2.2.10 Electronegativity and Bond Polarity

Lesson Outcomes:

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

I can explain how a permanent dipole can result in a polar bond

I can link electronegativity to the bonding type on a compound.

2.2.10 Electronegativity and Bond Polarity

2.2.10 Electronegativity and Bond Polarity

Definition: Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons in a covalent bond.

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Therefore: The greater the electronegativity of an atom, the more it attracts electrons towards it.

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Electronegativity and Polarity

Challenge: What factors will affect how electronegative an atom is?

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

The factors affecting electronegativity are:

- Atomic charge;

- Distance from the nucleus;

- Electron shielding

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Electronegativity and Polarity

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

• In a covalent bond, electrons are shared.
• When the 2 atoms are identical, the electrons are shared equally.

H-H

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Cl-Cl

Bonds between identical atoms

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

• The more electronegative atom will have a greater share of the electrons.
• E.g. Cl is more electronegative than H so in a molecule of HCl, the electrons are pulled towards the Chlorine.

This symbol is called delta and means “slightly”

Bonds between different atoms

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

• The charge difference between H and Cl is called a permanent dipole.
• HCl is therefore a polar covalent bond.

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Polar Molecules

I can explain how a permanent dipole can result in a polar bond

• Symmetrical molecules are non-polar even though they contain polar bonds.

4 dipoles acting in different directions cancel each other out.

Non-polar molecules

I can explain how a permanent dipole can result in a polar bond

• The greater the difference in electronegativity between the bonded atoms, the greater the permanent dipole.

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• The more electronegative atom will take the δ- charge.

Key Concept

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

I can explain how a permanent dipole can result in a polar bond

1932 – Linus Pauling invented Pauling scale to measure electronegativity on an atom.

Linus Pauling

I can link electronegativity to the bonding type on a compound

Pauling Scale

Question: Explain why electronegativity increases across a period.

Across a Period

As you go across a period the electronegativity increases. The chart shows electronegativities from sodium to chlorine - you have to ignore argon. It doesn't have an electronegativity, because it doesn't form bonds.

Electronegativity increases across a period because the number of charges on the nucleus increases. That attracts the bonding pair of electrons more strongly.

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

Both sodium and chlorine have their bonding electrons in the 3-level. The electron pair is screened from both nuclei by the 1s, 2s and 2p electrons, but the chlorine nucleus has 6 more protons in it. 

Across a Period

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

Question: Explain why electronegativity decreases down a group.

Down a Group

As you go down a group, electronegativity decreases. If it increases up to fluorine, it must decrease as you go down.

As you go down a group, electronegativity decreases because the bonding pair of electrons is increasingly distant from the attraction of the nucleus.

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

The bonding pair is shielded from the fluorine's nucleus only by the 1s² electrons. In the chlorine case it is shielded by all the 1s²2s²2p⁶ electrons.

Down a Group

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

Covalent → Ionic

No difference between electronegativity

non polar covalent bond

Large difference in electronegativity

ionic bond

Small difference in electronegativity

polar covalent bond.

A and B would usually have to be the same atom.

e.g. H₂ or Cl₂

Challenge: Could you give an example if each?

One end is slightly positive and the other slightly negative.

e.g. most covalent bonds, HCl, O-H in water

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

I can describe electronegativity in terms of an atom attracting bonding electrons in a covalent bond

• Check your answer using a green pen.

What is electronegativity?

http://www.chemguide.co.uk/atoms/bonding/electroneg.html