Physical Chemistry: A Very Short Introduction
One way to understand how a physical chemist thinks and contributes to chemistry is to start at the interior of an atom and then to travel out into the world of bulk matter. The interior of an atom is where much of the explanation of matter is to be found and it is here that a chemist is most indebted to physics. Within this realm, within an atom, explanations necessarily draw on quantum mechanics, that perplexing description of the behaviour of the very small. That quantum mechanics is central to their description should not be taken to be a warning that the rest of this chapter will be incomprehensible! I shall distil from that theory only the qualitative essence of what we need.
The ancient Greeks speculated that matter was composed of atoms. That was pure speculation unsupported by any experimental evidence and so cannot be regarded as the beginning of physical chemistry. Experimental evidence for atoms was accumulated by John Dalton (1766â€“1844) in the very early 19th century when the use of the chemical balance allowed quantitative measurements to be made on the reactions that matter undergoes. Dalton inferred the existence of atoms from his measurements but had no way of assessing their actual sizes. He had no notion that nearly two centuries later, in the late 20th century, scientists would at last be able to see them.
For a physical chemist, an atom consists of a central, tiny, massive, positively charged nucleus surrounded by a cloud of much lighter, negatively charged electrons. Chemists have little interest in the details of the structure of the nucleus itself and are content to think of it as a tightly bound collection of two types of fundamental particle, positively charged protons and electrically neutral neutrons. The number of protons in the nucleus, the atomâ€™s â€˜atomic numberâ€™, determines the identity of the element (1 for hydrogen, 2 for helium, and so on up to, currently, 118 for livermorium). The number of neutrons is approximately the same as the number of protons (none for ordinary hydrogen, 2 for ordinary helium, and about 170 for livermorium). This number is slightly variable, and gives rise to the different isotopes of the element. As far as a physical chemist is concerned, a nucleus is a largely permanent structure with three important properties: it accounts for most of the mass of the atom, it is positively charged, and in many cases it spins on its axis at a constant rate.
One particular nucleus will play an important role throughout this account: that of a hydrogen atom. The nucleus of the most common form of hydrogen is a single proton, a single ceaselessly spinning, positively charged fundamental particle. Although so simple, it is of the utmost importance in chemistry and central to the way that physical chemists think about atoms in general and some of the reactions in which they participate. There are two further isotopes of hydrogen: deuterium (â€˜heavy hydrogenâ€™) has an additional neutron bound tightly to the proton, and tritium with two neutrons. They will play only a slight role in the rest of this account, but each has properties of technical interest to chemists.
The electronic structure of atoms
Physical chemists pay a great deal of attention to the electrons that surround the nucleus of an atom: it is here that the chemical action takes place and the element expresses its chemical personality. The principal point to remember in this connection is that the number of electrons in the atom is the same as the number of protons in the nucleus. The electric charges of electrons and protons are equal but opposite, so this equality of numbers ensures that the atom overall is electrically neutral. Thus, a hydrogen atom has a single electron around its nucleus, helium has two, livermorium has a crowded 118, and so on.
Quantum mechanics plays a central role in accounting for the arrangement of electrons around the nucleus. The early â€˜Bohr modelâ€™ of the atom, which was proposed by Neils Bohr (1885â€“1962) in 1913, with electrons in orbits encircling the nucleus like miniature planets and widely used in popular depictions of atoms, is wrong in just about every respectâ€”but it is hard to dislodge from the popular imagination. The quantum mechanical description of atoms acknowledges that an electron cannot be ascribed to a particular path around the nucleus, that the planetary â€˜orbitsâ€™ of Bohrâ€™s theory simply donâ€™t exist, and that some electrons do not circulate around the nucleus at all.
Physical chemists base their understanding of the electronic structures of atoms on SchrÃ¶dingerâ€™s model of the hydrogen atom, which was formulated in 1926. Erwin SchrÃ¶dinger (1887â€“1961) was one of the founders of quantum mechanics, and in what he described as an episode of erotic passion whilst on vacation with one of his mistresses, he formulated the equation that bears his name and solved it for the location of the electron in a hydrogen atom. Instead of orbits, he found that the electron could adopt one of a variety of wave-like distributions around the nucleus, called wavefunctions, each wave corresponding to a particular energy.
Physical chemists adopt SchrÃ¶dingerâ€™s solutions for hydrogen and adapt it as the starting point for their discussion of all atoms. That is one reason why the hydrogen atom is so central to their understanding of chemistry. They call the wave-like distributions of electrons atomic orbitals, suggesting a link to Bohrâ€™s orbits but indicating something less well-defined than an actual path.
We shall need some nomenclature. The lowest energy atomic orbital in a hydrogen atom is an example of an s-orbital. An electron in an s-orbital (physical chemists say â€˜inâ€™ when they mean having a distribution described by a particular orbital) can be pictured as a spherical cloudlike distribution that is densest at the nucleus and declines sharply with distance from it (Figure 1). That is, the electron is most likely to be found at the nucleus, and then progressively less likely to be found at points more distant from it. Incidentally, an electron in this orbital has no sense of rotation around the nucleus: it is effectively just hovering over and surrounding the nucleus. An atom is often said to be mostly empty space. That is a remnant of Bohrâ€™s model in which a point-like electron circulates around the nucleus; in the SchrÃ¶dinger model, there is no empty space, just a varying probability of finding the electron at a particular location
List of illustrations
1 Matter from the inside
2 Matter from the outside
3 Bridging matter
4 States of matter
5 Changing the state of matter
6 Changing the identity of matter
7 Investigating matter
Appendix: the Periodic Table
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