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Chemistry LibreTexts

4.8: Polyatomic Molecules- Water, Ammonia, and Methane

  • Page ID
    153799
  • Learning Objectives

    • Determine the number of bonds formed by common nonmetal elements.
    • Illustrate covalent bond formation with Lewis electron dot diagrams.

    More than two atoms can participate in covalent bonding, although any given covalent bond will be between two atoms only. Water, ammonia, and methane are common examples that will be discussed in detail below. Carbon is unique in the extent to which it forms single, double, and triple bonds to itself and other elements. The number of bonds formed by an atom in its covalent compounds is not arbitrary. Hydrogen, oxygen, nitrogen, and carbon have very strong tendencies to form substances in which they have one, two, three, and four bonds to other atoms, respectively (Table \(\PageIndex{1}\)).

    Table \(\PageIndex{1}\) The Number of Bonds That Selected Atoms Commonly Form to Other Atoms
    Atom Number of Bonds
    H (group 1) 1
    O (group 16) 2
    N (group 15) 3
    C (group 14) 4

    Water

    Consider H and O atoms:

    The H and O atoms can share an electron to form a covalent bond:

    The H atom has a complete valence shell. However, the O atom has only seven electrons around it, which is not a complete octet. We fix this by including a second H atom, whose single electron will make a second covalent bond with the O atom:

    (It does not matter on what side the second H atom is positioned.) Now the O atom has a complete octet around it, and each H atom has two electrons, filling its valence shell. This is how a water molecule, H2O, is made.

    Ammonia

    The N atom has the following Lewis electron dot diagram:

    It has three unpaired electrons, each of which can make a covalent bond by sharing electrons with an H atom. The electron dot diagram of NH3 is as follows:

    Methane

    The C atom has the following Lewis electron dot diagram:

    Image result for carbon lewis dot

    It has four unpaired electrons, each of which can make a covalent bond by sharing electrons with an H atom. The electron dot diagram of CH4 is as follows:

    clipboard_ee9bd5f574529f3ff037fb445900dcb6b.png

    Summary

    In polyatomic molecules, there is a pattern of covalent bonds that different atoms can form.

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