Bonding: Bond Polarity

Question 1

Definition of Electronegativity

Answer 1

• Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond

Question 2

Factors Affecting Electronegativity

Answer 2

• Atomic radius

• As atomic radius increases, the bonding pair of electrons become further from the nucleus
• They are therefore less attracted to the positive charge of the nucleus, resulting in a lower electronegativity
• The smaller the atom the greater the electronegativity

• Nuclear charge

• The greater the number of protons in a nucleus, the greater the attraction to the electrons in the covalent bond, resulting in higher electronegativity
• However, full energy levels of electrons shield the electrons in the bond from the increased attraction of the greater nuclear charge, thus reducing electronegativity
• The bigger the nuclear charge the bigger the electronegativity but only if shielding is constant

Question 3

Trends in Electronegativity Across a Period

Answer 3

• Electronegativity increases across a period

• Nuclear charge increases, shielding remains constant
• Decrease in atomic radius, as more protons mean electrons are more attracted to nucleus and pulled closer - reducing atomic radius
• Therefore there’s a greater attraction for the bonding electrons in a covalent bond

Question 4

Trends in Electronegativity Down a Group

Answer 4

• Electronegativity decreases down a group

• The atomic radius increases, shielding also increases
• This outweighs the increase in nuclear charge
• Therefore less attraction for the bonding pair of electrons in a covalent bond

Question 5

Definition of a Polar Bond

Answer 5

• An unequal distribution of the pair of electrons in a covalent bond
• This occurs in covalent bonds between elements with different electronegativities

Question 6

Partial Charges

Answer 6

• In a polar bond, the two atoms have partial charges
• ẟ+ = delta positive, slightly positive
• ẟ- = delta negative, slightly negative
• The more electronegative atom will always have a ẟ-charge and the less electronegative atom will have a ẟ+ charge

Question 7

Definition of a Dipole

Answer 7

• A dipole is an uneven distribution of charge

Question 8

How does the difference in electronegativity affect the polarity of a bond?

Answer 8

• The greater the electronegativity difference between the two atoms in a bond the greater the polarisation of the bond

Question 9

Non-polar Bonds

Answer 9

• Non-polar bonds form if the electronegativity of both atoms in a covalent bond is identical
• The electrons in the bond will be equally attracted to both of them
• Bonding in diatomic gases (for example O2 or Cl2) is always non-polar because the electronegativity of the atoms in each molecule is the same

Question 10

Polar Molecules

Answer 10

• If charge is distributed unevenly across a whole molecule, then the molecule will have a permanent dipole
• Molecules that have a permanent dipole are known as polar molecules
• If the polar bonds are arranged so that they point in roughly the same direction and don’t cancel out then the molecule will have an overall permanent dipole moment

Question 11

Non-polar Molecules

Answer 11

• If polar bonds are arranged symmetrically so that the dipoles cancel each other out, then the molecule has no permanent dipole and is non-polar

Question 12

How is electronegativity measured?

Answer 12

• Electronegativity is measured using the Pauling Scale

- A higher number means an element has a higher electronegativity

Question 13

Using Electronegativity Values to Predict Bonding

Answer 13

• By looking at the difference in electronegativity between two elements, you can predict the nature of the bonding between them
• Very small differences in electronegativity are associated with predominantly covalent bonding.
• Differences close to the maximum difference of 3.3 are associated with predominantly ionic bonding