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Shapes of Molecules

Key Concepts

  • A molecule consists of 2 or more atoms joined by covalent bonds.

  • The shape of a molecule is a description of the way the atoms in the molecule occupy space.

  • A diatomic molecule, a molecule composed of only 2 atoms, must always be linear in shape as the centers of the 2 atoms will always be in a straight line.

  • 'Electron Cloud' Repulsion Theory (Valence Shell Electron Pair Repulsion, VSEPR) is used to predict shapes and bond angles of simple molecules
  1. an 'electron cloud' may be a single, double or triple bond, or a lone pair of electrons
  2. a lone pair of electrons is a non-bonding pair of electrons
  3. 'electron clouds' are negatively charged since the electrons are negatively charged, so electron clouds repel one another and try to get as far away from each other as possible
  4. lone pairs of electrons exert a greater repelling effect than bonding pairs do
  5. lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion
  6. lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion
Total Number of electron pairs Arrangement of electron pairs Number of bonding pairs of electrons Number of lone pairs of electrons Shape of Molecule Name of Shape Bond Angle Examples
not applicable linear 1 not applicable linear 180o H2, HCl
2 linear 2 0 linear 180o CO2, HCN
3 trigonal planar 3 0 triganol planar 120o BCl3, AlCl3
4 tetrahedral 4 0 tetrahedral 109.5o CH4, SiF4
3 1 trigonal pyramidal <109.5o (bond angles in ammonia, NH3, are 107o) NH3, PCl3
2 2 bent <109.5o (bond angles in water, H2O, are 105o) H2O, SCl2
5 trigonal bipyramidal 5 0 trigonal bipyramidal 120o in the trigonal planar part of the molecule, 90o for the others PCl5
6 octahedral 6 0 octahedral 90o SF6

Examples

  1. hydrogen chloride, HCl

    HCl is composed of only 2 atoms, 1 atom of hydrogen and 1 atom of chlorine covalently bonded.
    HCl is, therefore, diatomic.
    All diatomic molecules are linear in shape.

    H-Cl is linear in shape

  2. hydrogen cyanide, HCN

    HCN is composed of 3 atoms, 1 atom of hydrogen, 1 atom of carbon and 1 atom of nitrogen covalently bonded.
    Carbon, C, is the central atom in the molecule.
    Carbon has 4 valence electrons (electrons that can be used in bonding).
    1 of carbon's valence electrons will be used to form a covalent bond with hydrogen.
    3 of carbon's valence electrons will be used to form 3 covalent bonds with nitrogen ( a triple bond).
    The central carbon atom therefore has no lone pairs of electrons.
    The bonding pairs will repel each other as much as possible, so the molecule will be linear.

    HCN is linear in shape.

  3. aluminium chloride, AlCl3

    AlCl3 is composed of 4 atoms, 1 atom of aluminium and 3 atoms of chlorine.
    Aluminium, Al, is the central atom in the molecule.
    Aluminium has 3 valence electrons (electrons that can be used in bonding).
    Each of the aluminium's valence electrons will be used to form a covalent bond with each chlorine atom.
    The central aluminium atom will therefore have no lone pairs of electrons and 3 bonding pairs of electrons.
    The bonding pairs of electrons will repel each other as much as possible, so the molecule will be trigonal planar.

    AlCl3 is trigonal planar in shape.

  4. methane, CH4

    CH4 is composed of 5 atoms, 1 atom of carbon and 4 atoms of hydrogen covalently bonded.
    Carbon, C, is the central atom in the molecule.
    Carbon has 4 valence electrons (electrons that can be used in bonding).
    Each of carbon's 4 valence electrons will form a bonding pair with 1 of hydrogen's electrons.
    The central carbon atom will therefore have no lone pairs of electrons and 4 bonding pairs of electrons,
    The bonding pairs of electrons will repel each other as much as possible, so the molecule will be tetrahedral.

    CH4 is tetrahedral in shape.

  5. ammonia, NH3

    NH3 is composed of 4 atoms, 1 nitrogen atom and 3 hydrogen atoms covalently bonded.
    Nitrogen, N, is the central atom in the molecule.
    Nitrogen has 5 valence electrons.
    3 of nitrogen's valence electrons will be used to form bonding pairs of electrons with hydrogen (3 covalent bonds).
    These bonding pairs repel each other equally and will try to get as far away from each other as possible.
    2 of nitrogen's valence electrons will be unused for bonding, these are a lone pair of electrons.
    lone pair-lone pair repulsion is greater than bonding pair-bonding pair repulsion, so the lone pair pushes the bonding pairs closer together than in a tetrahedral arrangement of the 'electron clouds'.
    This distorted tetrahedral arrangement is called trigonal pyramidal.

    NH3 is trigonal pyramidal in shape.

    In this representation, the solid triangles represent bonds coming out of the plane of the screen, the broken lines represented a bond going behind the plane of the screen.

  6. water, H2O

    Water is composed of 3 atoms, 1 atom of oxygen and 2 atoms of hydrogen covalently bonded.
    Oxygen, O, is the central atom.
    Oxygen has 6 valence electrons.
    2 of oxygen's electrons will be used to form bonding pairs of electrons with hydrogen (2 covalent bonds).
    These bonding pairs repel each other equally and will try to get as far away from each other as possible.
    4 of oxygen's valence electrons will not be used for bonding, these will remain as 2 lone pairs of electrons.
    These lone pairs of electrons repel each other equally and will try to get as far away from each as possible.
    lone pair-lone pair repulsion is greater than lone pair-bonding pair or bonding pair-bonding pair repulsion, so the lone pairs of electrons push the bonding pairs of electrons closer together than in a tetrahedral arrangement of the 'electron clouds'.
    This distorted tetrahedral arrangement is call bent.

    H2O is bent in shape.

    In this representation, solid lines represent bonds that are in the same plane as the screen.


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