Question

predict the molecular geometries of all the following compounds using vsepr:
a) sif4
b) of2
c) ocf2 (with respect to the carbon atom, which is central)
d) pcl3
e) co2

Explanation:

Part (a): $\boldsymbol{\ce{SiF4}}$

Step 1: Determine the central atom and its valence electrons

The central atom is $\ce{Si}$. Silicon has 4 valence electrons. Each $\ce{F}$ atom contributes 1 valence electron, and there are 4 $\ce{F}$ atoms, so total valence electrons from $\ce{F}$: $4\times1 = 4$. Total valence electrons: $4 + 4 = 8$.

Step 2: Determine the number of bonding and non - bonding pairs

The Lewis structure of $\ce{SiF4}$ shows that $\ce{Si}$ forms 4 single bonds with $\ce{F}$ atoms. So, the number of bonding pairs ($BP$) around the central $\ce{Si}$ atom is 4, and the number of non - bonding pairs ($LP$) is 0.

Step 3: Apply VSEPR theory

The VSEPR formula is $AX_nE_m$, where $A$ is the central atom, $X$ is the surrounding atom, $n$ is the number of $X$ atoms, and $m$ is the number of lone pairs. For $\ce{SiF4}$, the formula is $AX_4E_0$. According to VSEPR theory, a molecule with 4 bonding pairs and 0 lone pairs around the central atom has a tetrahedral molecular geometry.

Part (b): $\boldsymbol{\ce{OF2}}$

Step 1: Determine the central atom and its valence electrons

The central atom is $\ce{O}$. Oxygen has 6 valence electrons. Each $\ce{F}$ atom contributes 1 valence electron, and there are 2 $\ce{F}$ atoms, so total valence electrons from $\ce{F}$: $2\times1=2$. Total valence electrons: $6 + 2=8$.

Step 2: Determine the number of bonding and non - bonding pairs

The Lewis structure of $\ce{OF2}$ shows that $\ce{O}$ forms 2 single bonds with $\ce{F}$ atoms. The number of bonding pairs ($BP$) is 2. The number of non - bonding pairs ($LP$) on $\ce{O}$: $\frac{8-(2\times2)}{2}=2$ (using the formula: $LP=\frac{V - n\times1}{2}$, where $V$ is the valence electrons of the central atom, $n$ is the number of bonding atoms).

Step 3: Apply VSEPR theory

The VSEPR formula for $\ce{OF2}$ is $AX_2E_2$. According to VSEPR theory, a molecule with 2 bonding pairs and 2 lone pairs around the central atom has a bent (or angular) molecular geometry.

Part (c): $\boldsymbol{\ce{OCF2}}$ (central atom $\ce{C}$)

Step 1: Determine the central atom and its valence electrons

The central atom is $\ce{C}$. Carbon has 4 valence electrons. The $\ce{O}$ atom (in a double - bond situation, but for VSEPR, we consider the number of electron groups) and 2 $\ce{F}$ atoms. The Lewis structure of $\ce{OCF2}$ (with $\ce{C}$ as central) shows that $\ce{C}$ forms a double bond with $\ce{O}$ and two single bonds with $\ce{F}$ atoms. The number of electron groups (bonding groups) around $\ce{C}$: the double bond with $\ce{O}$ counts as 1 electron group, and each single bond with $\ce{F}$ counts as 1 electron group. So, total electron groups (bonding pairs) around $\ce{C}$ is 3, and the number of lone pairs on $\ce{C}$ is 0.

Step 2: Apply VSEPR theory

The VSEPR formula is $AX_3E_0$. According to VSEPR theory, a molecule with 3 bonding pairs and 0 lone pairs around the central atom has a trigonal planar molecular geometry.

Part (d): $\boldsymbol{\ce{PCl3}}$

Answer:

s:
a) $\ce{SiF4}$: tetrahedral
b) $\ce{OF2}$: bent
c) $\ce{OCF2}$ (central $\ce{C}$): trigonal planar
d) $\ce{PCl3}$: trigonal pyramidal
e) $\ce{CO2}$: linear