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What are molecules and compounds?

Atoms are joined to form compounds and molecules.

  • A compound contains 2 or more different kinds of atoms chemically bonded in particular proportions. Water is one of the simplest compounds. It contains atoms of oxygen and hydrogen. The ratio of hydrogen atoms to oxygen atoms is 2 to 1. So we could call it dihydrogen oxide. Carbon dioxide is another familiar chemical substance. Although we cannot see or smell it you are exhaling it right now. As its name suggests there are twice as many oxygen atoms as carbon atoms. We have all heard of carbon monoxide, the poisonous gas released by faulty burners. Carbon monoxide contains carbon and oxygen atoms in equal numbers, one for one.
  • Molecules consist of atoms joined together by chemical bonds. Molecules can consist of atoms of the same element. Oxygen, for example, usually exists in the molecule 02, which is two oxygen atoms bonded together. Molecules can also consist of the atoms of different elements. These types of molecules are the components of compound substances. For example a water molecule (H2O) consists of 2 hydrogen atoms and an oxygen atom bonded together.
  • A mixture is 2 or more substances which have been mixed together but which have not chemically bonded. Air is a mixture. It contains nitrogen, oxygen, carbon dioxide, a small amount of inert gases and some moisture. The proportions of these substances are not fixed. The air we breathe out contains rather more carbon dioxide and moisture than the air we breathe in. Seawater is also a mixture. It is mostly water, but has a lot of salt dissolved in it.

How do atoms bond?

Atoms bond together by sharing electrons from their outer shells in order to get a full or empty outer shell of electrons. What occurs in a chemical bond is actually an electrostatic force of attraction between 2 atoms or ions. There are 3 main ways that atoms bond:

  • Ionic bonding is when atoms transfer electrons to another atom to form ions. Ions are formed when atoms lose or gain electrons and become charged: atoms that lose electrons form a positive ion (usually metal e.g. Fe2+); atoms that gain electrons form a negative ion (usually a non-metal element e.g. Cl-).
  • Covalent bonding is when a pair of electrons are shared between 2 atoms. The electron pair is usually one electron from each atom. Electrostatic forces between the shared pair of electrons and the nuclei of the 2 atoms hold the atoms together. This type of bond is usually formed between 2 non-metal elements. The atoms may be from one single element or from different elements chemically combining to form a compound. Most of the compounds we come across are covalent compounds – wood, plastic, clothing, food - the atoms of the majority of these kinds of materials are joined by covalent bonds. Sometimes atoms share more than one pair of electrons, for example a double covalent bond can occur, as in the carbon dioxide molecule where carbon shares 2 pairs of electrons with each oxygen atom: O=C=O (where = represents a double bond).
    • Another kind of covalent bonding is called co-ordinate or dative covalent. This occurs when both electrons in a covalent bond come from just one of the atoms. Carbon monoxide is bonded in this way: the oxygen atom donates 4 electrons, whilst carbon donates 2.
  • Metallic bonding occurs only between pure metals. Metalic bonds are giant structures of atoms held together by neither ionic nor covalent bonding. The outer shell of metal atoms has either 1, 2 or 3 electrons which actually move around from one atom to another, in a kind of sea of electrons, and the structure is all held together by the positive attraction in the nucleus.

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See the reactions in molecule matters for some examples of ions, and discover the molecular structure and properties of ozone.

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The states of matter!

All chemical substances consist of particles. These may be individual atoms as in the case of neon gas, small individual molecules as in the case of hydrogen or oxygen gas, larger molecules of compounds ranging from methane CH4 or glucose C6H12O6, very large molecules such as proteins and DNA, or giant structures such as diamonds. In every case the individual atoms vibrate and the atomic or molecular particles will move around. How much these particles vibrate and move depends on how much energy they have. Cooling substances down involves removing energy and the more energy is removed the less the particles will move. In theory, at absolute zero, i.e. minus 273 degrees Celsius, the particles will have absolutely no energy and will stop moving, although in practice this is impossible to achieve. In contrast, heating a substance will make the particles vibrate and move more rapidly.

There are 3 main states of mattersolid, liquid and gas, and 2 other curious states – super-cooled liquid and plasma. The main states of matter are obvious to everyone. We all know what solids, liquids and gases look like though we may not appreciate how the particles in them are behaving or how they change from one state to another.

In a gas the particles have so much energy they move around freely: each particle is free to vibrate and move around. Although there is an attraction between individual particles in a gas, it is not strong enough to hold the particles together. When a gas is cooled down, the particles have less energy and start to stick together: individual particles are attracted to one another. The gas is said to condense as it turns into a liquid. In a liquid, the particles are still free to move around, but the attractions between them prevent them from moving so freely. Squeezing a gas also helps it to condense. In fact, turning oxygen gas into liquid oxygen is only possible if it is cooled and squeezed. The opposite of condensation is evaporation. When a liquid is heated, individual particles have enough energy to escape from the attraction of other particles. When this happens the liquid evaporates. Liquids will evaporate at lower temperatures if the pressure is reduced. The temperature at which a gas condenses or the liquid evaporates is its boiling point. The change is a change of phase.

