BONDING AND STRUCTURES

TYPES OF BONDING

Hydrogen Bonding Hydrogen is an exceptional element in that when it forms a covalent bond its electron is held to one side of the nucleus leaving the other side relatively bare. Any approaching negatively charged group can get very close to the hydrogen nucleus and produce an unexpectedly large electrostatic attraction. These electrostatic attractions are exaggerated when H is bonded to a more electronegative element e.g. N, F or O. Such electrostatic attractions are called hydrogen bonds.

Both are explained by the increased attraction between molecules caused by hydrogen bonding making it more difficult to separate them. For example, in water. Black dots represent oxygen atoms and white dots represent hydrogen atoms.

Permanent Dipole Forces -Dipole Interactions One part of the molecule is always slightly positive or negative compared with another part When molecules that have permanent dipoles come together, they will arrange themselves so that the negative and the positive ends of the molecules attract one another

The attractions are called dipole-dipole interactions. The molecules eventually align in order to find the best compromise between attraction and repulsion.

Instantaneous Dipole Forces These are small electrostatic forces that are caused by movement of electrons within the covalent bonds of molecules that would otherwise have no permanent dipole. As one molecule approaches another the electrons of one or both are temporarily displaced owing to their mutual repulsion. This movement causes small, temporary dipoles to be set up which attract one another.

These dispersion forces increase with the size of the molecule and with its surface area. Large molecules with big surface areas have more electrons, and more dispersion forces and greater attraction and therefore higher boiling points.

Question Which type of dipole do the following molecules have (explain your reasoning) Cl2 HCl CH4 CH3Cl Poly(ethene) Poly(propene) Poly(chloroethene) (PVC)

Structures The properties of substances are decided by their bonding and structure. Structure – The way atoms are arranged relative to one another, e.g. Giant Lattice, Molecular (Simple and Macro). How does bonding and structure decide properties?

For a solid substance – there are three main factors Type of particle it contains Atoms, ions or molecules. If a substance contains ions or polar molecules, it may dissolve in water. How the particles are bonded together Ionic, covalent, metallic or weak intermolecular bonds. The stronger the bonds, the higher the mp/bp of the substance, and the greater the hardness. How the particles are arranged relative to each other One-dimensional chains, two-dimensional sheets or one of many three-dimensional arrangements.

Classification of Substances According to Structures (CI 5.6)

Structure and the Periodic Table As you go across a period of the Periodic Table, there is a trend in the structures of the element. (fig 37 p.118 CI). Metallic  covalent network  covalent molecules  monatomic Note Carbon exists in two different forms; covalent network (graphite & diamond) and covalent molecules (fullerenes).

Trends in reactions across a period Tables 12, 14, 15 p.121 CI 5.6 Reactions with oxygen In period 3 all elements except argon form oxides Na2O, MgO, Al2O3, SiO2, P4O10/P4O6, SO3 /2, Cl2O/ Cl2O7 Reactions with chlorine In period 3 all elements except argon form chlorides NaCl, MgCl2, AlCl3, SiCl4, PCl5/PCl3, S2Cl2, Cl2

Reactions with water Na – Vigorous, forms NaOH and H2 Mg – Only with steam, forms Mg(OH)2 and H2 Al – No reaction unless protective oxide layer removed from surface, forms Al(OH)3 and H2 Si – No reaction P – No reaction S – No reaction Cl – Dissolves in water (some reacts to form an acidic solution (HCl + HClO)) Ar – No reaction

Acid-base properties of oxides Acid-base character of oxides is linked to their structure. Giant ionic lattices are basic (Na2O & MgO). Covalent oxide structures are basic (P4O6, SO3 & Cl2O7). Bonding in Al2O3 has both ionic and covalent character so it can behave as both an acid and a base – it is Amphoteric.

Behaviour of chlorides in water Behaviour of chlorides in water is linked to their structure. Chlorides with giant ionic lattices simply dissolve in water with no chemical reaction (NaCl & MgCl2). Chlorides with covalent molecular structures react (hydrolysed), producing fumes of HCl and forming acidic solutions (AlCl3, SiCl4, PCl5, S2Cl2). Complete CI 5.6 q2, 4,5