Bonding in compounds Overview Learn how the elements can form bonds in compounds.

Van der Waals forces Learning intention An introduction to the variety of intermolecular forces which exist between molecules.

Relating physical properties to intermolecular forces Learning intention Learn how to explain differences in physical properties such as viscosity, melting point and boiling point in terms of differences in strength of intermolecular forces.

Metallic Covalent Polar covalent Ionic Hydrogen bonding Permanent Dipole- Permanent Dipole interactions London’s forces The Chemical Bond

Ionic Compounds

Ions - metals lose electrons and form positive ions - non-metals gain electrons to form negative ions - electrons are transferred from metals to non-metals Ionic Compounds

Na atom + Cl atom Na + ion + Cl - ion (2.8.1) (2.8.7) (2.8) (2.8.8) Cl Na Cl transfer + - +

Ionic Compounds The positive and negative ions are attracted (electrostatic bond ) to each other. Na + This ionic network compound has many ionic bonds so ionic compounds have high m.p.s Cl - A giant lattice structure is formed. Each Na + ion is surrounded by 6 Cl - ions. While each Cl - is surrounded by 6 Na + ions. Ionic bonding is the electrostatic force of attraction between positively and negatively charged ions. Ionic bond (electrostatic attraction)

Ionic Compounds NaCl

Ionic Compounds A giant lattice structure is formed when each Na + ion is surrounded by 6 Cl - ions and each Cl - ion is surrounded by 6 Na + ions. Sodium Chloride The formula of sodium chloride is NaCl, showing that the ratio of Na + to Cl - ions is 1 to 1. The m.p. of NaCl is C The size of the ions will effect the strength of the ionic bond and how the ions pack together. e.g. NaF m.p o C, NaI 660 o C

Molecular Ions, e.g. SO 4 Cu Cu e Copper sulphate contains the Cu 2+ and the SO 4 2- ions. There is, therefore, covalent bonding and ionic bonding in copper sulphate A single covalent bond. A solution of copper sulphate can conduct electricity. Molten ionic compounds can also conduct electricity. 2 additional electrons Oxygen Sulphur e.g. Copper can donate the extra 2 electrons needed. S OO O O

Bond Strengths Bond TypeStrength (kJ mol –1 ) Metallic80 to 600 Ionic100 to 500 Covalent100 to 500 Hydrogen40 Dipole-Dipole30 London’s forces1 to 20

Covalent Molecular Compounds

Covalent Bonding Sharing electrons takes place between non-metal and non-metal shared electrons count as part of the outer shell of both Atoms shared electrons attract the nuclei of both atoms this attraction is called the covalent bond

Cl Cl (linear) HClH Hydrogen chloride H

N Ammonia H H H N (pyrimidal) NH 3 H H H

O Water H H O (bent) H 2 O H H

Draw electron dot cross diagrams for the following molecules and structural formula 1.SCl 2 2.CO 2 3.CH 4 XX XX S Cl-S-Cl O=C=O H H C H H

Bond Strengths Bond TypeStrength (kJ mol –1 ) Metallic80 to 600 Ionic100 to 500 Covalent100 to 500 Hydrogen40 Dipole-Dipole30 London’s Forces1 to 20

Discrete molecules are formed when two or more atoms share electrons. The atoms are non-metal elements. An example is methane. C H H H H Methane: CH 4 Methane has strong intra-molecular and weak inter-molecular. It’s b.p. is -183 o C HCH H H Covalent Molecular Compounds

Non- metals elements can form double and triple covalent bonds. C H H CH ethane C 2 H 6 H H H HC CCH H HH H H ethene C 2 H 4 C H H C H H Double covalent bond CC H HH H Covalent Molecular Compounds Covalent molecular compounds have low m.p.’s because the weak forces holding the molecules together require only small amounts of thermal energy to break them.

Properties Low m.p.’s and b.p.’s., this increases with size of the molecule and the increasing number of atoms in the molecule. Temp / o C m.p.’s increase because the strength of the London’s forces forces increase with the increasing size of the molecule. So more Energy is needed to separate molecules m.p.’s of the carbon halides CF 4 CCl 4 CBr 4 CI 4 Covalent Molecular Compounds

Covalent Network Compounds

Silicon Carbide SiC Si CC C C The 4 carbon atoms are available to bond with another 4 silicon atoms. This results in a COVALENT NETWORK COMPOUND Covalent Bond Tetrahedral shape Silicon, like carbon, can form giant covalent networks. Silicon carbide exist in a similar structure to diamond.

Silicon carbide (carborundum) has a chemical formula is SiC. As this compound is linked by strong covalent bonding, it has a high m.p. (2700 o C). It is a hard substance as it is very difficult to break the covalent lattice. Silicon Carbide SiC Each Si is bonded to 4 C’s and each C is bonded to 4 Si’s. SiC is used as an abrasive for smoothing very hard materials. Hence the chemical formula, SiC Video

Silicon Dioxide SiO 2 Silicon and oxygen make up nearly 75% of the Earth’s crust. They are therefore the most common elements in the Earth’s crust. They combine together to make a covalent network compound called silicon dioxide. This is usually found in the form of sand or quartz. Each Si atom is bonded to 4 O atoms, and each O atom is bonded to 2 Si atoms. Hence the chemical formula, SiO 2. Silicon dioxide (silica) also has a high m.p. (1610 o C) and like SiC, it is very hard and used as an abrasive. It is relatively un-reactive. New Higher Chemistry E Allan J Harris Video