attraction while particles of the same charge repel each other. With regard to the atom, we must look at subatomic particles to understand the origin of the atomic charge. Every atom consists of a small nucleus that carries most of its mass. The nucleus consists of positively charged protons and neutrons that carry no charge. The latter are of little importance in our context and therefore will not be discussed further. Around this nucleus, negatively charged lightweight electrons move in orbits. Each electron bears a charge that exactly balances that of a corresponding proton. In its natural state, the atom contains as many protons as electrons. As a whole, an atom is therefore, with respect to its electrical charge, a neutral body. By chemical interaction it is possible to abstract or to add electrons to an atom and this leads to charged particles that are called ions. Positively charged ions having more protons than electrons are cations; negatively charged ions with an excess of electrons are anions.

We know a number of substances that are composed only of one type of atom. These are, for example, the noble gases helium (He) and neon (Ne); two others, oxygen (O₂) and nitrogen (N₂), constitute the main components of the earth’s atmosphere. These pure substances are called elements. They exist as monoatomic gas, for example helium, or as diatomic molecular gas, for example oxygen or nitrogen, at ambient temperature and pressure. Other substances that exist in the elemental state are carbon in the form of diamond or graphite, and the precious metals gold and silver.

In this chapter, we will focus on a selection of elements that are relevant to paper and water. They will be discussed especially regarding their function as parts of compounds, i.e. in a state in which they are combined with other elements. These elements are hydrogen, carbon and oxygen that build the cellulose molecule, nitrogen that is present in proteins such as gelatine, calcium and magnesium present in deacidification agents, and aluminium and sulphur that occur in the papermaking additive alum (Neimo 1999).

Elements can be differentiated by their number of protons and electrons. They are grouped according to their number of protons and electrons in the periodic table of elements, where the horizontal rows feature the elements according to their atomic number (Appendix A1, p. 493). The atomic number indicates the number of protons present in the atomic nucleus. In the first row, we only find hydrogen and helium (Table 1.1). The simplest element, hydrogen (H), consists of one proton and one electron. The next element is the noble gas helium (He) with two protons balanced by two electrons. In a simple model it is assumed that electrons are arranged in shells, each of which surrounds the nucleus at a certain distance. However, the