Who Discovered Isotopes and Why do some chemical elements have different atomic weights?

Isotopes of an element are slightly different forms of that element.

Isotopes have the same chemical, physical, and electrical properties as the original element, but have a different number of neutrons in their nucleus. The discovery of isotopes created a new dimension and concept for physics and chemistry.

This discovery answered baffling problems that had stymied physics researchers studying radioactive elements. The study of isotopes became a key foundation for the development of atomic power and weapons. Isotopes are also critical to geology since carbon dating and other rock-dating techniques all depend on the ratios of specific isotopes.

This one discovery removed roadblocks to scientific progress, opened new fields of physics and chemistry research, and provided essential research tools to earth science research.

Frederick Soddy was born in 1877 in Sussex, England. In 1910 Soddy accepted a position at the University of Glasgow as a lecturer in radioactivity and chemistry.

The study of radioactive elements was still exciting and new. Radioactive elements were identified by differences in their mass, atomic charge, and radioactive properties, including the kinds and energies of different particles they emitted.

However, using this system, scientists had already identified 40 to 50 radioactive elements. But there existed only 10 to 12 places for all of these radioactive elements on the periodic chart of elements. Either Mendeleyev’s periodic chart was wrong or, for some unknown reason, radioactive elements fell outside the logic and order of the periodic chart.

Neither answer made any sense, and radioactive research ground to a halt.

Soddy decided to study the three known subatomic particles emitted by the various radioactive elements (alpha, beta, and gamma particles). Soddy found that alpha particle held a positive charge of two (as would two protons) and a mass equal to four protons. Gamma rays had neither charge nor mass, only energy, so they didn’t affect the nature of the atom at all.

Beta particles had no measurable mass but held a negative charge of one. They were apparently just electrons.

When an atom emitted a beta particle, it lost a negative charge. Soddy realized that was the same as gaining a positive charge. Emit an alpha particle and lose two positive charges from the nucleus. Emit a beta particle and gain one.

Because the periodic table was organized by the number of protons in the nucleus of an atom, from the lightest element (hydrogen) up to the heaviest known element (uranium), Soddy realized that the emission of an alpha particle would, in effect, shift the atom two spaces to the left on the periodic chart and the emission of a beta particle shifted it one place to the right.

It must be, he concluded, that atoms of many elements could exist in several different spaces on the periodic chart. Soddy used new spectrographic research techniques (discovered by Gustav Kirchhoff and Robert Bunsen in 1859) to show that, even though they had a different atomic mass and so occupied different spaces on the periodic chart, atoms of uranium and thorium were still the same, original element.

This meant that more than one element could occupy the same spot on the periodic chart and that atoms of one element could occupy more than one spot and still be the same, original element. Soddy named the versions of an element that occupied spots on the periodic chart, other than that element’s “normal” spot, isotopes, from the Greek words meaning “same place.”

Later that same year (1913), American chemist Theodore Richards measured the atomic weights of lead isotopes resulting from the radioactive decay of uranium and of thorium and proved Soddy’s theory to be true.

However, Soddy’s explanation of his discovery was not completely accurate. Chadwick’s discovery of the neutron (in 1932) was needed to correct Soddy’s errors and to complete the understanding of Soddy’s concept of isotopes.

Soddy had tried to explain his isotopes using only protons and electrons. Chadwick discovered that as many neutrally charged neutrons existed in the nucleus as did positively charged protons. Gaining or losing neutrons didn’t change the electric charge of or the properties of the element (since elements were defined by the number of protons in the nucleus). It did, however, change the atomic mass of the atom and so created an isotope of that element.

Soddy discovered the concept of isotopes. But an understanding of neutrons was needed in order to fully understand them.

Isotopes are more important than most people think. Every ancient rock, fossil, human remain, or plant ever dated was dated using isotopes of various elements. Natural radioactivity is created by isotopes. The atomic bomb uses an isotope of uranium.