Topic 2: Bonding, structure, and the properties of matter

Almost every property you can observe in a substance, whether it melts easily, conducts electricity, or bends without breaking, traces back to two things: which particles it contains and how those particles are bonded. This topic links the three states of matter to particle arrangement and then examines the three main types of strong chemical bonding, the structures they produce, and modern materials such as polymers, carbon allotropes, and nanoparticles.

States of matter and changes of state

The particle model pictures all matter as made of tiny particles. In a solid the particles are packed closely in a regular pattern and only vibrate about fixed positions, so solids have a fixed shape and volume. In a liquid the particles are still close together but can move past one another, giving a fixed volume but no fixed shape. In a gas the particles are far apart and move quickly in all directions, so a gas fills its container. The amount of energy stored in the particles decides the state: heating adds energy and weakens the forces between particles. Melting (solid to liquid) and boiling or evaporating (liquid to gas) happen as particles gain energy, while condensing and freezing happen as they lose it. The stronger the forces between particles, the more energy is needed to separate them, so the higher the melting and boiling points. The particle model is useful but limited: it treats particles as solid spheres with no forces shown and ignores their actual size and shape.

Ionic bonding

Ionic bonding happens between metals and non-metals. A metal atom loses one or more outer electrons to form a positive ion (a cation), and a non-metal atom gains those electrons to form a negative ion (an anion). Both ions then have full outer shells, like a noble gas. For example, sodium gives an electron to chlorine: Na loses one to become Na+ and Cl gains one to become Cl-, forming NaCl. The oppositely charged ions are held together by strong electrostatic forces of attraction acting in all directions. This force is the ionic bond. The charge on an ion can usually be predicted from the group number: Group 1 forms 1+, Group 2 forms 2+, Group 6 forms 2-, and Group 7 forms 1-. Dot-and-cross diagrams are used to show which atom each transferred electron came from.

Ionic compounds and their properties

Ionic compounds do not exist as single molecules. Instead the ions arrange themselves into a giant ionic lattice, a regular three-dimensional structure repeated many times where each ion is surrounded by ions of opposite charge. Because the electrostatic forces throughout the lattice are strong, a lot of energy is needed to break them, so ionic compounds have high melting and boiling points and are solid at room temperature. They do not conduct electricity when solid because the ions are locked in place and cannot move. However, when melted or dissolved in water the ions become free to move and carry charge, so molten or dissolved ionic compounds do conduct electricity.

Covalent bonding and simple molecules

Covalent bonding occurs between non-metal atoms. Instead of transferring electrons, the atoms share pairs of electrons so that each atom achieves a full outer shell. A shared pair of electrons is a single covalent bond. Examples include H2, Cl2, O2 (a double bond), H2O, NH3, CH4, and CO2. Many covalent substances exist as small simple molecules. The covalent bonds within each molecule are very strong, but the forces between separate molecules, called intermolecular forces, are weak. When a simple molecular substance melts or boils it is only these weak intermolecular forces that are overcome, not the bonds inside the molecules. This is why simple molecular substances have low melting and boiling points and are often gases or liquids at room temperature. As molecules get larger the intermolecular forces increase, so melting and boiling points rise. Simple molecular substances do not conduct electricity because the molecules have no overall charge and there are no free electrons or ions.

Giant covalent structures

Some covalently bonded substances do not form small molecules but instead form giant covalent structures, also called macromolecules, in which huge numbers of atoms are joined by strong covalent bonds in a continuous network. Examples include diamond, graphite, and silicon dioxide (silica). Because every atom is held by strong covalent bonds, very large amounts of energy are needed to break the structure, so giant covalent substances have very high melting and boiling points and are solid at room temperature. Most do not conduct electricity because there are no free charged particles, although graphite is an important exception.

Metallic bonding and metal properties

Metals consist of a giant structure of positive metal ions arranged in regular layers, surrounded by a sea of delocalised electrons that have come from the outer shells of the atoms. These delocalised electrons are free to move throughout the whole structure. The strong electrostatic attraction between the positive ions and the negative sea of electrons is the metallic bond, and it gives most metals high melting and boiling points. Metals conduct electricity and heat well because the delocalised electrons are free to move and carry charge and energy through the metal. Metals can be bent and shaped because the layers of ions can slide over one another without breaking the bonding. An alloy is a mixture of a metal with other elements; the different sized atoms distort the regular layers so they cannot slide as easily, which makes alloys harder than the pure metal.

Polymers

Polymers are very large molecules made when many small molecules called monomers join together in long chains. Although the chains are held to each other only by intermolecular forces, these chains are so long that the total intermolecular attraction between them is fairly strong. As a result polymers are usually solid at room temperature and have higher melting points than small simple molecules, but lower than ionic or giant covalent substances. In structural diagrams a polymer is shown by drawing the repeating unit inside brackets with the letter n outside, meaning the unit is repeated many times. Poly(ethene) is a common example made from ethene monomers.

Allotropes of carbon

Allotropes are different structural forms of the same element. Carbon has several important allotropes. Diamond is a giant covalent structure in which each carbon atom forms four strong covalent bonds, making it extremely hard with a very high melting point; it does not conduct electricity because all four outer electrons are used in bonding. Graphite has each carbon bonded to only three others, forming layers of hexagonal rings; the layers are held together by weak forces so they can slide, making graphite soft and slippery, and the spare electron from each atom is delocalised, so graphite conducts electricity. Graphene is a single layer of graphite, one atom thick, that is strong, light, and an excellent conductor. Fullerenes are molecules of carbon arranged as hollow shapes such as spheres or tubes; buckminsterfullerene (C60) is a hollow ball, while carbon nanotubes are cylinders with very high strength and useful electrical properties.

Nanoparticles

Nanoparticles are extremely small particles, roughly 1 to 100 nanometres across, containing only a few hundred atoms. They sit between individual molecules and fine particles in size. A key feature of nanoparticles is their very high surface area to volume ratio: as a particle gets smaller, the proportion of its atoms on the surface rises sharply. This makes nanoparticles very effective as catalysts and means smaller amounts can be used. They are used in sun creams, cosmetics, medicines, and self-cleaning surfaces. Because they are so new and behave differently from the bulk material, there are concerns that their effects on health and the environment are not yet fully understood.