Bonding In Elements And Compounds

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Bonding in Elements and Compounds
Structure of solids, liquids and gases

Ionic

Types of bonding between atoms and molecules

Covalent

Many compounds between metals & nonmetals (salts), e.g.

Metallic
Metals and alloys, e.g. sodium, magnesium, iron, steel, copper, zinc, tin, brass, bronze

NaCl, MgCl2, MgSO4

Simple molecular Giant molecular Covalent bonds hold all the atoms or molecules together in a giant molecule, e.g. diamond (C), quartz (SiO2), silicon (Si), silicon carbide
(SiC)

Covalent bonds between atoms within each molecule, weak intermolecular forces between molecules, e.g sulphur (S8),
H2O, halogens (F2, Cl2, Br2, I2)

Intermolecular Forces (weak) instantaneous dipole – induced dipole attractions

van der Waal’s monoatomic Nobel gases (He,
Ne, Ar, Xe, Kr)

simple molecular covalent
Elements, e.g. halogens, sulphur (S8)

permanent dipole – hydrogen-bonds permanent dipole attractions
H-{O, N, F} e.g.
H2O, NH3, HF,
CH3COOH,
simple molecular covalent
CH3CH2OH
Compounds, e.g. HCl(g),
H2S

Ionic Bonding
•An ionic bond is the force of electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).
•Unlike charges attract, like charges repel.
•Many compounds of metals and non-metals are ionic
(why?) and all Group I and Group II metal / non-metal compounds are ionic, e.g. NaCl, MgCl2, CaSO4, KNO3.
Above: the giant ionic lattice of NaCl – which spheres represent the sodium ions (large or small)? Note: the sticks in this diagram DO NOT represent the ionic bonds since ionic bonds act in all directions!

Physical Properties of Ionic Compounds:
•Hardness: ionic bonds are strong and rigid and so ionic compounds tend to be hard, high melting point solids.
•Brittleness: unlike metallic bonds, ionic bonds are rigid and so break suddenly when enough force is applied – ionic solids tend to be brittle.
•Electrical conductivity: to conduct electricity we need mobile charge carriers. In metals these are the delocalised electrons. In ionic solids the ions are held in a rigid ionic lattice and are not free to move, so ionic solids are poor conductors of electricity. However, when molten
(fused) ionic solids do conduct electricity.
•Solubility: so long as the ionic bonds are not too strong, ionic solids dissolve in water (and other polar solvents) since polar water molecules surround the ions and partially screen the ionic attraction, ripping them from the lattice:
Attraction between ions and the charges on water molecules causes them to become surrounded by shells of water molecules, called solvation shells.
NaCl(s) → Na+(aq) + Cl–(aq)

•Conductivity in solution: although the solvation shells partially screen the electric charge of the ions, enough charge remains to enable the ions in solution to conduct electricity, since ions in solution are mobile charge carriers. The positive ions move toward the negative electrode (cathode) and are called cations; negative ions move toward the positive electrode and are called anions.

Covalent Compounds
Giant covalent molecules / lattices
•All the atoms are bonded together into a giant molecule.
•Some elements, e.g. diamond (carbon), silicon – both have the ‘diamond’ structure:

Diamond

Some compounds also have the ‘diamond’ structure:
e.g. quartz (SiO2, crystalline silica / silicon dioxide)
This lattice can extend indefinitely – a single crystal of diamond or quartz is a single giant molecule!

These lines do represent covalent bonds, each formed by one pair of shared electrons.

Physical Properties of Giant Covalent Substances
•Poor electrical conductivity: the atoms are neutral and held together by rigid covalent bonds and the electrons are not free to move, so these materials are poor electrical conductors and are insulators.
•Hardness and strength: covalent bonds are strong, in diamond each carbon makes 4 covalent bonds
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