3.4 Electronegativity

Question 1

What is electronegativity

Answer 1

. The power of an atom to attract the electron density in a covalent bond towards itself

Question 2

What element has the highest electronegativity

Answer 2

Flourine is the best at attracting electrons, it is better than hydrogen at it so is more electronegative

Question 3

What does electron density mean

Answer 3

. When chemists consider the electrons as charge clouds
Describes the way the negative charge is distributed in a molecule

Question 4

What is the Pauling scale

Why don’t noble gases have an electronegative number

Answer 4

It is a measure of electronegativity.
It runs from zero to four

The greater the number, the more electronegative the atom is.

Noble gases don’t have a number because they do not form covalent bonds

Question 5

What 2 things does electronegativity depend on

Answer 5

. Nuclear charge

. Size of atom:

• Distance between the nucleus and outer shell electrons
• Shielding of nuclear charge by electrons in inner shells

Question 6

How does the size of an atom affect the electronegativity of it

Answer 6

. the smaller an atom, the closer the nucleus is to the shared outer main level electrons so they are more attracted to each other and it is easier to attract electron density

Question 7

How does nuclear charge affect electronegativity

Answer 7

. The larger the nuclear charge (for a given shielding effect), the greater the electronegativity, because it can attract the outer shell more and so attract electron density.

Question 8

How does electronegativity change going down a group

Answer 8

It decreases because the atoms get bigger so there is more shielding from electrons, and distance between nucleus and outer shell increases.
As a result it is less able to attract electron density.

Question 9

How does electronegativity change across a period

Answer 9

. It increases because nuclear charge increases as atomic radii decreases and the number of energy levels remain the same.
As a result it is more able to attract electron density in a covalent bond

Question 10

So where are the most and least electronegative elements found in the periodic table

Answer 10

. Most electronegative: top right eg fluorine, oxygen etc.
But ignore the noble gases which form few compounds

. Least electronegative elements are bottom left

Question 11

What is the polarity of covalent bonds

Answer 11

. It is the unequal sharing of electrons between atoms that are covalently bonded.
It is a property of the bond

Question 12

What type of covalent bonds are between two of the same element

Answer 12

. They are the same, eg in fluorine F2, the electrons in the bond must be shared equally between the atoms.
This is because both atoms have the same electronegativity and the bond is non polar.

Question 13

What type of covalent bond is between two different atoms and why

Eg hydrogen fluoride

Answer 13

.Polar

In a covalent bond between two atoms of different electronegativity, the electrons in the bond will be shared unequally between the atoms.
So there will be an unsymmetrical distribution of them.
Eg HF hydrogen fluoride

. Hydrogen has an electronegativity of 2.1 and fluorine has 4.0.
This means the electrons in the covalent bond will be more attracted to the fluorine than the hydrogen.
So as a result the electron cloud is distorted towards the fluorine.

The fluorine end of the atom is relatively negative, and that of hydrogen is relatively positive/electron deficient.

Question 14

What is the relationship between polarity and electronegativity of atoms

Answer 14

The greater the difference in electronegativity between two atoms, the more polar the covalent bond is

Question 15

What does f+ and f- mean

Answer 15

Delta positive and delta negative

They represent the partial charges in a formula, so a small charge of less than 1 electron.

Because it is only a small change as the electron pair has only shifted towards the more electronegative atom.

They represent the relative polarity of each atom.

Eg in H-F there will be a f+ above the H and an f- above the F

Question 16

What are the different intervals in electronegativity for each bond formed on the Pauling scale

Answer 16

If the difference in electronegativity is less than 0.4 for two atoms, they have a non polar bond. So pure and the electrons are shared equally.

If the difference in electronegativity is between 0.4 and 2.0, it is a polar covalent bond so the electrons aren’t distibuted evenly

If the difference in electronegativity between two atoms is from 2 to 4, it is an ionic bond so the electrons are so attracted to the other atom that they go completely.

Question 17

When can a molecule with a large difference in electronegativity become non polar

Answer 17

. Polar covalent bonds have an unequal distribution of electron density with the more electronegative atom having a greater electronegativity.

. However non polar molecules are formed when the bond dipoles cancel each other out, so when the molecule is symmetrical around the central atom.

Question 18

What is a dipole

Answer 18

When the electron density goes to one atom with a greater electronegativity than the other.

This separation of charge is called a dipole

Question 19

What does

——I———-> mean

Answer 19

Same as delta (f+ and f-)
~The arrow points towards the more electronegative element

Question 20

Describe the bonding in the molecule carbon dioxide

Answer 20

O==C==O
It has two carbon=oxygen double bonds, and both of these bonds are polar.
However these bonds point in opposite directions in a straight line, so their dipoles cancel each other out.

As a result, there is no overall polarity in carbon

Question 21

Describe bonding in molecule tetrachloromethane CCl4

Answer 21

This molecule has 4 chlorine and one carbon, so 4 carbon to chlorine bonds which are polar

However, the molecule is symmetrical in all directions so the dipoles cancel each other out.

Overall, the molecule is non polar

Question 22

Describe bonding in trichloromethane
CHCl3

Answer 22

This molecule has 3 chlorine and 1 carbon so overall 3 carbon-chlorine bonds.

The carbon to hydrogen bond is basically non polar as carbon and hydrogen have very similar Electronegativities.

In this case, the dipoles on the carbon to chlorine can’t cancel out as it is not symmetrical
So it is a polar molecule.

The side with hydrogen is positive and the side with chlorines is negative

Question 23

Describe the bonding in a water molecule

Answer 23

It consists of two hydrogen atoms bonded to one oxygen atom.
It is polar.

This is because a water molecule has a non linear shape, so although the bonds point in opposite directions, they are not acting in a straight line.
As a result the bond polarities can’t cancel.

So bond angles are 104.5 degrees