A-level Chemistry: 3.1.3 Bonding Part 2

Trigonal Pyramidal

Bent

Trigonal Bipyramidal

Seesaw

T-shaped

Octahedral

Square planar

1. Find central atom (one that all other atoms are bonded to)

2. Work out no. of electrons in outer shell of central atom (use periodic table)

3. Add one to this number for every atom that central atom is bonded to

4. Divide by 2 to find no. of electron pairs on central atom

5. Compare no. of electron pairs to no. of bonds to find no. of lone pairs and no. of bonding pairs on central atom

• Central atom: Sulfur (S)

• Valence electrons on S: 6

• Bonded atoms: 2 Hydrogen atoms

• Lone pairs on S: 2

• Electron domains: 4 (2 bonding + 2 lone pairs)

• Electron geometry: Tetrahedral

• Molecular shape: Bent

• Approximate bond angle: ~104.5°

• Final shape: Bent

The power of an atom to attract a pair of electrons in a covalent bond

Fluorine

In a covalent bond between 2 atoms of different electronegativities, bonding electrons will be pulled towards the more electronegative atom = makes bond polar

non-polar

non-polar

permanent dipole

Difference in charge between 2 atoms caused by shift in electron density in bond

more polar the bond

When a molecule contains polar bonds that give an uneven distribution of charge across the whole molecule

When polar bonds are arranged symmetrically in molecule = charges cancel out & there's no permanent dipole

• Permanent dipole-dipole forces

• Van der Waals forces or induced dipole-dipole forces

• Hydrogen bonding

Polar molecules

In a substance made from molecules with permanent dipoles = they'll be weak electrostatic forces of attraction between δ+ and δ- charges on neighbouring molecules

1. ∵ polar liquids contain molecules with permanent dipoles

2. (Doesn't matter if rod is postively or negatively charged)

3. Polar molecules in liquid can turn around so the opposite charged end is attracted towards the rod

Found between all atoms and molecules

1. Electrons in charge clouds = always moving quickly

   1. At any moment, electrons in atom are likely to be more to one side than the other

   2. At this moment = atom has temporary dipole

2. Dipole causes another temporary dipole in opposite direction on neighbouring atom

   1. 2 dipoles are attached to each other

3. 2nd dipole causes another dipole in 3rd atom

4. Dipoles are being created and destroyed constantly ∵ electrons are constantly moving

   1. Overall effect = atoms are attracted to each other

lattice

1. Iodine atoms are held together in pairs by strong covalent bonds to from I2 molecules

2. Molecules held together in molecular lattice arrangement by weak van der Waals attractions (this causes iodine to be solid at room temp.)

1. Size of molecules

2. Shape of molecules

3. Number of electrons

Larger molecules = larger electron clouds = stronger van der Waals forces

Long, straight molecules lie closer than branched ones = closer together 2 molecules get = stronger the forces between them are

When hydrogen is covalently bonded to fluorine, nitrogen or oxygen

Strongest

1. F, N & O = very electronegative ∴ they draw bonding electrons away from hydrogen atom

2. Bond is polarised + hydrogen has high charge density = hydrogen atoms form weak bonds with lone pair of electrons on F, N or O atoms or other molecules

-OH or -NH groups

Substances with h-bond have higher boiling/melting points than similar molecules

∵ of extra energy needed to break h-bonds

Anomalously high boiling points of H2O, NH3 & HF are
caused by hydrogen bonding between molecules

1. As liquid water cools to form ice, molecules make more h-bonds & arranged themselves into regular lattice structure

2. In this structure, H2O molecules are further apart on average than molecules in liquid water

1. To melt/boil simple covalent compound = just need to overcome the van der Waals forces between molecules

2. These forces are weak

3. To melt/boil macromolecules = many covalent bonds have to be broken

4. Covalent bonds = strong

• Water = polar solvent

• ∴ substances that are polar or charged will dissolve in it

• Whereas non-polar or uncharged substances won't

• Around carbon = 4 bonding pairs of electrons

• ∴ these repel equally & spread as far apart as possible

Means a giant molecule with covalent bonding

1. Solvent has low boiling point or weak intermolecular forces

2. Solvent needs energy, taken from the skin, to overcome intermolecular forces and evaporate

3. Perfume molecule slowly spreads through the room

4. By random diffusion of the perfume

Dative Covalent Bonding

Ionic Bonding

• PH3 has dipole-dipole

• between molecules

• stronger than in CH4

• Difference in electronegativity / F more electronegative than H

• Bonding pair of electrons attracted (drawn) towards F (nucleus) in the covalent bond