Chemistry chap 6 Shapes of molecules and intermolecular forces

Module 2

Describe the electron- pair repulsion theory. (3)

• The shape of a molecule is determined by repulsions between all of the electrons appears in the valency shell.

• Electrons pairs in the valency shells around the nucleus repel each other due to their negative charges and they pushes themselves as far as possible to minimise the repulsive forces to be more stable.

• These repulsive forces are equal so the bonds will also be equally spaced

Name each regular shapes according to their number of bonds and suggests eacg of their bond angles.

2 bonds- linear- 180 degrees

3 bonds- trigonal planar- 120 degrees

4 bonds- tetrahedral- 109.5 degrees

5 bonds- trigonal bipyramidal- 90 and 120 degrees

6 bonds- octahedral- 90 degrees

How can you show 3 dimensional shapes? (2)

By drawing bonds that goes into the paper by dotted wedges and bonds that goes out of the paper by solid wedges.

What does irregular shapes means?

ireegular shapes often occurs when lone pairs occurs in a molecule

How does a lone pair affect the shape of a molecule?

Lone pairs have more and extra repulsion which make bond angles to be smaller by 2.5 degrees (for each lone pairs)

Why do lone pairs repel more strongly than regular bonding pairs( in covalent bonds)?

This is because lone pairs are closer to the central atom and require more space as they are not shared between two atoms which allow their electron cloud to spread out more freely.

Whereas bonding pairs are more localized and confined between atoms

In what case do lone pairs not affect the shape?

When they are opposite each other - (eg. square planar- 2 lone pairs and 4 bonding pairs with a total of 6 reigions)

What is the difference between trigonal planar and trigonal pyramidal?

Trigonal planar is the flat regular shape with 3 atoms whereas trigonal pyramidal is the 3D irregular shape with 3 atoms and a lone pair.

The lone pair in this shape pushes the rest of the bonding electrons to the base.

What are the three main types of irregular shapes?

1. Trigonal pyramidal

2. Angular or non linear

3. Square planar

Define electronegativity.

The ability or tendency for an atom to attract the bonding/ sharing electrons in a covalent bond

Define pauling electronegativity values

A measure of the atoms’ ability to pull shared electrons in a covalent bond towards itself.

Define bond polarity.

The uneven distribution of electrical charges across a chemical bond. Which occurs when two atoms with different electronegativities form a covalent bond.

Define polar covalent bond

When there is an uneven sharing of electrons between two atoms, creating a charge seperation

Define non polar bond

When two electrons are shared equally between two atoms (electronegativity are similar)

Define dipole

When two atoms in a molecule have different electronegativity with one end partially negative and the other partially positive. Or the seperation of charge.

Define polar molecules

Molecules with an un even distribution of electrical charge

Describe the differences between a polar covalent bonds, dipole and polar molecules?

In a molecule, if the two or more atoms have different electronegativities, then this happens to be dipole. Then the covalent bond within the molecule (eg. HCl) will be described as polar covalent bond.

Whereas if the electronegativity in between atoms are the same, then it would be a pure covalent bond.(eg. Cl- Cl)

Polar molecule is the whole molecule with an overall uneven charge distribution- when the shape and polar bonds are asymmetrical- net dipole

What are the three factors that affect electronegativity?

• Atomic / nuclear charge- number of protons

• Distance from nucleus/ atomic radius

• Inner shells/ electron shielding

Why does the inner shells affect electronegativity?

because the inner shells blocks the attraction between nucleus and the outer electrons.

How does electronegativity acts in the periodic table? (2)

• Increases across the period- increasing number of protons- more attraction, atomic radius decreases as the inner electron shells are closer to each other

• Descrease down a group- larger atomic radius as more layers of shells are added so more shielding layers of inner shells blocks the attraction

Why are identical atoms non polar?

Because they have the same or similar electronegativity that have no differences

How do you define which element in the polar molceule is slightly more negative?

Depends which one is more electronegative (in periodic table)- can pull the shared electrons more towards to itself.

Define intermolecular forces

Weak forces between dipoles of different molecules

What are the three types of intermolecular forces?

1. Induced dipole- dipole interactions

2. Permanent dipole- dipole interactions

3. Hydrogen bonding

Define induced dipole- dipole interactions.

Another word of London forces. These are weak intermolecular forces that exist between any molecules- (polar or non polar)

Describe how induced dipole- dipole interactions work. (3)

1. At any instant, an instantaneous dipole will exist when electrons move around randomly. There is a chance where most electrons are concentrated in one side, making it slightly negative.

2. The instantaneous dipole will induce a neighbouring molecule because the slightly negative side will cause the electrons in the neighbouring molecule to shift to the further place due to repulsion.

3. The induced dipole in the atom 2 then induce further dipoles on its neighbouring molecule which cause molecules attract each other.

Define permanent dipole- dipole interactions.

The weak intermolecular forces that occurs between a partially negative charge area of one molecule is attracted to partially positvely charged area of another another molecule. This attraction is always here.

How can polar molecules be proved? (eg.H2O)

Testing by charged rods with running tap with the solution filled up in the burette

Why can non polar molecules dissolve in non polar solvents?

Due to “like dissolve like” principle. They have similar attractions and weak intermolecular forces so this makes them easy to mix, spread and interact

Why can’t non polar molecules dissolve in polar substances?

Because polar molecules have stronger intermolecular forces

They are more likely to stick together than bond with non polar molecules

As non polar molecules usually can’t offer strong enough attractions to break these strong intermolecular attractions.

Why can polar dissolve in polar?

Because polar molecules have opposite charges that can attract and seperate the bondings and lattice.

Allow to mix and interact

Define hydrogen bonding

A strong intermolecular force between a partially positive hydrogen atom (bonded to N /O /F or highly electronegative atom) and a partially negative N /O /F atom on another molecule.

What are the two main conditions for hydrogen bonding to occur?

• The hydrigen atom is needed to be bonded to a strongly electronegative element (nitrogen/ oxygen/ flourine) - as they have a strong pull to the shared electrons, forming partially positive and negative sides leaving the hydrogen highly attractive to the lone pairs in the neighbouring electronegative atoms.

• The electronegative atom must have at least one lone pair of electrons

Describe the evidence for hydrogen bonds. (3)

• Usually, smaller molecules have lower boiling points and larger molecules have higher boiling points (size increase down the group as there are more shells and electrons)

• But molecules like NH3, H2O and HF all have higher boiling points than expected according to their molecular sizes

• These higher boiling points are because of additional intermolecular forces- hydrogen bondings, rather than only weak induced dipole- dipole interactions

Describe the anomalous properties of water. (2)

• Water has relatively high boiling points as it has both london forces and hydrogen bonding so extra energy is needed to break these energy

• Ice are less dense than water- usually, solid are denser than liquid as its structure is packed more tightly together but in ice, water molecules are further apart as they have an open and hexagonal lattice structure.