BONDINGRecap bonding in solids from year 12Compare types of intermolecular forces

SORT THESE INTO TYPES OF SOLIDS Magnesium Magnesium chloride SOCl2 Brass Graphite I2 Wax Diamond Sugar

calcium Zinc iodide Magnesium sulfate steel Glass (SiO2) CO2

PCl5 HgCl2 Al3Mg2

COVALENT BONDS Form between non – metals Formed by strong electrostatic attraction between two nucleii and the

shared electrons between nucleii of bonding atom. Forms either giant covalent solids or discrete molecules that have weak

intermolecular forces. Covalent bonds can be polar (different electronegativities) or non- polar. Strength of these bonds decreases down as size of the atom increases.

Strength is seen by bond dissociation energies. Triple bonds are stronger than single bonds.

(More details on these later – these are the focus of this topic.)

METALLIC BONDS Forms between metal atoms Strong electrostatic attraction between positive metal ions and the sea of

neighbouring valence electrons Forms a structure with a lattice of metal cations in a regular array

surrounded by delocalised valence electrons Strength of bond depends on number of valence electrons (more =

stronger bond) and distance of valence electrons from nucleus (more shells = weaker bond)

Valence electrons cause the properties: malleable, ductile, electrical conductivity and heat conductivity.

IONIC BONDING Between metal and non-metal ions Strong electrostatic force between a cation and

an anion. NB: Polyatomic ions occur when two or more atoms bond covalently then lose or gain extra electrons to become more stable. Eg: SO4

2-

Form giant 3D lattice structures with each ion attracted to the oppositely charged ions surrounding it.

Formulae give ratio of ions found in structure. Strength of bond depends on size of charge (eg:

AlPO4 is stronger bond than NaCl) and size of ion (LiF has higher mp than KI as bigger ions = weaker bonds)

WORK TIMEPg 88 – 89 Questions 1-4

A BIT EXTRA IF YOU WANT MORE….

FORCES BETWEEN MOLECULES Physical properties of molecular compounds such as melting points, are

due to the strength of intermolecular forces These intermolecular forces are electrostatic – an attraction between a

positive and negative end of a molecule caused by an imbalance of electrons creating a net dipole.

Three types of intermolecular forces1. Temporary dipole-dipole2. Permanent dipole-dipole3. Hydrogen bonding

Temporary dipole-Dipole attractions

Permanent dipole-dipole attractions

Hydrogen bonding attractions

TEMPORARY DIPOLE-DIPOLE ATTRACTIONS

These occur in all molecules – polar and non-polar. Occur as electrons are in constant motion so there is a probability that at

any one instant there will be a greater density of electrons on one side of the molecule than the other, creating a temporary dipole.

If this molecule approaches another molecule, it can cause an induced dipole in another molecule and causes an attraction.

Strength of attraction increases with number of electrons (leading to a greater ability to become polarised) and shape – (spherical molecules have reduced contact area).

KEY POINT – ALL MOLECULES HAVE THESE FORCES>>>>>>>

PERMANENT DIPOLE-DIPOLE ATTRACTIONS Polar molecules have an uneven distribution of charge which forms a

permanent dipole. Electrostatic attraction occurs between negative and positive ends of the molecule.

Polar substances have two forces of attraction – temporary dipoles forces and permanent dipole forces. Polar molecules of higher mass will have stronger attractive forces than polar molecules of lower mass.

HYDROGEN BONDS Occur when the dipole-dipole attractions are very strong – compounds with

H-F, H-O, H-N. F,O,N are all highly electronegative so form highly polar molecules with a

very positive H atom which is attracted to a lone pair electron on N, O or F on a neighbouring atom.

WATER – A SPECIAL CASE Water is less dense in the solid state so ice floats in liquid water. This occurs because water forms a tetrahedral shape with four hydrogen

bonds for each molecule forming an open shape that is less dense than liquid water.

OVERVIEW – STRENGTH OF INTERMOLECULAR FORCES DEPENDS ON Number of electrons (compare molar masses) Shape of molecule (how close can it get to other molecules) Polarity of molecule (electronegativity differences) Availability for hydrogen bonding (H with N,O or F)

WORKTIME Pg 94 8B

Pg 97 8C

Exam Q