There are 118 different chemical elements of which 92 occur naturally on earth. Each is represented by a single letter or two letters, such as H for hydrogen or Au for gold. Chemistry is mainly concerned with the way these elements act and combine to form compounds.
Materials usually consist of several compounds, and the picture can therefore become very complex. A traditional telephone can contain as many as 42 different elements (Altenpohl, 1980; Karsten, 1989). Materials exist as solids, liquids or gases and the same chemical compound can exist in any of these three states, depending on temperature and pressure. Water (H2O) freezes at 0 °C and boils, or evaporates at 100 °C without changing its chemical composition.
The smallest unit a material can break down into is a molecule. Every molecule consists of a certain number of atoms. These atoms represent the different elements and can be obtained through chemical reactions ().
Each element has its own characteristic atomic structure, mainly described by its weight: the relative atomic weight. Hydrogen has the lowest relative atomic weight, 1, while oxygen has a relative atomic weight of 16.
The molecular weight of water is found through adding up the different atomic weights:
Calcium carbonate (CaCO3) consists of calcium (Ca), with a relative atomic weight of 40, carbon (C) with a relative atomic weight of 12, and oxygen (O) with 16. The relative molecular weight is therefore:
CaCO3 = Ca + C + O + O + O = 40 + 12 + 16 + 16 + 16 = 100 (2)
The relative atomic weights of the different elements are given in the periodic table (Figure 4.1). The elements are also given a ranking in the table of 1-118. The number of the elements in the ranking order is equivalent to the number of protons in the nucleus of the atom.
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