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A theory of chemical combination was first stated by John Dalton in 1804. It involves the following postulates: (1) Elements consist of indivisible small particles (atoms). (2) All atoms of the same element are identical; different elements have different types of atoms. (3) Atoms can neither be created nor destroyed. -
The first atomic theory was discovered back in 1804 by John Dalton
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The first atomic theory model was made by John Dalton.
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The atomic theory helped us understand what an atom is and how it behaves. Before the theory was developed, scientists thought that atoms were indivisible, but as more evidence was gathered, it became clear that atoms were made up of smaller particles. The theory helped us to understand how these particles interact with each other and how they can combine to form molecules.
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In 1897, the British physicist J. J. Thomson (1856–1940) proved that atoms were not the most basic form of matter. He demonstrated that cathode rays could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles
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It was Thomson who made the breakthrough however, concluding through his experimentation that particles making up the rays were 1,000 times lighter than the lightest atom, proving that something smaller than atoms existed. Ten years later, Ernest Rutherford discovered that atoms have a very dense nucleus, which contains protons. In 1932, James Chadwick discovered the neutron, another particle located within the nucleus.
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Thomson atomic model, the earliest theoretical description of the inner structure of atoms, was proposed about 1900 by William Thomson (Lord Kelvin) and strongly supported by Sir Joseph John Thomson, -
In 1911, Rutherford described the atom as having a tiny, dense, and positively charged core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. The light, negatively charged, electrons circulated this nucleus, much like planets revolving around the Sun. -
The Bohr model postulates that electrons orbit the nucleus at fixed energy levels. Orbits further from the nucleus exist at higher energy levels. When electrons return to a lower energy level, they emit energy in the form of light. -
In 1924 Louis de Broglie introduced the idea that particles, such as electrons, could be described not only as particles but also as waves. This was substantiated by the way streams of electrons were reflected against crystals and spread through thin metal foils. -
The quantum mechanical model of the atom was made by Erwin Schrödinger, an Austrian physicist, who tells us that electrons orbit the atom in random ways and pictures the atom as being surrounded by an electron cloud containing all the possible places that the electron might be. -
James Chadwick, through his extensive research, found that the atoms consisted of protons and electrons and another sub-atomic particle called the neutron. He further discovered that the neutron has the same mass as that of a proton, and it occupies the area called the nucleus of the atom.