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General Chemistry Related to Textiles
1.1 Introduction
This chapter provides a background to the chemical principles involved in coloration processes, which will be beneficial to those with little working knowledge of dyeing chemistry. Chemistry has been classically divided into three branches: inorganic chemistry, organic chemistry and physical chemistry. Inorganic chemistry is the study of elements and their compounds. However carbon is so unique in the breadth of the compounds it forms (chiefly with hydrogen, oxygen, nitrogen and, to a lesser extent, sulphur) that it has its own branch – organic chemistry. Physical chemistry is concerned with the influence of process conditions such as temperature, pressure, concentration and electrical potential on aspects of chemical reactions, such as how fast they proceed and the extent to which they occur.
There are no clear distinctions between the three branches. For example, organometallic compounds are important substances that combine organic and inorganic chemistry, and the principles of physical chemistry apply to these two branches as well. Fundamental to all these branches of chemistry is an understanding of the structure of matter, so the chapter begins with this important aspect.
1.2 Atomic Structure
Modern chemistry is based on the belief that all matter is built from a combination of exceedingly minute particles (atoms) of the various chemical elements. Many different elements are found in nature, each possessing characteristic properties; the atoms of any one element are all chemically identical. An element is a substance made up of only one type of atom, for example, carbon is only made up of carbon atoms, and sodium is only made up of sodium atoms. Atoms combine together to form molecules of chemical compounds. A molecule is the smallest particle of a chemical element or compound that has the chemical properties of that element or compound.
A single atom consists of a very dense central core or nucleus, which contains numbers of positively charged particles called protons and uncharged particles, called neutrons. Protons and neutrons have equal mass and together they account for the atom’s mass. A number of very small negatively charged particles, called electrons, circulate around the nucleus in fixed orbits or ‘shells’, each orbit corresponding to a certain level of energy: the bigger the shell (the further away from the nucleus it is), the greater the energy. These shells are labelled n = 1, 2, 3, etc., counting outwards from the nucleus, and each can hold a certain maximum number of electrons, given by 2n2. The movement of an electron from one energy level to another causes the absorption or emission of a definite amount of energy. Atoms are electrically neutral, so the number of electrons in an atom is exactly the same as the number of protons in its nucleus. The total number of electrons within an atom of a particular element is called the atomic number of the element. This is the same as the number of protons in its nucleus. It is the arrangement of the electrons around the nucleus of an atom that determines the chemical properties of an element, especially the electrons in the outermost shells.
It is possible that some of the atoms of an element have a different number of neutrons in their nucleus, but their numbers of protons and electrons are still the same. These atoms are called isotopes, and although they have the same chemical properties as the other atoms, their atomic masses are different. Also recent research into atomic structure has shown that the three subatomic particles are themselves made up of other smaller particles such as quarks, but for this book it is sufficient to only consider atoms in terms of protons, neutrons and electrons.
The simplest atom is that of hydrogen, which has a nucleus consisting of just one proton with one electron orbiting around it and has an atomic number of 1. In deuterium, an isotope of hydrogen, there is one neutron and one proton in its nucleus. So its atomic mass is 2, but its atomic number is still only 1. There are roughly 6400 atoms of ‘normal’ hydrogen for every atom of deuterium. Another example is chlorine, which has two stable isotopes – one with 18 neutrons and the other with 20 neutrons in the nucleus. Because each has 17 protons, their atomic weights (the combined weights of protons and neutrons) are 35 and 37, respectively. These two forms are labelled 35Cl and 37Cl. Approximately 75.8% of naturally occurring chlorine is 35Cl and 24.2% is 37Cl, and this is the reason why the periodic table of the elements shows the atomic weight of chlorine to be 35.45.
Within a shell there are orbitals, each of which can hold a maximum of two electrons. Within an orbital, the two electrons are distinguished by the fact that they are spinning around their own axis, but in opposite directions. In illustrating this diagrammatically the electrons in an orbital are often shown as upward and downward arrows ↑↓, for example, as in Figure 1.3. The orbital nearest the nucleus is called an s orbital, followed by p, d and f orbitals, which are occupied in the larger atoms. These orbital types have different shapes. The s orbitals are spherical, whilst the p orbitals have two lobes and are dumbbell shaped. The three p orbitals are all perpendicular to each other, in x, y, z directions around the nucleus, so are often labelled px, py and pz (Figure 1.1). There are five d and seven f orbitals and these have more complex shapes.
Figure 1.1 The three p orbitals.
The first shell (n = 1) can accommodate only two electrons (according to the 2n2 rule) and there is just the s orbital. The next element, that of atomic number 2 (helium), has two electrons, both occupying the s orbital. In lithium (atomic number 3), its first shell contains two s electrons, but because that is now full, the third electron goes into the s orbital of the next shell. This second shell (n = 2) now fills up, and after the s orbital is full, further electrons go into the p orbitals, as shown in Table 1.1. The p orbitals can hold a maximum of six electrons and after they are full the third shell (n = 3) begins to fill.
Table 1.1 Filling of the shells by electrons in the first 30 elements.
| Atomic number | Element | Orbit, n |
| 13 | Aluminium | 2s | 2s 6p | 2s 1p | |
| 14 | Silicon | 2s | 2s 6p | 2s 2p | |
| 15 | Phosphorus | 2s | 2s 6p | 2s 3p | |
| 16 | Sulphur | 2s | 2s 6p | 2s 4p | |
| 17 | Chlorine | 2s | 2s 6p | 2s 5p | |
| 19 | Potassium | 2s | 2s 6p | 2s 6p | 1s |
| 20 | Calcium | 2s | 2s 6p | 2s 6p | 2s |
| 21 | Scandium | 2s | 2s 6p | 2s 6p 1d | 2s |
| 30 | Zinc | 2s | 2s 6p | 2s 6p 10d | 2s |
Table 1.1 shows that once the three p orbitals of the third shell (n = 3) are full in argon, the electron of the next element, potassium, goes into the fourth shell, instead of continuing to fill the third shell, which can hold a maximum of 18 electrons. However, after calcium, further electrons go into the third shell, into its d orbital, of which there are five, thus holding a total of 10 electrons. After the d orbitals are all filled, at zinc, further electrons then fill up the 4p orbitals from gallium to krypton. Thereafter electrons go on to occupy the 5th orbit in a similar...
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