Chemical Bonding

Why do atoms bond?

• Electron configuration

Three ways of bonding

• Accepting or donating electronsIONIC

• Sharing electronsCOVALENT

• Forming lattices (electrons move)METALLIC

Ionic Bonding

• Losing or gaining electrons• NaCl

Properties of Ionic Substances

Phase/State

Solid at room temp• Strong

electrostatic forces much energy required to break up arrangement of ions

• High melting point• High boiling point

HardnessStrong electrostatic forces • Hard• Repulsion causes shattering brittle

ConductivityPhase/Sta

teConductiv

e? Why?

Solid Ions tightly bound in crystal structure unable to move

towards charge

Liquid(melted)

Ions now free to move

Aqueous (dissolved in

water) Ions now free to

move

Covalent Bonding

• The sharing of electrons between atoms of non-metallic elements.

Covalent Bonding

• A single covalent bond between two Chlorine atoms.

How would we represent:Bromine gas?

Water?Ethene? (C2H4)

Compare and contrast

AlCl3and

NCl3

Covalent Molecular Substances

Bromine Liquid

Covalent Molecular Substances• Strong covalent bonds• Weaker intermolecular forces• Boiling & Melting:

• Overcomes intermolecular forces• Does not break covalent bonds

• BP = 58 °C for Liquid Bromine (relatively low)

In molecular compounds:

Stronger intermolecular forces:

higher melting and boiling points.

Properties

Solids: • Soft• Non-conductive

Liquids (melted)• Non-conductive

Aqueous (dissolved in water)• Some form ions after reacting with water

conductive (e.g. HCl, H2SO4, HNO3)

CovalentNetworkSolids

Covalent Network Solids

• AKA Covalent lattice

• Many atoms in fixed positions

• Covalent bonds• E.g. Diamond

(Carbon)

Properties

• Melting point • very high (typically well above

1000°C)

• Conductivity • no ions no conduction.

Electrons all held or shared (localised)

• Exception: Graphitedelocalised electrons

Solids and their properties

PropertyMolecular

Solids

Lattice Solids

Metallic IonicCovalent network

Melting and boiling points

Low Variable High high

Conductivity No YesSolid: no

Molten: yesno

Hardness and/or

workabilitySoft

Variable hardness; malleable

and ductile

Hard and brittle

Hard and brittle

Forces holding particles

together in the soid

Intermolecular

Delocalised electrons (metallic bonding)

Electrostatic

Covalent bonding

throughout crystal

Homework

• P254• Q13-15

Covalent Molecular & IonicProperty Ionic

Covalent Molecular

Phase/state at room temperature

Solid

Generally gases (N2, SO2, NH3) or

liquids (H2O, CCl4, CH3OH)

Melting PointHigh melting points (typically above 400°C)

and

Low melting points (generally below 200°C)

and

Boiling Pointhigh boiling points

(typically above 1000°C)

Low boiling points (generally below 400°C)

Solid properties Hard, brittle Soft

Conductivity (solid)

Conductivity (Molten or aqueous)

Molten: non-conductiveAqueous: only if they

react with water to form ions

Exercises1.Phosphorus trichloride is a liquid with a boiling point of 74°C; it does

not conduct electricity. Calcium chloride is a solid with a melting point of 772°C; when molten it conducts electricity. Explain, in terms of bonding, why these two compounds have such different properties.

2.The boiling points of the fluorides of the elements of Period 2 of the Periodic Table are: LiF (1720°C), BeF2 (1175°C), BF3 (–101°C), CF4 (–

128°C), NF3 (–120°C), OF2 (–145°C) and F2 (–188°C). Explain why the

boiling points of lithium and beryllium fluorides are so much higher than the others.

3.Calcium and arsenic both form hydrides. Calcium hydride is a solid with a melting point of 600°C, while the hydride of arsenic, called arsine, is a gas at room temperature.

a.What do these facts imply about the bonding in the two compounds?

b.What do you expect the formulae of these compounds to be?

PCl3 is covalent, CaCl2 is ionic.

They are ionic compounds whereas the others are covalent.

a. Calcium hydride is ionic whereas arsenic hydride is covalent.

b. CaH2, AsH3

Activity

• P254 q19-22• Q13-15