Atoms & Molecules…

There are about 100 different kinds of atom naturally occurring on Earth. They are the defining structure of an element and cannot be broken down by any chemical means. They consist of a dense nucleus, containing protons and neutrons, surrounded by a cloud of electrons.

They may exist naturally as single atoms but more commonly they join up to make molecules. For example, argon (a gas present in air) is totally unreactive, so exists as single atoms of argon. Oxygen is much more reactive so usually exists as O2 – an oxygen molecule. These examples are of elements; molecules or single atoms of one particular type of atom from the periodic table. If a molecule forms that contains two or more different kinds of atom then we call it a compound.

Compounds can be simple, like water (H2O) or carbon dioxide (CO2), or they can be more complicated like glucose (C6H12O6). The numbers refer to how many atoms of each kind there are in the compound. Water contains two hydrogen atoms and one oxygen atom. Glucose has six carbon atoms, twelve hydrogen atoms and six oxygen atoms.

Different elements have different physical and chemical properties. For example, hydrogen is a gas at room temperature (physical property) and reacts violently with oxygen (chemical property). Sodium is a soft, shiny metal (physical property) that reacts vigorously with water (chemical property). Chlorine is a green gas at room temperature (physical property) that reacts rapidly with metals to form compounds called chlorides (chemical property).

When compounds form, they have their own unique properties that are usually very different to the properties of the elements from which they formed. For example, combine hydrogen and oxygen to make water and you have a substance that is a liquid at room temperature and will not burn. This is quite different to a mixture of oxygen and hydrogen. Combine sodium and chlorine and you have sodium chloride (table salt) which is a very stable, harmless white powder that tastes great on chips. The millions of different combinations possible between atoms accounts for the huge variety of different materials we see in the world; from stone to glass to skin to plastic. These are all subtle combinations of different chemical elements.

Slightly more technical stuff

Protons have a positive charge, neutrons no charge and electrons are negatively charged. The atom is thus held together by the electromagnetic force. Normally an atom has the same number of protons as electrons so overall it has no charge. If it loses electrons, during a chemical reaction or as a result of being bashed into, it becomes positively charged. If it gains electrons then it becomes negatively charged. These charged atoms are known as ions.

The properties of the atom are governed by the number of protons it has. Hydrogen is the simplest atom with just one proton and one electron. Helium, element number two on the periodic table, has two protons and two neutrons. The mass of an atom comes from the number of protons and neutrons that it contains. Electrons are so small that they contribute an insignificant mass to the total. Hydrogen is said to have a mass of 1 and so helium has a relative mass of 4.

Carbon has six protons and six neutrons so has a relative atomic mass (RAM) of 12. A small proportion of carbon atoms (about one atom in a trillion) has two extra neutrons and is called carbon-14. Carbon-14 is known as an isotope of carbon-12. This nucleus is unstable and breaks down slowly over time. It takes about 5,730 years for 50 percent of the carbon-14 atoms in a sample to decay into carbon-12. Living organisms are constantly recycling carbon so the level of carbon-14 in their bodies is consistent with the level of carbon-14 in the environment. When they die, they stop exchanging carbon, but the carbon-14 slowly breaks down. By comparing the level of carbon-14 present in the dead organism against the level normally found in the environment it is possible to predict roughly when the organism died. For example, if a fragment of wood dug out of a glacier was found to have half the level of carbon-14 found in living trees then you could estimate that the tree died 5,730 years ago. Other elements have radioactive isotopes with longer or shorter half-lives so you can check your results for consistency.

Questions…

  1. What elements do the following compounds contain?
    1. copper chloride
    2. aluminium iodide
    3. sulphuric acid
    4. calcium carbonate
    5. silver nitrate
  2. What could you add to an acid to release hydrogen?
  3. Broadly speaking, how do metal oxides and non-metal oxides differ chemically?
  4. Name an element that exists as single atoms.
  5. Name an element that normally exists as a diatomic molecule.