A Level Chemistry - Bonding

Metallic bonding

The attraction between delocalised outer shell electrons and positive metal ions in a lattice

Giant

Endlessly reprating

Delocalised

Not associated with a particular atom

What does the number of delocalised electrons depend on?

How many electrons are lost from each metal ion

Metallic bonding of sodium

Metallic bonding of magnesium

Ionic bonding

The electrostatic attraction between oppositely charged ions in a lattice

Diagram of a giant ionic lattice

Boron bonding

Don't for ions (but will form covalent compounds)

Carbon and silicon bonding

Don't form ions

Tin bonding

Sn4+ is most stable but can form Sn2+

Lead bonding

Pb2+ is most stable but can form Pb4+

Transition metal bonding

Form more than one stable ion

Ammonium

NH4 +

Hydroxide

OH -

Nitrate (III)

NO2 -

Nitrate (V)

NO3 -

Cyanide

CN -

Hydrogen carbonate

HCO3 -

Hydrogen sulfate

HSO4 -

Do hydrogen phosphate

H2PO4 -

Carbonate

CO3 2-

Sulfate (IV)

SO3 2-

Sulfate (VI)

SO4 2-

Hydrogen phosphate

HPO4 2-

Phosphate (V)

PO4 3-

Covalent bond

A shared pair of electrons

Lone pair

A pair of electrons which is not bonded

Non-octet molecules

Molecules where the central atom doesn't have a noble gas electron arrangement eg BF3, SF6

Coordinate bond

A covalent bond in which both electrons of the shared pair come from the same atom

How do coordinate bonds form?

The atom that donates electrons has a lone pair. The atom that accepts the electron pair doesn't have a full outer shell (it is electron deficient) and can fit a lone pair

How are coordinate bonds shown?

Using an arrow pointing from the the atom donating the electron

What is a molecules shape based on?

The total number of electron pairs around the central atom
The number of bonding pairs of electrons
The number of lone pairs of electrons

Electron Pair Repulsion Theory

Pairs of electrons repel each other so that they as far apart as possible
Lone pairs are more compact so repel more
The molecule takes up a shape which minimises repulsion
Lone pairs reduce bond angles by 2.5° each

Bond on the plane of the paper

Straight line

Bond coming out of the paper

A wedge

Bond going into the paper

Dotted line

2 bond pairs 0 lone pairs

Linear - 180°

3 bond pairs 0 lone pairs

Trigonal planar - 120°

4 bond pairs 0 lone pairs

Tetrahedral - 109.5°

5 bond pairs 0 lone pairs

Trigonal Bipyrimidal - 90° and 120°

6 bond pairs 0 lone pairs

Octahedral - 90° (and 180°)

2 double bonds 0 lone pairs

Linear - 180°

3 double bonds 0 lone pairs

Trigonal planar - 120°

2 bond pairs 1 lone pair

V-shaped/bent - 117.5°

3 bond pairs 1 lone pair

Trigonal pyramidal - 107°

2 bond pairs 2 lone pairs

V-shaped/bent - 104.5°

3 bond pairs 2 lone pairs

Trigonal planar - 120°

4 bond pairs 2 lone pairs

Square planar - 90°

How to explain molecule shape

Number of lone pairs and bond pairs and where. Electron pairs repel. Lone pairs repel more. State the bond angle and why

Define electronegativity

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

Trend of electronegativity across a period

Increases as nuclear charge (proton number) increases

Trend of electronegativity down a group

Decreases as the number of shells increases so atomic radius and shielding increases

What is the most electronegative element?

Fluorine

What makes a bond polar?

When there is an uneven distribution of electrons

What makes a non-polar covalent bond?

No or very small difference in electronegativity. The electron pair is shared equally with no dipole in the bond

Non polar covalent bond examples

H2, Cl2, O2, NaCl, CH

What makes a polar covalent bond?

A difference in electronegativity. The electron pair is closer to the more electronegative atom, making it slightly negative

Polar covalent bond examples

HCl
CCl
BeCl

What makes an ionic bond?

A large difference in electronegativity. Electrons are pulled so far towards the electronegative atom that the less electronegative atom loses the electron

Permanent dipole

A small charge difference across a bond resulting from a difference in electronegativities of the bonded atoms

What can happen if there are multiple dipoles in a molecule?

