<P> In 1917 Rutherford bombarded nitrogen gas with alpha particles and observed hydrogen nuclei being emitted from the gas (Rutherford recognized these, because he had previously obtained them bombarding hydrogen with alpha particles, and observing hydrogen nuclei in the products). Rutherford concluded that the hydrogen nuclei emerged from the nuclei of the nitrogen atoms themselves (in effect, he had split a nitrogen). </P> <P> From his own work and the work of his students Bohr and Henry Moseley, Rutherford knew that the positive charge of any atom could always be equated to that of an integer number of hydrogen nuclei . This, coupled with the atomic mass of many elements being roughly equivalent to an integer number of hydrogen atoms - then assumed to be the lightest particles - led him to conclude that hydrogen nuclei were singular particles and a basic constituent of all atomic nuclei . He named such particles protons . Further experimentation by Rutherford found that the nuclear mass of most atoms exceeded that of the protons it possessed; he speculated that this surplus mass was composed of previously - unknown neutrally charged particles, which were tentatively dubbed "neutrons". </P> <P> In 1928, Walter Bothe observed that beryllium emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles . It was later discovered that this radiation could knock hydrogen atoms out of paraffin wax . Initially it was thought to be high - energy gamma radiation, since gamma radiation had a similar effect on electrons in metals, but James Chadwick found that the ionization effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction . In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring the energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton . Chadwick now claimed these particles as Rutherford's neutrons . For his discovery of the neutron, Chadwick received the Nobel Prize in 1935 . </P> <P> In 1924, Louis de Broglie proposed that all moving particles--particularly subatomic particles such as electrons--exhibit a degree of wave - like behavior . Erwin Schrödinger, fascinated by this idea, explored whether or not the movement of an electron in an atom could be better explained as a wave rather than as a particle . Schrödinger's equation, published in 1926, describes an electron as a wavefunction instead of as a point particle . This approach elegantly predicted many of the spectral phenomena that Bohr's model failed to explain . Although this concept was mathematically convenient, it was difficult to visualize, and faced opposition . One of its critics, Max Born, proposed instead that Schrödinger's wavefunction described not the electron but rather all its possible states, and thus could be used to calculate the probability of finding an electron at any given location around the nucleus . This reconciled the two opposing theories of particle versus wave electrons and the idea of wave--particle duality was introduced . This theory stated that the electron may exhibit the properties of both a wave and a particle . For example, it can be refracted like a wave, and has mass like a particle . </P>

Historical development of the modern model of the atom