.

Subpart (b)

$S{O}_3$

In the molecule sulfur trioxide, the steric number is 3.

This is because all the six valance electrons of a sulfur atom are used to form three double bonds to the oxygen ligands. There are no lone pairs.

The geometry of the center is trigonal planar.

The image below depicts the 3D structure.

Subpart (c)

$Se{F}_6$

In the molecule selenium hexafluoride, the steric number is 6.

This is because six valance electrons of a selenium atom are used to form single bonds to the fluorine ligands. There areno lone pairs.

The geometry of the center isoctahedral.

The image below depicts the 3D structure.

Subpart (d)

$SOC{l}_2$

In the molecule thionyl chloride (sulfur oxide dichloride), the steric number is 4.

This is because only four of the six valence electrons of a sulfur atom are used to form a double bond to the oxygen and two single bonds to the chlorine ligands while there is a lone pair remaining.

The geometry of the center is tetrahedral but the molecule is considered (slightly distorted) trigonal pyramidal.

The image below depicts the 3D structure.

Subpart (e)

$IC{l}_3$

In the molecule iodine trichloride, the steric number is 5.

This is because only three of the seven valence electrons of an iodine atom are used to form single bond to the chlorine ligands while there are two loan pair remaining.

The geometry of the center is trigonal bipyramidal but the molecule is considered(distorted) T-shaped.

The distortion occurs because thelone pairsare present and are closer to the center than the ligands and so, they can force the axial positions closer to the third equatorial.

The image below depicts the 3D structure.