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Encyclopedia of Physical Science and Technology EN011H-551 July 25, 2001 18:33

Periodic Table (Chemistry) 679

1. However, one atom in 6500 has a relative mass of 2, spectral lines of hydrogen. Here, at last, was an atomic
an isotope called deuterium. In addition, there is a third explanation for a phenomenon which had been used to
isotope of hydrogen, a radioactive form called tritium, identify the rare gases and many other elements.
which has a mass of 3. The atomic mass for hydrogen, The experimentalist Henry G. J. Moseley (1887–1915)
1.0079, includes small contributions due to deuterium and was measuring the frequency of spectral lines—not in the
tritium. For elements where the natural mixture of isotopes visible region of the spectrum, but in the high-frequency
is less strongly biased towards a single species, the average X-ray region. He found that the frequencies of the most in-
atomic masses deviate considerably from whole numbers. tense lines fell into a pattern. When the square roots of the
Chlorine (atomic mass 35.453) and copper (atomic mass frequencies are plotted against the atomic masses of the
65.546) are cases in point. elements, a reasonably straight line is obtained. However,
the line is not straight enough to satisfy a scientist’s pas-
sion for order. So instead, Moseley plotted the same data
B. Nuclear Charge and Atomic Numbers
against the ordinal numbers of the elements. These points
The existence of isotopes clearly indicates that, the peri- exhibit almost perfect linearity, hence the ordinal number
odic law notwithstanding, the properties of the elements or atomic number, as Moseley called it, must represent
cannot be a simple function of atomic mass. Atomic mass a real physical property. “This quantity,” he wrote, “can
is not a sufficiently fundamental property. However, if only be the charge on the central positive nucleus.” The
atomic mass does not determine the identity and prop- atomic number, not the atomic mass, determines the ele-
erties of an element, what does? The answer again came mentary identity of an atom. Thanks to Moseley’s work,
from Rutherford’s laboratory. In 1909 he performed one and that of his successors, we now know that the atomic
of his most famous experiments, firing a stream of alpha number of an element is equal to the positive charge on the
particles at a thin gold foil and observing their trajectories. nucleus, determined by the number of positive particles or
Most of the positively charged projectiles passed through protons it contains. This number also equals the number
the foil with very little change in path. Occasionally, how- of electrons in the electrically neutral atom. It increases
ever, the alpha particles would be deflected through large integrally from hydrogen, atomic number 1, through ura-
angles, in some cases almost completely reversing direc- nium, atomic number 92, and beyond. All the isotopes of
tion. “It was quite the most incredible event that has ever any given element have the same atomic number, that is,
happened to me in my life,” Rutherford observed, as an old the same number of electrons and protons per atom. Thus,
and honored scientist looking back at this crucial experi- all atoms of carbon (atomic number 6) are made up of six
ment. “It was almost as incredible as if you fired a 15-inch electrons surrounding a nucleus containing six protons.
shell at a piece of tissue paper and it came back and hit However, carbon atoms differ in mass because of differ-
you.” From this bizarre behavior, Rutherford calculated ences in the number of neutrons, the neutral particles also
that an intense positive charge and most of the mass of the found in atomic nuclei. Therefore, the different isotopes
atom must be concentrated in a very small region of space. of carbon have different amounts of neutrons in the nuclei.
He assumed this extremely dense region would be at the For example, a carbon-13 atom is made of seven neutrons,
center, or nucleus, of the atom. Thus was born the familiar six protons, and six electrons, as opposed to carbon-12,
and somewhat inaccurate picture of the atom as a minia- which has six of each type of particle.
ture solar system, with electrons orbiting the positive and The fact that the atomic number of an element is a more
massive nucleus. fundamental property than its atomic mass means that the
Four years later, two young men working with Ruther- periodic law must be modified: the properties of the ele-
ford made giant theoretical and experimental strides to- ments are periodic functions of their atomic numbers, not
wards elaborating that model. The theoretician was the their atomic masses. This dependence also explains the
DaneNielsBohr(1885–1962)whoborrowedarevolution- several instances in the periodic table where the correct
ary idea from the German physicists Max Planck (1858– elementary placement results in a deviation from the nor-
1947) and Albert Einstein (1879–1955). These two had mal trend of increasing atomic mass. Argon (Ar) has an
previously postulated that energy is quantized. Bohr then atomic mass of 39.948, while potassium (K) has an atomic
assumed that the energy of the single electron in a hydro- mass of 39.0983. Yet no one who knows anything about
gen atom is also quantized; that is, it can only have certain the properties of these elements would think of putting
values, each corresponding to a circular orbit of specif- the highly reactive metal in Group 18 along with the inert
ically defined radii. Moreover, he derived mathematical gases or vice versa. Although argon does have a higher av-
expressions for both the energy and the radius. Bohr’s erage atomic mass than potassium, its atomic number is 18
calculations reproduced with amazing accuracy the fre- andthatofpotassiumis19.Similaratomicmassinversions
quencies which had been observed and measured for the occur for tellurium (Te, atomic number 52, atomic mass