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Milk proteins can be made visible by an electron microscope There are molecules which consist of two linked atoms, and others composed of hundreds of atoms. The number of atoms in a molecule can vary enormously. The formula means that the molecule is made up of three carbon atoms, six hydrogen atoms and three oxygen atoms. If the molecule consists mainly of carbon (C), hydrogen (H 2) and oxygen (O 2) atoms, the compound formed is said to be organic, i.e. iron (Fe) or siliceous sand (SiO 2), liquids, e.g. The molecules can then form solid substances, e.g. MoleculesĪtoms of the same element or of different elements can combine into larger units, which are called molecules. Common salt consists of sodium (Na) and chlorine (Cl) ions and has the formula NaCl (sodium chloride). in solutions as cations (positive charge) and anions (negative charge) or in solid form as salts. Positive and negative ions are always present at the same time i.e. If the ion contains more electrons than protons it is negatively charged, but if it has lost one or more electrons it is positively charged. Such an atom is no longer electrically neutral. IonsĪn atom may lose or gain one or more electrons. The nucleus is so small in relation to the atom that if it were enlarged to the size of a tennis ball, the outer electron shell would be 325 metres from the centre. If we call the diameter of the nucleus one, the diameter of the whole atom is about 10 000. There are about as many atoms in a small copper coin as there are seconds in a thousand million million years! Even so, an atom consists mostly of empty space. The atom is therefore electrically neutral.Īn atom is very small, Figure 2.2. The electrons, which orbit the nucleus, carry a negative charge equal and opposite to the unit charge of the protons.Īn atom contains equal numbers of protons and electrons with an equal number of positive and negative charges. The protons carry a positive unit charge, while the neutrons are electrically neutral. The nucleus of the atom consists of protons and neutrons, Figure 2.1.

Air, for example, is a mixture of oxygen, nitrogen, carbon dioxide and rare gases, while water is a chemical compound of the elements hydrogen and oxygen. However, most naturally-occurring substances are composed of several different elements. Examples are oxygen, carbon, copper, hydrogen and iron. A substance in which all the atoms are of the same kind is called an element. The atom is the smallest building block of all matter in nature and cannot be divided chemically. Chemical symbols of some common elements in organic matter: