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1 ‡ Christian Friedrich Schönbein (1799–1868) was born in Metzingen in Swabia, Germany and served as professor at the University of Basel from 1835 until 1868. He is best known for his discovery of ozone.

2 * We will be using System International Units as much as practicable throughout the book. A list of these units and their abbreviations can be found in the Appendix.

3 † Randomness can affect the outcome of any experiment (though the effect might be slight). By definition, the effect of this randomness cannot be predicted. Where the effects of randomness are large, one performs a large collection, or ensemble, of experiments and then considers the average result.

4 † The motion of Mercury is an exception: its motion differs slightly from predictions based on Newtonian mechanics. But relativity theory does accurately predict its motion. This successful prediction was one of several that led physicists to accept Einstein's new theory.

5 ‡ Dmitri Ivanovich Mendeleyev was born in Tobolsk, Russia in 1834. He became professor of chemistry at St Petersburg in 1866. His periodic table was the sort of discovery that noble prizes are awarded for, but it came before the prize was established. He was honored, however, by having element number 101, Medelevium, named for him. Mendeleyev died in 1906.

6 † By convention, the mass number, which is the sum of protons and neutrons in the nucleus, of an isotope is written as a preceding superscript. However, for historical reasons, it is often pronounced “helium–4.” Note also that the atomic number or proton number can be readily deduced from the chemical symbol (atomic number of He is 2). The neutron number can be found by subtracting the proton number from the mass number. Thus, the symbol 4He gives a complete description of the nucleus of this atom.

7 ‡ The actual mass of an atom depends on the number of electrons and the nuclear binding energy as well as the number of protons and neutrons. However, the mass of the electron is over 1000 times less than the mass of the proton and neutron, which have nearly identical masses, and the effect of nuclear binding energy on mass was too small for nineteenth-century chemists to detect.

8 * It is often convenient to think of the electrons orbiting the nucleus much as the planets orbit the Sun. This analogy has its limitations. The electron's position cannot be precisely specified as can a planet's. In quantum mechanics, the Schrödinger wave function, ψ (or more precisely, ψ2) determines the probability of the electron being located in a given region about the atom. As an example of failure of the classical physical description of the atom, consider an electron in the 1s orbital. Both quantum number specifying angular momentum, l and m, are equal to 0, and hence the electron has 0 angular momentum, and therefore cannot be in an orbit in the classical sense.