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    Distinguishing bonds - ionic or covalent ?

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    How do you work out which ionic molecules are more polarised and which are less covalent

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    When two different atoms share electron density to form a covalent bond, they generally do not share the electrons of this bond equally. The more electronegative atom will pull the electrons closer to it so that there will be a slight negative charge at this end of the bond (symbolized as -) and a slight positive charge at the other end (symbolized as +). We say that the bond is polarized toward this more electronegative atom, and we sometimes refer to this polar covalent bond as a dipole. Bonds between identical atoms, such as in the homonuclear diatomic molecule H2, will be nonpolar since the atoms are of equal electronegativity, but virtually all other bonds are at least slightly polar. As the electronegativity difference between the two atoms involved in a bond increases, the bond polarity increases and ultimately, when the electronegativity difference becomes large enough, the bonding becomes ionic. (NaCl.is an example)

    Two hydrogen atoms share the electrons of the covalent bond equally. A covalent bond between hydrogen and fluorine is polarized toward the more electronegative fluorine atom. The electronegativity difference between sodium and chlorine is so great that the bond is ionic.

    Deciding whether a molecule is polar or nonpolar can be complicated if there are pi bonds, or if atoms are close in electronegativity. In most simple cases, we can predict the polarity of a molecule by considering its overall shape and the sort of groups attached to the central atom.
    Recall from VSEPR theory that if a central atom contains three groups, these groups will be arranged 120 degrees apart to form a trigonal planar arrangement; four groups form a tetrahedron; five groups form a trigonal bipyramid; six groups form an octahedron. We can usually predict whether or not a molecule is polar using the guidelines listed below.
    ? If the central atom contains four or fewer groups, the molecule will be nonpolar only if all four groups are identical. An exception is a homonuclear diatomic molecule such as Cl2. Each chlorine has four groups attached (one bonding pair of electrons and three nonbonding pairs), but since the atoms are the same there is no electronegativity difference so the bonding electrons are shared equally.

    ? If the central atom possesses five groups, it will be nonpolar if the two axial groups are identical, and the three equatorial groups are identical. The axial and equatorial groups may be different from one another.

    ? If the central atom has six groups, then the groups on "opposite sides" of the central atom must be identical in order for the molecule to be nonpolar.


    The oxygen has four groups attached; two hydrogen atoms and two nonbonding pairs of electrons. Since the four groups are not identical, the molecule is polar.


    There are five groups (three F atoms and two pairs of nonbonding electrons) on the central chlorine atom. Remember that nonbonding electrons occupy the equatorial sites first, so this molecule is T-shaped. The two axial groups are identical (both are fluorine atoms), but the three equatorial groups are not identical (two are nonbonding pairs of electrons, the other is a fluorine atom) so this molecule is polar.

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