<P> Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons . As the chemical properties of the elements were known to largely repeat themselves according to the periodic law, in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner . Groups of electrons were thought to occupy a set of electron shells about the nucleus . </P> <P> The Stern--Gerlach experiment of 1922 provided further evidence of the quantum nature of the atom . When a beam of silver atoms was passed through a specially shaped magnetic field, the beam was split based on the direction of an atom's angular momentum, or spin . As this direction is random, the beam could be expected to spread into a line . Instead, the beam was split into two parts, depending on whether the atomic spin was oriented up or down . </P> <P> In 1924, Louis de Broglie proposed that all particles behave to an extent like waves . In 1926, Erwin Schrödinger used this idea to develop a mathematical model of the atom that described the electrons as three - dimensional waveforms rather than point particles . A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at a given point in time; this became known as the uncertainty principle, formulated by Werner Heisenberg in 1926 . In this concept, for a given accuracy in measuring a position one could only obtain a range of probable values for momentum, and vice versa . This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and spectral patterns of atoms larger than hydrogen . Thus, the planetary model of the atom was discarded in favor of one that described atomic orbital zones around the nucleus where a given electron is most likely to be observed . </P> <P> The development of the mass spectrometer allowed the mass of atoms to be measured with increased accuracy . The device uses a magnet to bend the trajectory of a beam of ions, and the amount of deflection is determined by the ratio of an atom's mass to its charge . The chemist Francis William Aston used this instrument to show that isotopes had different masses . The atomic mass of these isotopes varied by integer amounts, called the whole number rule . The explanation for these different isotopes awaited the discovery of the neutron, an uncharged particle with a mass similar to the proton, by the physicist James Chadwick in 1932 . Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus . </P>

Who made the current model of an atom
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