If they make the molecule electronically symmetrical eg CO2, the dipoles cancel out

Define intermolecular forces

The forces of attraction between molecules

What are van der Waals' forces?

These are the weakest intermolecular forces which occur between all substances between molecules of atoms

Instantaneous dipole

A constantly forming a disappearing dipole caused by the movement of electrons which unbalanced the charge distribution within the molecule

Why do non polar substances have low boiling points?

They only have van der Waals forces between molecules which are very weak

What determines the strength of van der Waals forces?

Bigger molecules have more electrons so the induced dipoles are large resulting in stronger forces

What causes van der Waals forces?

The random movement of electrons causes an instantaneous dipole across the molecule which indices a dipole in neighbouring molecules resulting in weak forces of attraction

Which substances only have van der Waals forces?

Simple molecules and simple atomic substances

What are permanent dipole-dipole forces?

These occur between molecules which have a permanent dipole in addition to van der Waals forces. The negative pole of one molecule attracts another's positive pole. These are generally stronger than van der Waals forces

What are hydrogen bonds?

These are the strongest intermolecular forces which occur between molecules which contain a hydrogen atom bonded to either F, O or N. The bond forms between the slightly positive H atom in one molecule and a lone pair in the other

Properties of hydrogen bonds

Higher boiling point than expected due to the strength of the hydrogen bonds
Substances which can hydrogen bond tend to dissolve in water because they form hydrogen bond with water

What must be broken to melt or boil an ionic, metallic or macromolecular structure?

The bonds

What must be broken to melt or boil a simple molecular or atomic substance?

The intermolecular forces

What is needed to conduct electricity?

Charged particles which are free to move such as delocalised electrons or ions

What is the solubility rule?

Substances can dissolve if dilute and solvent molecules attract one another - like dissolves like. Ionic and polar substances dissolve in polar solvent sand non polar substances dissolve in non polar solvents

Structure of ionic compounds

A giant ionic lattice structure where negative and positive ions alternate so each ion is surrounded completely by oppositely charged ion held together by ionic bonds

Why do ionic compounds have high melting points?

The ions are held together by many strong ionic bonds that need a lot of energy to break

Why are ionic compounds brittle?

If enough force is applied, the layers can slide over each other. Like charges will move next to each other, causing repulsion which breaks down the lattice structure

Why do ionic compounds only conduct electricity when molten or aqueous?

The ions are only free to move in theses states so can carry current

Why do metals have high melting and boiling points?

Metallic bonds are generally strong so large amounts of energy is needed to break them

Why can metals conduct electricity?

The delocalised electrons can move through the structure and carry the current

Why are metals strong?

Metallic bonds are strong and extend through the giant metallic lattice structure

What does metallic bond strength depend on?

The size and charge of the metal ion (it's charge density). Smaller and more highly charged ions form stronger bonds

Malleable

Can be hammered or pressed into shape without breaking or cracking

Ductile

Can be drawn into thin wires

Why are metals ductile and malleable?

The layers of ions in the giant metallic lattice can slide over each other into new positions without disrupting the metallic bond

Types of covalent substances

Simple molecular and macromolecular

Why do simple covalent molecules have low melting points?

The forces of attraction between molecules are weak. Not much energy is needed to break the intermolecular forces

Why can't simple covalent molecules conduct electricity?

They don't contain ions or delocalised electrons so can't carry a charge

Solubility rule of simple covalent molecules

Generally insoluble in water unless they can form hydrogen bonds with water or react with it

Structure of diamond

A giant arrangement of carbon atoms bonded to four other carbon atoms

Structure of graphite

A giant arrangement of carbon atoms bonded to three other carbon atoms in layers

Structure of diamond

A giant arrangement of carbon atoms bonded to four other carbon atoms

Why do diamond, graphite and graphene have high melting points?

Many covalent bonds between all the atoms which take lots of energy to break

Structure of graphene

A single sheet of graphite

Why do graphite and graphene conduct electricity?

One electron per carbon isn't involved in bonding and is delocalised along the layer allowing it to carry a current

Why are diamond, graphite and graphene insoluble in water?

The covalent bonds are very strong and the lattice does not break up when any solvent is added