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# 3.4.1.0: Molecular Shape (Problems)

PROBLEM $$\PageIndex{1}$$

Explain why the HOH molecule is bent, whereas the HBeH molecule is linear.

Answer

The placement of the two sets of unpaired electrons in water forces the bonds to assume a tetrahedral arrangement, and the resulting HOH molecule is bent. The HBeH molecule (in which Be has only two electrons to bond with the two electrons from the hydrogens) must have the electron pairs as far from one another as possible and is therefore linear.

PROBLEM $$\PageIndex{2}$$

Explain the difference between electron-pair geometry and molecular structure.

Answer

Space must be provided for each pair of electrons whether they are in a bond or are present as lone pairs. Electron-pair geometry considers the placement of all electrons. Molecular structure considers only the bonding-pair geometry.

PROBLEM $$\PageIndex{3}$$

Why is the H–N–H angle in NH3 smaller than the H–C–H bond angle in CH4? Why is the H–N–H angle in $$\ce{NH4+}$$ identical to the H–C–H bond angle in CH4?

Answer

NH3 has a lone pair of electrons, which forces the bonds closer together, lessening the bond angle compared to species with 4 bonds and no lone pairs.

PROBLEM $$\PageIndex{4}$$

Predict the electron pair geometry and the molecular structure of each of the following molecules or ions:

a. BeH2
b. $$\ce{CH3+}$$

Answer a

Both the electron geometry and the molecular structure are linear.

Answer b

Both the electron geometry and the molecular structure are trigonal planar.

PROBLEM $$\PageIndex{5}$$

Identify the electron pair geometry and the molecular structure of each of the following molecules or ions:

a. CF4
b. BF3
c. BeCl2

Answer a

Both the electron geometry and the molecular structure are tetrahedral.

Answer b

Both the electron geometry and the molecular structure are trigonal planar.

Answer c

Both the electron geometry and the molecular structure are linear.

PROBLEM $$\PageIndex{6}$$

What are the electron-pair geometry and the molecular structure of each of the following molecules or ions?

a. $$\ce{ClO2-}$$
b. PCl3
c. $$\ce{PH2-}$$

Answer a

electron-pair geometry: tetrahedral

molecular structure: bent

Answer b

electron-pair geometry: tetrahedral

molecular structure: trigonal pyramidal

Answer c

electron-pair geometry: tetrahedral

molecular structure: bent (109°)

PROBLEM $$\PageIndex{7}$$

Identify the electron pair geometry and the molecular structure of each of the following molecules:

a. ClNO (N is the central atom)
b. CS2
c. Cl2CO (C is the central atom)
d. Cl2SO (S is the central atom)
e. SO2F2 (S is the central atom)
f. (g) $$\ce{ClOF2+}$$ (Cl is the central atom)

Answer a

electron-pair geometry: trigonal planar, molecular structure: bent (120°)

Answer b

electron-pair geometry: linear, molecular structure: linear

Answer c

electron-pair geometry: trigonal planar, molecular structure: trigonal planar

Answer d

electron-pair geometry: tetrahedral, molecular structure: trigonal pyramidal

Answer e

electron-pair geometry: tetrahedral, molecular structure: tetrahedral

Answer f

electron-pair geometry: tetrahedral, molecular structure: trigonal pyramidal

PROBLEM $$\PageIndex{8}$$

Draw the Lewis structures and predict the shape of each compound or ion:

a. CO2
b. $$\ce{NO2-}$$
c. SO3
d. $$\ce{SO3^2-}$$

Answer a

linear

Answer b

trigonal planar (bent 120)

Answer c

trigonal planar

Answer d

tetrahedral (trigonal pyramidal)

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PROBLEM $$\PageIndex{9}$$

A molecule with the formula AB2, in which A and B represent different atoms, could have one of three different shapes. Sketch and name the three different shapes that this molecule might have. Give an example of a molecule or ion for each shape.

Answer

PROBLEM $$\PageIndex{10}$$

A molecule with the formula AB3, in which A and B represent different atoms, could have one of two different shapes. Sketch and name the three different shapes that this molecule might have. Give an example of a molecule or ion that has each shape.

Answer

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