The Three Dimensional Aspects of a Molecule
Explain how polar compounds differ from nonpolar compounds.
Determine if a molecule is polar or nonpolar.
Given a pair of compounds, predict which would have a higher melting or boiling point.
Bond Polarity
The ability of an atom in a molecule to attract shared electrons is called electronegativity. When two atoms combine, the
difference between their electronegativities is an indication of the type of bond that will form. If the difference between the
electronegativities of the two atoms is small, neither atom can take the shared electrons completely away from the other atom,
and the bond will be covalent. If the difference between the electronegativities is large, the more electronegative atom will take
the bonding electrons completely away from the other atom (electron transfer will occur), and the bond will be ionic. This is
why metals (low electronegativities) bonded with nonmetals (high electronegativities) typically produce ionic compounds.
A bond may be so polar that an electron actually transfers from one atom to another, forming a true ionic bond. How do we
judge the degree of polarity? Scientists have devised a scale called electronegativity, a scale for judging how much atoms of
any element attract electrons. Electronegativity is a unitless number; the higher the number, the more an atom attracts
electrons. A common scale for electronegativity is shown in Figure .
Figure : Electronegativities of the Elements. Electronegativities are used to determine the polarity of covalent bonds.
The polarity of a covalent bond can be judged by determining the difference of the electronegativities of the two atoms
involved in the covalent bond, as summarized in the following table:
Electronegativity Difference Bond Type
0–0.4 pure covalent
0.5–2.0 polar covalent
>2.0 likely ionic
Nonpolar Covalent Bonds
A bond in which the electronegativity difference is less than 1.9 is considered to be mostly covalent in character. However, at
this point we need to distinguish between two general types of covalent bonds. A nonpolar covalent bond is a covalent bond
in which the bonding electrons are shared equally between the two atoms. In a nonpolar covalent bond, the distribution of
electrical charge is balanced between the two atoms.
Learning Objectives
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