In 1803, John Dalton proposed an atomic theory. Three critical pieces of this theory included the following:
• Elements are composed of tiny particles called atoms.
• All atoms of a given element are identical.
• The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights.
J. J. Thomson developed one of the earliest models of an atom. In 1897 the British physicist discovered the electron in a series of experiments designed to study the nature of electric discharge in a vacuum cathode-ray tube. Cathode rays are constructed by sealing two electrodes in a vacuum glass tube. When the electrodes are connected to a high voltage, a beam of radiation is emitted from the negative electrode. These beams are called cathode rays. Thompson discovered that cathode rays travel in straight lines except when they are bent by electric or magnetic fields. Because the cathode rays bent away from a negatively charged plate, Thomson hypothesized that these cathode rays are made of negatively charged particles also. Today we call these negatively charged particles electrons. He concluded that electrons were found in all atoms and a thousand times smaller than protons. In 1904 Thomson proposed the plum pudding model of the atom as a sphere of positive matter in which negative electrons are embedded.
Ernest Rutherford was J. J. Thomson's first graduate student at the Cavendish Laboratory. Rutherford is known for his gold foil experiment of 1911. Alpha particles, which are heavy and have a positive charge, were fired at a very thin layer of gold foil. Most of the alpha particles passed straight through. According to the plum pudding model all of the particles should have slowed as they passed through the "pudding", but none should have been deflected. Rutherford noted that a few alpha particles were deflected back the way they came. Rutherford’s results lead him to the conclusion that the plum pudding model is wrong. He stated that:
• Atoms have a very small, dense nucleus with a positive charge and most of the atom's mass.
• The atom consists of mostly empty space.
• The electrons are attracted to the nucleus, but remain far outside it.
Niels Bohr improved on Rutherford’s model. The Bohr model can be summarized by the following four principles:
• Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits.
• Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.
• Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.
• The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron.
Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.
According to the passage, what is the correct order of discovery of the three subatomic particles?
A) proton, neutron, electron
B) electron, neutron, proton
C) electron, proton, neutron
D) proton, electron, neutron