6.4 Bohr’s model of the hydrogen atom  (Page 3/37)

The key to unlocking the mystery of atomic spectra is in understanding atomic structure. Scientists have long known that matter is made of atoms. According to nineteenth-century science, atoms are the smallest indivisible quantities of matter. This scientific belief was shattered by a series of groundbreaking experiments that proved the existence of subatomic particles, such as electrons, protons, and neutrons.

The electron was discovered and identified as the smallest quantity of electric charge by J.J. Thomson in 1897 in his cathode ray experiments, also known as β-ray experiments: A β -ray is a beam of electrons. In 1904, Thomson proposed the first model of atomic structure, known as the “plum pudding” model, in which an atom consisted of an unknown positively charged matter with negative electrons embedded in it like plums in a pudding. Around 1900, E. Rutherford , and independently, Paul Ulrich Villard, classified all radiation known at that time as $\alpha$ -rays , β -rays, and γ -rays (a γ -ray is a beam of highly energetic photons). In 1907, Rutherford and Thomas Royds used spectroscopy methods to show that positively charged particles of $\alpha$ -radiation (called $\alpha$ -particles ) are in fact doubly ionized atoms of helium. In 1909, Rutherford, Ernest Marsden, and Hans Geiger used $\alpha$ -particles in their famous scattering experiment that disproved Thomson’s model (see Linear Momentum and Collisions ).

In the Rutherford gold foil experiment (also known as the Geiger–Marsden experiment), $\alpha$ -particles were incident on a thin gold foil and were scattered by gold atoms inside the foil (see Types of Collisions ). The outgoing particles were detected by a $360\text{°}$ scintillation screen surrounding the gold target (for a detailed description of the experimental setup, see Linear Momentum and Collisions ). When a scattered particle struck the screen, a tiny flash of light (scintillation) was observed at that location. By counting the scintillations seen at various angles with respect to the direction of the incident beam, the scientists could determine what fraction of the incident particles were scattered and what fraction were not deflected at all. If the plum pudding model were correct, there would be no back-scattered $\alpha$ -particles. However, the results of the Rutherford experiment showed that, although a sizable fraction of $\alpha$ -particles emerged from the foil not scattered at all as though the foil were not in their way, a significant fraction of $\alpha$ -particles were back-scattered toward the source. This kind of result was possible only when most of the mass and the entire positive charge of the gold atom were concentrated in a tiny space inside the atom.

In 1911, Rutherford proposed a nuclear model of the atom    . In Rutherford’s model, an atom contained a positively charged nucleus of negligible size, almost like a point, but included almost the entire mass of the atom. The atom also contained negative electrons that were located within the atom but relatively far away from the nucleus. Ten years later, Rutherford coined the name proton for the nucleus of hydrogen and the name neutron for a hypothetical electrically neutral particle that would mediate the binding of positive protons in the nucleus (the neutron was discovered in 1932 by James Chadwick ). Rutherford is credited with the discovery of the atomic nucleus; however, the Rutherford model of atomic structure does not explain the Rydberg formula for the hydrogen emission lines.