The discovery of x-rays by William Conrad Roentgen in
November of 1895 excited the imagination of a generation of scientists who rushed to study
this phenomenon. Within a few months, Henri Becquerel found that both uranium metal and
salts of this element gave off a different form of radiation, which could also pass
through solids. By 1898, Marie Curie found that compounds of thorium were also
"radioactive." After pain-staking effort she eventually isolated two more
radioactive elements
polonium and
radiumfrom ores that contained
uranium.
In 1899 Ernest Rutherford found that there were at least
two different forms of radioactivity when he studied the absorption of radioactivity by
thin sheets of metal foil. One, which he called alpha (
) particles, were absorbed by metal foil that was a
few hundredths of a centimeter thick. The other, beta (� particles, could pass
through 100 times as much metal foil before they became absorbed. Shortly thereafter, a
third form of radiation, gamma (
) rays, was discovered that could penetrate as much as
several centimeters of lead.
The results of early experiments on these three forms of
radiation are shown in the figure below. The direction in which -particles were deflected
by an electric field suggested that they were positively charged. The magnitude of this
deflection suggested that they had the same charge-to-mass ratio as an He2+
ion. To test the equivalence between -particles and He2+ ions, Rutherford built an
apparatus that allowed -particles
to pass through a very thin glass wall into an evacuated flask that contained a pair of
metal electrodes. After a few days, he connected these electrodes to a battery and noted
that the gas in the flask did indeed give off the characteristic emission spectrum of
helium.
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| The effect of an electric field on -, �, and -radiation. |
Experiments with electric and magnetic fields demonstrated that �/i>-particles were negatively charged. Furthermore, they had the same charge-to-mass ratio as an electron. To date, no detectable difference has been found between �/i>-particles and electrons. The only reason to retain the name "�/i>-particle" is to emphasize the fact that these particles are ejected from the nucleus of an atom when it undergoes radioactive decay.
The fact that -rays are not deflected by either electric or magnetic fields
suggests that these rays don't carry an electric charge. Since they travel at the speed of
light, they are classified as a form of electromagnetic radiation that carries even more
energy than x-rays.
At the turn of the century, when radioactivity was discovered, atoms were assumed to be indestructible. Ernest Rutherford and Frederick Soddy, however, found that radioactive substances became less active with time, as shown in the figure below. More importantly, they noticed that radioactivity was always accompanied by the formation of atoms of a different element. By 1903, they concluded that radioactivity was accompanied by a change in the structure of the atom. They therefore assumed that radiation was emitted when an element decayed into a different kind of atom.
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| Growth and decay curves reported by Rutherford and Soddy for "uranium X" produced when uranium undergoes radioactive decay. Curve B shows decay in the activity after "uranium X" is extracted from uranium. Curve A shows growth in the activity of uranium as "uranium X" is replenished by radioactive decay. |
By 1910, 40 radioactive elements had been isolated that were associated with the process by which uranium metal decayed to lead. This created a problem, however, because there was space for only 11 elements between lead and uranium. In 1913, Kasimir Fajans and Fredick Soddy proposed an explanation for these results based on the following rules.
- �/i>-particles are emitted when an element is formed that belongs one space to the right in the periodic table. Actinium (Z = 89) emits a �/i>-particle when it decays to form thorium (Z = 90).
- Radioactive elements that fall in the same place in the periodic table are different forms of the same element. The radioactive thorium produced by the
Soddy proposed the name isotope to describe different radioactive atoms that occupy the same position in the periodic table. J. J. Thomson and Francis Aston then used a mass spectrometer to show that isotopes are atoms of the same element that have different atomic masses.
The discovery of the electron in 1897 by J. J. Thomson
suggested that there was an internal structure to the "indivisible" building blocks of
matter known as atoms. This raised an obvious question: How many electrons does an atom
contain? By studying the scattering of light, x-rays, and -particles, Thomson
concluded that the number of electrons in an atom was between 0.2 and 2 times the weight
of the atom.
In 1911, Rutherford concluded that the scattering of -particles by extremely
thin pieces of metal foil could be explained by assuming that all of the positive charge
and most of the mass of the atom were concentrated in an infinitesimally small fraction of
the total volume of the atom, for which he proposed the name nucleus. Rutherford's
data also suggested that the nucleus of a gold atom carries a positive charge that is
about 80 times the charge on an electron.
The discovery of the neutron in 1932 explained the discrepancy between the charge on the nucleus and the mass of an atom. A neutral gold atom that has a mass of 197 amu consists of a nucleus that contains 79 protons and 118 neutrons surrounded by 79 electrons. By convention, this information is specified by the following symbol, which describes the only naturally occurring isotope of gold.
This convention can also be applied to subatomic particles. The only difference is the use of lowercase letters to identify the particle.
Because anyone with access to a periodic table can find the atomic number of an element, a shorthand notation is often used that reports only the mass number of the atom and the symbol of the element. The shorthand notation for the naturally occurring isotope of gold is 197Au.
| Practice Problem 1: Determine the number of protons, neutrons, and electrons in a 210Pb2+ ion. |
A particular combination of protons and neutrons is called a nuclide. Nuclides with the same number of protons are called isotopes. Nuclides with the same mass number are isobars. Nuclides with the same number of neutrons are isotones.
| Practice Problem 2: Classify the following sets of nuclides as examples of either isotopes, isobars, or isotones: (a) 12C, 13C, and 14C (b) 40Ar, 40K, and 40Ca (c) 14C, 15N, and 16O |