Chemistry GCSE Revision: Bonding, Structure, And The Properties Of Matter

The bonding, structure, and properties of matter determine the dynamics of interactions of the elements and the compounds they form. It’s possible to predict the behaviour of matter under certain conditions. Chemists can synthesise natural substances or create new materials based on their understanding of the properties of matter.

Chemical bond of water molecule

The Three States Of Matter

Under the normal range of conditions on earth, matter has three states: solid, liquid, and gas. Most chemical reactions and chemical manipulations are done in these states. They’re also the most stable states of matter on earth.

The states or phases of matter mainly have to do with the way the particles are moving and their distances from one another. For example, at room temperature, water is liquid. However, at 0°C or lower, water is solid because the molecules slow down and become more compact by hooking onto each other. Similarly, at 100°C or higher, water molecules begin to move more quickly because of heat energy, which causes it to boil. When water is boiled, steam is produced because there’s too much energy for the molecules to stay in the liquid phase. So, by producing steam, water changes into a gaseous state.

Here’s a bit more information about the three states of matter:

  • Solids: The molecular particles of solids are locked into place because the particles are strongly bonded and cannot slide past one another. Because there’s very little free space between the particles, solid matter becomes rigid. It retains its shape and volume, making it virtually impossible to compress.
  • Liquids: Substances in liquids have fixed volume but they take the shape of the container they occupy. This is because the particles can easily move or slide past one another. Like solid substances, liquids are also very hard to compress. Liquids can flow, but fluid dynamics vary depending on viscosity.
  • Gases: Substances in gaseous form do not have fixed volumes. They assume the shape and volume of the containers they occupy. Gas particles have a lot of room to move, making them easily compressible.

Molecules in solid, liquid, and gas substances all vibrate, but at varying degrees. For example, molecules in solid objects barely move, while the vibrations are actually observable in liquid and in gas, which is known as Brownian motion. Even in calm, undisturbed liquid, such as water, random motions of particles of dust can be observed. Similarly, diffusion of gases occurs in an undisturbed container.

Plasma And BEC

In addition to solids, liquids and gases, there are two additional states of matter that appear under special conditions: plasma and Bose-Einstein condensate (BEC). These states cannot be used to produce new materials:

  • Plasma: Plasma is basically ionised gas. It’s also the most abundant high-energy form of matter found in stars. In plasma, some or all of the orbital electrons are removed, making them free to move within the gas. In stars, plasma is created by the extreme heat of nuclear fusion, and held in place by the gravitational force and strong magnetic force of stars. It can be artificially created by heating gases or applying electricity. Some gases can more easily be ionised than others because of their loosely held electrons. Gases that have strongly held electrons in their outer shells require higher ionisation energy
  • Bose-Einstein condensate: While plasma exists at high temperature, the Bose-Einstein condensate is the opposite: it can only exist at a temperature very near absolute zero. When near-zero temperature is reached, atoms barely move – there’s almost no free energy available to do so. In this state, the clump of atoms acts as one atom. The condensate can then be made from a cloud of diffuse gas. Experiments typically use rubidium atoms by cooling them with lasers to take energy away. Evaporative cooling is applied to further cool the rubidium atoms to near-absolute zero.

Click here for GCSE revision tips on the three states of matter.

Vector illustration showing the states of matter

Ionic Compounds

Ionic compounds are composed of ions held together by electrostatic forces known as ionic bonds. This is a type of bond that dissociates when dissolved in polar solvents like water. Here, the ions split apart into the positively charged and negatively charged components. For example, if sodium chloride or table salt is dissolved in water, the positively charged sodium ions separate from the negatively charged chloride ions.

Ionic compounds are typically salt, base, and acid compounds. Salt compounds are solid and crystalline in form. Each ion typically has several ionic neighbours that form regular crystal lattice patterns. Therefore, they are not considered as part of individual molecules but part of continuous, three-dimensional crystal structures.

Click this to see samples of GCSE test questions for ionic compounds.

Small Molecules

Small molecules are simple, lightweight molecules composed of two or more atoms that are covalently bonded. This could either be an elemental molecule or a compound molecule. In elemental or compound substances, the number of atoms of elements per molecule remain the same. Substances that have ionic bonds do not form molecules.

Substances with small molecules contain few atoms per molecule. They are also lighter compared to other molecules, such as polymer molecules. Their average size is around 0.1 nm or 1 × 10-10 metres across. An example of a substance containing small molecules is water, where they measure about 0.3 nm across. By comparison, DNA is 2 nm across and can stretch up to several meters. A human DNA molecule in a chromosome, for instance, has a length of about two meters when stretched. 

Click this for the GCSE revision for small molecules.

Giant Covalent Molecules 

Giant covalent molecules are covalently bonded and contain a large number of atoms that form giant covalent lattice structures. The best known examples of this are elemental silicon, silicon dioxide, and two elemental forms of carbon: diamond and graphite.

At room temperature, substances that have giant covalent structures are solid in form. Their melting points and boiling points are very high because large amounts of energies are needed to overcome the covalent bonds. For example, graphite has a melting point of 3,600°C. In comparison, iron has a melting point of 1,538°C and a boiling point of 2,862 °C.

Giant covalent structures are mostly not electrically conductive because no charged particles can freely move in the structures. Some, however, are semiconductors, like silicon. Graphite is the only giant covalent structure that is a very good electrical conductor. This is because it has spare electrons: each carbon atom in graphite has one free valence electron, and these are delocalised between the layers. 

Follow this to revise for GCSE questions on giant covalent molecules.

Vector illustration of bonding between atoms and molecules

Metals And Alloys 

Most elements are metallic or metalloids. Approximately 95 out of the 118 elements in the periodic table of elements are classified as metals. However, the boundaries between metals, nonmetals and metalloids slightly fluctuate because of the inexactness of the definitions of the categories: there are no universally accepted definitions of the boundaries.

The main characteristic of metallic elements is their electrical conductivity. Many elements that are normally not considered metallic at room temperature become metallic at a temperature near absolute zero. Other elements and their compounds also become metallic under high pressures. Iodine, for instance, becomes a metal when subjected to a pressure between 40,000 and 170,000 times the normal atmospheric pressure at sea level.

Metals and alloys that have industrial and commercial value are generally hard and malleable. They are also good conductors of electricity and thermal energy. Alloys are combinations of at least one metal and other elements. Typically, two or more metals are combined to form alloys, such as brass and steel. Alloys have different or improved intended properties than any of their constituents. For example, stainless steel is an alloy of iron, chromium, and carbon. Pure iron is soft and can easily become corroded.

Follow this to revise for GCSE questions on metals and alloys.

Nanoscience

Nanoscience is the study of structures and materials on an ultra-small scale. A nanometre is one billionth of a metre. To have an idea how small a nanometre is, bacteria sizes range from 0.5–5.0 micrometres (0.05–0.5 nm) – in fact, a billion bacteria can fit on the head of a pin. Nanoscience deals with objects that are submicroscopic, or objects that cannot be seen even with the aid of a light microscope. At this nano level, physical and chemical properties of matter can change.

Nanoscience focuses on ways to use nanoscale molecules as machines. One of the smallest molecular machines created is a molecular elevator for an experimental microchip, and it measures about 0.7 nm. For further reading, the scientist who created the first nanomachines won the Nobel Prize in Physics in 2016. Nanotechnology is important because it has the potential to revolutionise a diverse range of fields, from health care to manufacturing.

Vector illustration of nanoparticles

Follow this to revise for GCSE questions on nanoscience.

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