Electronegativity and Bond Polarity

Covalent Bonding

Define electronegativity.

The ability of an atom to attract a bonding pair of electrons in a covalent bond.

What is the general trend in electronegativity across a period?

Electronegativity increases from left to right across a period.

Why does electronegativity increase across a period?

Nuclear charge increases while shielding remains similar, so the nucleus attracts the bonding pair of electrons more strongly.

What is the general trend in electronegativity down a group?

Electronegativity decreases down a group.

Why does electronegativity decrease down a group?

Atomic radius and electron shielding increase, reducing the attraction between the nucleus and the bonding pair of electrons.

Which element has the highest electronegativity?

Fluorine.

Why is fluorine more electronegative than chlorine?

Fluorine has fewer electron shells, less shielding and a smaller atomic radius, so its nucleus attracts a bonding pair of electrons more strongly.

What happens to electron density when two identical atoms form a covalent bond?

It is distributed symmetrically between the two nuclei.

Why is a bond between two identical atoms non-polar?

The atoms have identical electronegativities, so they attract the bonding pair of electrons equally.

Give an example of a non-polar covalent bond.

Cl–Cl.

What is another name for a non-polar covalent bond?

A pure or normal covalent bond.

Define a polar covalent bond.

A covalent bond in which the bonding electrons are unequally distributed, giving one atom a slight negative charge and the other a slight positive charge.

What causes a covalent bond to become polar?

A difference in electronegativity between the bonded atoms.

Which atom attracts the bonding electrons more strongly in a polar bond?

The more electronegative atom.

What charge does the more electronegative atom develop?

A partial negative charge, δ−.

What charge does the less electronegative atom develop?

A partial positive charge, δ+.

What does the symbol δ mean?

A partial or slight charge.

How is HCl polarised?

Hδ+–Clδ−.

Why is HCl a polar molecule?

Chlorine is more electronegative than hydrogen, so it attracts the bonding pair more strongly and gains δ− while hydrogen gains δ+.

What does an electron density map of a polar bond show?

Greater electron density around the more electronegative atom.

What do closely spaced contour lines on an electron density map represent?

High electron density.

How can the direction of electron drift in a polar bond be represented?

By an arrow pointing towards the more electronegative atom.

Which direction does the polarity arrow point?

Towards the atom with the greater electronegativity.

Give examples of polar covalent bonds.

C–Cl, H–O, H–N and H–C.

How can you predict which end of a bond is δ−?

The atom with the higher electronegativity is δ−.

How can you predict which end of a bond is δ+?

The atom with the lower electronegativity is δ+.

What determines the degree of polarity of a covalent bond?

The difference in electronegativity between the two bonded atoms.

What happens to bond polarity as the electronegativity difference increases?

Bond polarity increases.

What happens when the electronegativity difference is zero?

The electrons are shared equally and the bond is non-polar covalent.

Which bond is more polar: one with a small or large electronegativity difference?

One with a large electronegativity difference.

How do you compare the polarity of several bonds?

Calculate or compare the electronegativity difference between the bonded atoms; the largest difference gives the most polar bond.

What is meant by the continuum of bonding?

Ionic and covalent bonding are two extremes, with many bonds having characteristics between the two.

What are the two extremes of the bonding continuum?

Pure (100%) covalent bonding and pure (100%) ionic bonding.

How can a polar covalent bond be described in terms of the bonding continuum?

A covalent bond with some degree of ionic character.

What happens to ionic character as electronegativity difference increases?

Ionic character increases.

When does bonding become mainly ionic?

When the electronegativity difference between the atoms becomes sufficiently large.

Is there a sharp boundary between ionic and covalent bonding?

No. Bonding exists on a continuum.

What is meant by percentage ionic character?

An approximate measure of how much ionic character a bond has.

What relationship exists between electronegativity difference and percentage ionic character?

A larger electronegativity difference gives a greater percentage ionic character.

Put C–F, C–Cl, C–Br and C–I in decreasing order of bond polarity.

C–F > C–Cl > C–Br > C–I.

Why is C–F the most polar of the carbon–halogen bonds?

Fluorine has the highest electronegativity, so C and F have the greatest electronegativity difference.

Why does C–X bond polarity generally decrease from F to I?

Electronegativity of the halogens decreases down Group 7, reducing the electronegativity difference with carbon.