Key discoveries about the atomJoseph J. Thomson - kd 1897 to 1899

Thomson's claim to be the discoverer of the electron rests on two key observations. First, he found that the value of e/m was of the order of 1000 times larger than its value for the lightest particle then known, which was the hydrogen ion in electrolysis. On the assumption that the charge was the same for both particles, this meant that the mass of the new particle was of the order of 1000 times less than that of the hydrogen atom. (We now know that it is 1837 times less.) Secondly, he repeated the measurements for different gases and for different materials for the electrodes, and found that the value of e/m was independent of both the nature of the gas and the material of the electrode. In other words the particle he had discovered was a universal constituent of matter.

Hans Geiger, Ernest Marsden and Ernest Rutherford - kd 1909

In Ernest Rutherford's laboratory, Hans Geiger and Ernest Marsden (a 20 yr old undergraduate student) carried out experiments to study the scattering of alpha particles by thin metal foils. In 1909 they observed that alpha particles from radioactive decays occasionally scatter at angles greater than 90°, which is physically impossible unless they are scattering off something more massive than themselves. This led Rutherford to deduce that the positive charge in an atom is concentrated into a small compact nucleus. During the period 1911-1913 in a table-top apparatus, they bombarded the foils with high energy alpha particles and observed the number of scattered alpha particles as a function of angle. Based on the Thomson model of the atom, all of the alpha particles should have been found within a small fraction of a degree from the beam, but Geiger and Marsden found a few scattered alphas at angles over 140 degrees from the beam.

Rutherford's remark "It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you." The scattering data was consistent with a small positive nucleus that repelled the incoming positively charged alpha particles. Rutherford worked out a detailed formula for the scattering (Rutherford formula), which matched the Geiger-Marsden data to high precision.

Henry Moseley - kd 1913

Henry Moseley wan an English physicist who determined the number of positive charges in the nucleus (the first concept of the atomic number) by measuring the wavelength of the x-rays given off by certain metals in 1913.

Henry Moseley was an English physicist born in 1887. As a student working for Rutherford at the University of Manchester, Moseley determined that elements fell into a precise order on Mendeleev's period table. By studying the x-rays emitted by certain consecutive elements, under specific conditions, he concluded that the positive charge of an atom had characteristic values. "

By photographing the x-ray spectrum of 12 elements, 10 of which occupied consecutive places on the periodic table, he discovered that frequency of the K-lines in the spectrum of each element was directly proportionate to the squares of the integer representing the position of each successive element in the table." He said "there is a fundamental quality, which increases by regular steps as we pass from one element to the next." This he called the Atomic Number.

James Chadwick - kd 1932

In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical cha rge. Chadwick observed that beryllium, when exposed to bombardment by alpha particles, released an unknown radiation that in turn ejected protons from the nuclei of various substances. Chadwick interpreted this radiation as being composed of particles of mass approximately equal to that of the proton, but without electrical charge--neutrons.

This discovery provided a new tool for inducing atomic disintegration, since neutrons, being electrically uncharged, could penetrate undeflected into the atomic nucleus. Chadwick was knighted in 1945.