Module 2 - Electronegativity and Polarity

Question 1

Define Electronegativity.

Answer 1

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.

Question 2

Going down a Group…

Answer 2

-electron shells increase
-atomic radius increases
-nuclear charge increases
-effect: increases shielding of nuclear charge…
=nuclear attraction decreases.

Question 3

Going across a Period.

Answer 3

-electron shells remain the same
-atomic radius decreases
-nuclear charges increases
-effect: more charge in a smaller area…
=nuclear attraction increases

Question 4

The Pauling scale…

Answer 4

• The Pauling scale is used to compare the electronegativity of the atoms of different elements.
• The non-metals have the most electronegative atoms.
• The Group 1 metals have the least electronegative atoms.

Question 5

Electronegativity sharing:

Answer 5

-same atom = identical electronegativity
^meaning bonded pair of electrons is shared equally.
-similar electronegativities (within 0.4)
^meaning bonded pair of electrons is shared equally.
-different electronegativities = shared more towards δ-
^meaning distributed unequally.

Question 6

Ionic or covalent:

Answer 6

• If electronegativity difference is large, one bonded atom will have a much greater attraction for the shared pair than the other bonded atom.
• The more electronegative atom will have gained control of the electrons and the bond will be ionic rather than covalent.

Question 7

Bond type VS Electronegativity difference

Answer 7

Covalent = 0
Polar Covalent = 0 - 1.8
Ionic = greater than 1.8

Question 8

Non-polar bonds…

Answer 8

-bonded atoms are the same.
-bonded atoms have the same/similar electronegativity.
=The electron pair is shared equally.
=The bond is a pure covalent bond.

Question 9

Polar bonds…

Answer 9

-bonded atoms are different.
-bonded atoms have different electronegativities.
=The electron pair is shared unequally, closer to the atom with the larger electronegativity (δ-).

Question 10

Polarisation and Dipoles:

Answer 10

-When polarised, there are partial charges on the atoms.
-The atom with the larger electronegativity value has the δ- charge.
-The atom with the smaller electronegativity value has the δ+ charge.
=This separation of opposite charges is called a dipole.
=A dipole in a polar covalent bond does not change and is called a permanent dipole. (contrasted to an induced dipole.)

Question 11

Polar molecules:

Answer 11

Depending on the shape of the molecule, the dipoles may reinforce one another to produce a larger dipole over the whole molecule, or cancel out if the dipoles act in opposite directions.

Question 12

Polar molecule: H2O

Answer 12

• H2O is polar.
• The two O-H bonds each have a permanent dipole.
• The two dipoles act in different directions but do not oppose each other.
• O = δ- , H = δ+

Question 13

Non-polar molecule: CO2

Answer 13

• CO2 is non-polar.
• The two C=O bonds each have a permanent dipole.
• The two dipoles act in opposite directions and exactly oppose one another.
• Over the whole molecule, the dipoles cancel and the overall dipole is zero.

Question 14

Polar solvents and solubility: NaCl

Answer 14

• NaCl(s) + aq = Na^+(aq) + Cl^-(aq)
• Water molecules attract Na^+ and Cl^- ions.
• Ionic lattice breaks down as it dissolves.
• Water molecules surround the ions.
• Na^+ ions are attracted to the oxygen of the water molecules.
• Cl^- ions are attracted to the hydrogen of the water molecules.