bonding of carbon, oxygen and nitrogen exist on different energy levels.

Elements with the same number of valence electrons in their outermost electron shell, the valence shell, form groups that are arranged in vertical columns in the periodic table (1A–8A in Table 1.1). By interaction of their outermost electrons, atoms are linked together to form molecules. This is the process we call chemical bonding. The number of electrons determines the chemistry of an atom. The elements that have the same number of electrons in their outermost shell and belong to one group or chemical family characterised by their similar chemical reactivity. When different atoms undergo a chemical reaction and form a new compound, its properties will be different from the properties of any of its constituent elements.

Many of the substances we are confronted with are built from molecules, such as water, alcohol, sugar or cellulose. In the case of water, the compound consists of two elements, hydrogen and oxygen. In the case of alcohol, sugar or cellulose the building elements for their molecules are, in each case, hydrogen, carbon and oxygen. However, in each of the molecules, the elements are arranged differently, so the resulting substances have different properties and chemical behaviour. Alteration of the chemical bonding always produces a modification of the material itself. With respect to conservation, the material substance of an object may already be changed chemically through natural ageing at the time it is considered for treatment. However, conservation treatment tends to avoid changes to the chemical structure of the object material unless this is essential for its preservation. Indeed, the paper conservator deals only with a small number of conservation interventions that include chemical reactions. One particularly pertinent example of natural ageing of a material that results in an undesirable change of its chemical bonding is the acid hydrolysis and oxidation of cellulose that produce a loss of paper strength and brightness (see Chapter 8). One conservation intervention that seeks to counteract acid hydrolysis and involves the chemical transformation of the reactive acids is called deacidification. With this treatment, acids are neutralised by a chemical reaction within the cellulose matrix (see Chapter 13).

Besides chemical bonding, there are other weaker electric forces acting between molecules. These forces result mainly from an uneven distribution of the electrons across a molecule. This causes the formation of a molecular structure where the negative charge accumulates on one side, leaving at least one atomic nucleus on the other side nearly unshielded by the electron cloud that should envelop the molecule evenly. One can describe this as a particle with a small