When a liquid is cooled sufficiently it will become a solid. The particles are no longer free to move around. Heating the solid will cause it to melt. The extra energy of the particles allows them to escape the attraction of other particles. Individual particles in a solid still have some energy and continue to vibrate, but they cannot escape from their neighbours. The change from liquid to solid and solid to liquid is another change of phase and the temperature at which this happens is referred to as the melting point, or sometimes freezing point.

Boiling points of substances are affected by pressure. Increasing pressure will increase the temperature at which the substance boils. Tables of boiling points give figures for standard pressure. The temperature at which substances melt and freeze is not affected by pressure in the same way. However, impurities will affect both melting and boiling points.

The general tendency is for smaller molecules to change phase at lower temperatures and larger molecules to change phase at much higher temperatures. For example, the hydrocarbons methane CH4, ethane C2H6, propane C3H8, and butane C4H10 are all gases at ordinary room temperature. Pentane C5H12 is a liquid at room temperature. Hydrocarbons (molecules made up of only hydrogen and carbon atoms) with chains of more than about 20 carbon atoms are solids at room temperature. The longer the chains are, the more difficult it is to melt them.

For every day purposes a substance like glass is obviously a solid. However, glass does not behave like other solids. The convenient division of matter into solid, liquid and gas does not fit every situation. Although glass is hard, and it feels and looks like other solids, the molecules actually move around rather like the particles in a liquid. For this reason it is sometimes called a super-cooled liquid. Our perception of solids, liquids and gases is not sufficient to explain what the matter in a star is really like. The Sun consists of a mixture of elements, but is mainly hydrogen and helium. The nuclear process converting hydrogen into helium only happens at exceedingly high temperatures and pressures. Individual particles in the Sun (atoms of hydrogen and helium) have an enormous amount of energy. Scientists refer to this material as plasma rather than gas. The particles are very close to each other as in a liquid, but moving around as in a gas.

Allotropes

Some elements can exist in different forms but in the same physical state. This is known as allotropy. Take carbon, for example. There are 2 types of solid carbon: diamond and graphite, so it is said to exist in allotropic forms. Diamond and graphite are the most commonly known allotropes of carbon, but there are actually more: fullerene is also a solid form of carbon. This is crystalline carbon which is made up of clusters of carbon atoms.

Why are there different versions of the same element in the same state?

The answer to this question lies with an understanding of how the atoms have bonded. In a diamond, the carbon atoms are strongly bonded (covalent bonds) to 4 other carbon atoms. This makes up a tetrahedral giant structure. The structure is uniform and extremely strong, making it a tough, resistant material.

Graphite, on the other hand, is bonded in layers rather than in a tetrahedral structure as in the diamond. Each layer in graphite is bonded covalently, and bonds within the layers are strong, but the bonds between those layers are weak, which means the layers can slide over each other. This explains why graphite is a fairly brittle, flaky substance.

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Why does ice float?

When substances are heated or cooled there are usually changes in volume. Heating a gas causes it to expand and if it is in an enclosed container the pressure will increase. Cooling a gas has the opposite effect. Refrigerators use a similar process to keep the food inside cool.

Heating and cooling solids and liquids also results in changing volume. Heating metals makes them expand and cooling them makes them contract. Large metal structures must have special expansion joints. As the metal heats up and expands on a hot summer’s day or cools down and shrinks on a cold winter’s night, the expansion joints compensate. Metal bridges don’t collapse and railway lines don’t buckle from season to season.

Water has some very interesting properties. As water is cooled it contracts. Cold water is denser than hot water. Water molecules attract and repel each other at the same time: the hotter they are, the more they repel each other. If a given mass of water is heated and expands, it will become less dense. As water at the surface of the sea warms up in summer, it becomes less dense and floats at the top of the sea. The surface of the ocean in the summer may rise to 20 degrees celsius or more whilst the water beneath is still hovering above freezing point. This phenomenon is important in central heating systems. Hot water rises to the top of radiators. As it cools and heats the room, it will sink to the bottom of the radiator, return to the boiler and be reheated. This is known as a convection current. The same thing happens to the air in the room. Hot air rises and cold air sinks and air circulates around the room. In fact, on a much larger scale, convection currents are involved in producing our weather.

So why then, if cold water falls to the bottom of a lake, does ice float on top of it? When water is cooled below 4 degrees Celsius, the hydrogen bonds connecting different the water molecules begin to form a crystalline structure, and as the temperature drops further they join together to form ice. The resulting structure is less dense than water as a liquid - approxiamately 9% less dense. In other words, if a container with a volume of 1 litre was filled with ice, it would weigh less than if it was filled with liquid water, and since objects lighter than water will float, when you place a block of ice into water, it floats.

What is nanotechnology?

Until very recently it was impossible to see molecules. Now scientists are manipulating molecules at the level of a billionth of a metre. This is called nanotechnology. Research has focused on molecular manufacturing - the creation of tools and machines (called nanobots) that will eventually enable scientists to snap together the fundamental building blocks of nature. The uses of nanotechnology are endless, particularly in medicine where it could be used to fight viruses or even repair genetic diseases. All materials, from metals to wood to food, could be replicated using nanotechnology, or it could be used to halt pollution. However, nanotechnology is still very much in its infancy.

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