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KS4 Chemistry. Comparing Bonding. Contents. Comparing bonding. Bonding and structure. Melting and boiling point. Conductivity. Other properties. Summary activities. Types of bonding.
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KS4 Chemistry Comparing Bonding
Contents Comparing bonding Bonding and structure Melting and boiling point Conductivity Other properties Summary activities
Types of bonding There are three ways in which atoms can chemically bond.In each case, electrons are involved in different ways. • ionic bonding – electrons are transferred from metal to non-metal atoms to form positive and negative ions. These oppositely-charged ions are attracted to each other. • covalent bonding – electrons are shared betweennon-metal atoms. • metallic bonding – electrons become delocalized and form a sea of free electrons, which attracts positive metal ions.
Bonding and structures The type of bonding in a substance affects the structure of that substance. giant ionic millions of ions solid lattice simple few atoms usually liquid or molecular solid giant covalent millions of solid lattice atoms giant metallic millions of ions solid (except lattice mercury – liquid)
Bonding and properties The bonding and structure of a substance affects the properties of that substance. • Different bonds require different amounts of energy to be broken – this affects properties such as melting and boiling point, solubility and hardness. • Different bonds involve electrons in different ways – this affects properties such as conductivity of electricity and heat.
How do properties differ? How do the properties of substances differ according to the type of bonding they contain?
Contents Comparing bonding Bonding and structure Melting and boiling point Conductivity Other properties Summary activities
Melting and boiling point: giant structures + + + - - + + - - - + + + - - + + - - - + + + - - strong metallic bonds holds ions together Substances with giant structures generally have high melting and boiling points because all the atoms are strongly bonded together to form a continuous 3D lattice. A large amount of energy is needed to break these bonds. strong covalent bonds holds atoms together strong ionic bonds holds ions together
Melting and boiling point: molecules weak bonds between molecules strong bonds within molecules Simple molecular substances have low melting and boiling points because the forces that hold the molecules together are weak and easily broken. In general, the larger the molecule, the higher the melting and boiling point.
Contents Comparing bonding Bonding and structure Melting and boiling point Conductivity Other properties Summary activities
Conductivity – ionic substances + + + ions cannot move to carry a charge - - + + - - - + + + - - + + - - - + + + - - + + - - + ions are able to move and carry a charge + + - - - - + + - + - - + + - Ionic substances cannot conduct electricity when solid because the ions are bonded together in the lattice. When liquid (molten) or dissolved, however, the ions are able to break free of the lattice.
Conductivity – covalent substances Substances with covalent bonding do not conduct electricity because all the outer electrons are fixed in shells. electrons unable to move and carry a charge The exception is graphite, which has free electrons between layers of carbon atoms.
Conductivity – metals metal ions Metals can conduct electricity because the sea of delocalized electrons between the metal ions are free to move anywhere in the structure. sea of electrons electrons free to move and carry an electrical charge
Contents Comparing bonding Bonding and structure Melting and boiling point Conductivity Other properties Summary activities
Solubility Many ionic substances dissolve in water because water molecules have a slight electrical charge and can attract the ions away from the lattice. Simple molecular substances are usually insoluble in water but soluble in solvents such as petrol. Substances with giant covalent or metallic structures generally do not dissolve easily in water or other solvents. This is because dissolving involves breaking bonds, which needs lots of energy.
Strength Substances with giant structures are generally hard because a large amount of energy is needed to break ionic, covalent and metallic bonds. • Ionic substances are hard but brittle – when a force is applied, ions with like-charges are brought together. These repulse each other and cause the lattice to shatter. • Covalent substances are hard but brittle – the covalent bonds do not bend but break when enough force is applied. • Graphite is slippery because the weak forces between layers of carbon atoms can easily be broken. • Metals are hard but malleable and ductile – the delocalized electrons allow the metallic bonds to move.
Density strong bonds cause tight packing weak bonds cause loose packing The density of a substance depends on the type and strength of bonding and how the atoms/ions are arranged. Giant substances, especially metals, are dense because the atoms/ions are tightly packed together. Substances with simple molecular structures have lower densities. The weak forces between molecules cannot pull the molecules together tightly.
Contents Comparing bonding Bonding and structure Melting and boiling point Conductivity Other properties Summary activities
Glossary covalent bond –A strong bond between two atoms in which each atom shares one or more electrons with the other. delocalized–Electrons that are free to move throughout a structure. In metals, they form a ‘sea of electrons’. ionic bond –The strong electrostatic force of attraction between oppositely charged ions. giant structure –A structure containing millions of atoms or ions bonded together. The structure extends in three dimensions until all available atoms or ions are used up. lattice –A 3D structure of closely packed atoms or ions. metallic bonding –The strong attraction between positive metal ions and a sea of electrons. molecule –A simple structure containing two or more atoms covalently bonded together.
