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Chemistry LibreTexts

3.4.1.0: Molecular Shape (Problems)

  • Page ID
    210712
  • 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

    clipboard_e0a8df949265432712f1f1665cb53eec2.png

    linear

    Answer b

    clipboard_e26bf2c01cc4d7696034d39416e2d488d.png

    trigonal planar (bent 120)

    Answer c

    clipboard_efaadfed977b2a7f6e31bcc6ede76204b.png

    trigonal planar

    Answer d

    clipboard_ec260a29ea7d64dc303d8f19bec790af9.png

    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

    Three Lewis diagrams are shown. The first diagram shows the letter A single bonded to the left and right to the letter B. An example, “C O subscript 2,” and the term, “linear,” are written beside this diagram. The second diagram shows the letter A with two lone pairs of electrons, single bonded to the left and lower right to the letter B. An example, “H subscript 2 O,” and the term, “bent with an approximately 109 degree angle,” are written beside this diagram. The third diagram shows the letter A with one lone electron pair, single bonded to the left and lower right to the letter B. An example, “S O subscript 2,” and the term, “bent with an approximately 120 degree angle,” are written beside this diagram.

    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

    clipboard_e13790cd4eab11cbe9835da387ab0981d.png

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