3.1.3.6 Bond Polarity

Question 1

Define electronegativity

Answer 1

Electronegativity is the power of an atom to draw electron density (a pair of electrons) in a covalent bond, towards itself

-it can be thought of as the ‘pulling power’ of an atom on electrons.

Question 2

How does electronegativity occur?

Answer 2

It is due to the ability of the positive nucleus to attract negatively charged electrons in the outer shells towards itself. This means that in a covalent bond, the shared pair of electrons will lie closer to the atom with higher electronegativity.

Question 3

What is the Pauling Scale?

Answer 3

A scale which is used to assign a value of electronegativity for each atom.
Eg. Fluorine= 4.0

Question 4

What is the electronegativity in a metallic bond?

Answer 4

Difference Less than 1.9

Question 5

What is the electronegativity of a covalent bond?

Answer 5

Both have Electronegativity greater than 1.9 and a difference of less than 0.5 between the two atoms

Question 6

What is the electronegativity of a polar covalent bond?

Answer 6

Electronegativity greater than 1.9, and a difference more than 0.5 but less than 2.1

Question 7

What is the electronegativity in an ionic bond?

Answer 7

Difference Greater than 2.1

Question 8

How does nuclear charge affect electronegativity?

Answer 8

-attraction exists between the positive protons in the nucleus and the negative electrons in shells
-an increased proton number leads to increased nuclear attraction between electrons in the covalent bond and the nucleus
-therefore increased nuclear charge = increased electronegativity

Question 9

How does atomic radius affect electronegativity?

Answer 9

-atomic radius is the distance between the nucleus and the outermost shell electrons
-electrons closer to the nucleus are more strongly attracted
-therefore increased atomic radius = decreased electronegativity

Question 10

Explain why electronegativity decreases down a group

Answer 10

-increased nuclear charge but more shells increases shielding and atomic radius
-decreased attraction between nucleus and outer electrons bonding

Question 10

How does shielding affect electronegativity?

Answer 10

-filled energy levels can shield nuclear charge effect, causing outer electrons to experience less nuclear attraction
-therefore increased number of inner shells = decreased electronegativity

Question 11

Describe the trends in electronegativity on the periodic table

Answer 11

-electronegativity decreases down a group, but increases across a period.

Question 12

What is a homonuclear diatomic molecule?

Answer 12

-when electron density is shared equally
-they are non-polar as the electrons are evenly distributed as both atoms have the same electronegativity
-it occurs when the two bonding atoms are the same

Question 12

What is electron density?

Answer 12

Relates to the probability of finding electrons at a particular position in space

Question 12

Explain why electronegativity increases across a period

Answer 12

The nuclear charge increases with the addition of protons to the nucleus
-shielding remains constant
-nuclear force increases for the outer electrons which results in a decreased atomic radius
-increased attraction between the nucleus and outer electrons bonding.

Question 13

What is a heteronuclear diatomic molecule?

Answer 13

-when electron density may be focused towards one atom, creating a dipole
-the differing attraction for the pair of electrons allows there to be a small charge difference between the atoms
-it is said to be a polar molecule (as electronegativity means the electron density lies more towards the electronegative atom)
-uneven distribution of electrons leads to polarity
-this is known as a permanent dipole , as the slight charge difference is always present
-it occurs when the two bonding atoms are different

Question 14

What does delta mean?

Answer 14

-slightly
-so a small charge difference of less than one electrons worth

Question 15

Explain how symmetry affects the polarity of molecules

Answer 15

-the symmetry of polar bonds can cancel the effect of any permanent dipole
-dipoles are directional so will cancel out if the are in equal and opposite directions
-a non-symmetrical molecule is an overall dipole, and is polar.

Question 16

Give the electronegativity values for the following elements:

Hydrogen
Carbon
Nitrogen
Oxygen
Fluorine
Chlorine

Answer 16

H = 2.1
C = 2.5
N = 3.0
O = 3.5
F = 4.0
Cl = 3.0