Bond Polarity

Basic Concepts of Chemical Bonding

Bond Polarity and Electronegativity

Bond Polarity and Electronegativity

The electron pairs shared between two atoms are not necessarily shared equally

Extreme examples:

1. In Cl₂ the shared electron pairs is shared equally

2. In NaCl the 3s electron is stripped from the Na atom and is incorporated into the electronic structure of the Cl atom - and the compound is most accurately described as consisting of individual Na⁺ and Cl^- ions

For most covalent substances, their bond character falls between these two extremes

Bond polarity is a useful concept for describing the sharing of electrons between atoms

A nonpolar covalent bond is one in which the electrons are shared equally between two atoms
A polar covalent bond is one in which one atom has a greater attraction for the electrons than the other atom. If this relative attraction is great enough, then the bond is an ionic bond

Electronegativity

A quantity termed 'electronegativity' is used to determine whether a given bond will be nonpolar covalent, polar covalent, or ionic.

Electronegativity is defined as the ability of an atom in a particular molecule to attract electrons to itself

(the greater the value, the greater the attractiveness for electrons)

Electronegativity is a function of:

the atom's ionization energy (how strongly the atom holds on to its own electrons)
the atom's electron affinity (how strongly the atom attracts other electrons)

(Note that both of these are properties of the isolated atom)

For example, an element which has:

A large (negative) electron affinity
A high ionization energy (always endothermic, or positive for neutral atoms)

Will:

Attract electrons from other atoms
Resist having its own electrons attracted away

Such an atom will be highly electronegative

Fluorine is the most electronegative element (electronegativity = 4.0), the least electronegative is Cesium (notice that are at diagonal corners of the periodic chart)

General trends:

Electronegativity increases from left to right along a period
For the representative elements (s and p block) the electronegativity decreases as you go down a group
The transition metal group is not as predictable as far as electronegativity

Electronegativity and bond polarity
We can use the difference in electronegativity between two atoms to gauge the polarity of the bonding between them

Compound	F₂	HF	LiF
Electronegativity Difference	4.0 - 4.0 = 0	4.0 - 2.1 = 1.9	4.0 - 1.0 = 3.0
Type of Bond	Nonpolar covalent	Polar covalent	Ionic (non-covalent)

In F₂ the electrons are shared equally between the atoms, the bond is nonpolar covalent
In HF the fluorine atom has greater electronegativity than the hydrogen atom.

The sharing of electrons in HF is unequal: the fluorine atom attracts electron density away from the hydrogen (the bond is thus a polar covalent bond)

The H-F bond can thus be represented as:

The 'd+' and 'd-' symbols indicate partial positive and negative charges.
The arrow indicates the "pull" of electrons off the hydrogen and towards the more electronegative atom
In lithium fluoride the much greater relative electronegativity of the fluorine atom completely strips the electron from the lithium and the result is an ionic bond (no sharing of the electron)

A general rule of thumb for predicting the type of bond based upon electronegativity differences:

If the electronegativities are equal (i.e. if the electronegativity difference is 0), the bond is non-polar covalent

If the difference in electronegativities between the two atoms is greater than 0, but less than 2.0, the bond is polar covalent

If the difference in electronegativities between the two atoms is 2.0, or greater, the bond is ionic

1996 Michael Blaber