Starter for ten…Co-ordinate (dative) covalent bonds

Work out which molecules have dative covalent bonds and draw the structure

Electronegativity and polaritySpecification reference 3.1.3

Pages 52-53

Learning outcomesState what is meant by the term

electronegativity

State what makes one atom more electronegative than another

State what the symbols δ+ and δ- mean when placed above atoms in a covalent bond

Polar and non-polar bondsCovalent bond - ?

Ionic bond - ?

Vote now...Which of the three molecules is polar?

Why?

Non-polarCovalent bond – shared pair of electronsIn hydrogen, the two bonding atoms are

identicalThey have an equal share of the pair of

electrons in the bond which results in a perfect 100% covalent bond

Hydrogen is a non-polar molecule, the electrons are evenly distributed between the atoms that make up that bond

What happens if the atoms are different?

PolarIf the atoms bonding are different, one of the

atoms is likely to attract the bonding electrons more

The bonding atom with the greater attraction is more electronegative than the other atom

Example, HClThe Cl atom is more electronegative than the H

atomThe Cl atom has a greater attraction to the

bonding pair of electrons that the H atomThe bonding electrons are closer to the Cl atom

than the H atom

Trends in electronegativity

Li Be B C N O F

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Na Mg Al Si P S Cl

0.9 1.2 1.5 1.8 2.1 2.5 3.0

Br

2.8

Increasing electronegativity

Increasing electronegativity

Key definition – ElectronegativityElectronegativity is the power of an atom to attract the electron density in a covalent bond towards itself.

ElectronegativityElectron density is often used to describe the

way the negative charge is distributed in a molecule

The Pauling scale is used a measure of electronegativity (research opportunity…?)

Why do the noble gases not have a number assigned?They do not (in general) form covalent bonds

Electronegativity Electronegativity depends on three things:

The nuclear chargeThe distance between the nucleus and the outer

shell electronsThe shielding of the nuclear charge by electrons

in inner shellsNote – the smaller the atom, the closer to the

nucleus the shared outer shell electrons ∴ the greater the electronegativity

The larger the nuclear charge, the greater the electronegativity

Trends in electronegativity

Li Be B C N O F

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Na Mg Al Si P S Cl

0.9 1.2 1.5 1.8 2.1 2.5 3.0

Br

2.8

Increasing electronegativity

Increasing electronegativity

Small atomsLess shielding

PolarThis leads to a charge difference across the H-

Cl bond

This charge difference leads to a permanent dipole and we represent it with the δ+ or δ- symbol

We now have a polar covalent bond

Shows a small positive charge on the hydrogen

Shows a small negative charge on the chlorine

QuestionsIdentify which of these covalent bonds is/are

polar, and explain your answer.H – HH – FF – F

Add dipoles to the polar covalent bondsArrange the following covalent bonds in order

of increasing polarity: H – O, H – F, H – NExplain your answer

Polar moleculesPoles many cancel each other out.E.g. HCl is linear – it has polar bonds and is a

polar molecule. The molecule is non-symmetrical and so there is a charge across the whole molecule.

Symmetrical molecules, the dipoles of any bond may cancel out. E.g. Tetrachloromethane

Non polar polarEach C-Cl bond is polar

Symmetrical molecule(tetrahedral)

Dipoles act in different directions and cancel each other out

= a non-polar molecule with polar bonds!

Patterns

Trends in electronegativityWhich atoms form the most electronegative

atoms?

Which atoms form the least electronegative atoms?

Electronegativity and bonding type

What does this diagram describe to you?

QuestionsComplete the exam style questions

HomeworkComplete the relevant section of the

webquest