Atoms Joining Together
One of the greatest achievements of chemistry has been to show that all matter in the universe, be it a super nova, a starship enterprise, planet earth or a year six pupil, is built up from a combination of about 100 elements.
Elements cannot be broken down into simpler substances by heating, boiling, adding acid or any other method available to the chemist to change a substance. Peter Atkins suggests that to go further requires the aggressive techniques of the physicists who “can smash elements apart into electrons, protons and the other fundamental particles using high energy particle accelerators”. The smallest amount of an element that can exist is an atom. An element, such as metallic gold, is a collection of identical atoms and is identified by its atomic number, which is the number of protons in the nucleus For example only carbon atoms have six protons in their nucleus.
Pupils are often able to write or quote a textbook definition of the word ‘element’ but do not develop a mental model of the concept. Frequently too, pupils assign macroscopic properties to individual atoms and are, for example, unable to use the idea that copper atoms have very different properties from those of the copper element.
Compounds are held together by chemical bonds or links between atoms. Such links are known to arise from the deployment of the electrons of the outer shells of atoms, the so-called valence shells. For example, water is a combination of hydrogen and oxygen atoms bonded together and aspirin is a made up of carbon, hydrogen and oxygen. Many compounds consist of molecules which are discrete groupings of atoms in a definite geometrical arrangement. A suitable explanation at Key Stage 3 is that a compound contains two or more elements bonded together in fixed proportions. A compound usually has different properties from the elements from which it is made. For example sodium metal is a reactive metal and chlorine a toxic gas, but combined together they make sodium chloride which is added to food to preserve it or to enhance the flavour. Substances such as glass, steel, iron oxide, plastics and starch are compounds too but the proportions of elements making them up can vary. In each case atoms are attracted by all the adjacent atoms making them nearly uniform solids. Alloys are also compounds.
Mixtures contain elements or compounds in varying proportions. The components of mixtures have different physical properties and so can be separated. Ink for example can be separated into its components because the solvent used - water, boils at relatively low temperatures while the pigments added to colour the ink have very high melting and boiling points. It is possible to boil off the solvent at 100oc leaving the dyes in the ink behind. Nearly everything we eat is an example of a mixture, for example orange juice contains the compounds water, sugar and acid. Other mixtures include: air which is a mixture of gases and water vapour,; sea water which is a mixture of water, sodium chloride, magnesium chloride; and crude oil which is a mixture of different boiling points.
There are two principal varieties of chemical bonds - ionic bonds and covalent bonds. An ionic bond, as its name suggests, is an interaction between the ions that atoms form, stemming form the attraction between opposite charges of positively charged ions. Ions are charged atoms where there are unequal numbers of protons and electrons. Metals atoms lose electrons to form positively charged ions called cations . Non -metals take in electrons to form negatively charged ions called anions. A covalent bond is a discrete combination of atoms, effected by the sharing of electron pairs.
Ionic compounds are formed when a metal and a non-metal join together. When sodium metal is dropped into a gas jar of chlorine gas the elements react violently to form a new compound called sodium chloride joined by ionic bonds. To understand the influences leading to the formation of an ionic bond we need to consider the electronic arrangement of sodium and chlorine atoms. Sodium structure is 2,8,1 with one electron in the outer shell. Chlorine is 2,8,7, with seven electrons in the outer shell. The removal of the electron from the sodium outer shell requires 5.1 eV and in the process a sodium ion is formed. 3.6 eV are released when an electron enters the chlorine shell forming a chloride ion. At this stage two noble-gaslike structures have been formed, a sodium cation and a chlorine anion by the transfer of an electron from one type of atom to the other. It does not seem likely that the electrons will move if only the energy levels are considered. However the attraction between the ions is so strong that lowering the energy outweighs the investment which has to be made to extract the electron from the valence shell. The lowest energy of all is achieved if the cations and anions clump together with cations around each anion and anions around each cation. More detailed determination of the structures shows a giant lattice ionic solid structure.
When two non-metals join they form a covalent bond. The best that can happen is for the atoms effectively to retain their electrons but to enter into a sharing arrangement. When two electrons are shared between two neighbouring atoms, they are said to be joined by a covalent bond. When covalent bonds form between atoms, the resulting entity is called a molecule with a fixed characteristic geometry.