Bohr was born in Copenhagen on October 7, 1885. His father was a famous medicine professor. He has also a brother who graduated from Copenhagen University. His brother was a mathematic professor. Niels Bohr had six sons. He left for England to begin postdoctoral work under J. J. Thomson. After a few months, he left Cambridge to join Ruthherford at Manchester. In 2013, Bohr thought about electrons orbit around the atomic nucleus. He suggested that electron orbits were at a fixed a fixed energy orbit referred as an energy level. During his life, Niels Bohr received a lot of medals of honour and also he received Noble prize in 1922. He has a very famous quote: “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.” Niels Bohr died in Copenhagen on November 18, 1962.Democritus’s Atomic Model:This is Democritus’ atomic theory exactly:All matter consists of invisible particles called atoms.Atoms are indestructible.Atoms are solid but invisible.Atoms are homogenous.Atoms differ in size, shape, mass, position, and arrangement.Solids are made of small, pointy atoms.Liquids are made of large, round atoms.Oils are made of very fine, small atoms that can easily slip past each other. This was Democritus’ atomic model. It was simply a round sphere with no electrons, protons, or neutrons. Democritus created the first atomic model. His contribution helped people with understanding the idea of an atom, and helped other scientists further look into the science of the atom and its generic makeup.Dalton’s Atomic ModelDalton’s atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.Dalton based his theory on the law of conservation of mass and the law of constant composition.The first part of his theory states that all matter is made of atoms, which are indivisible.The second part of the theory says all atoms of a given element are identical in mass and properties.The third part says compounds are combinations of two or more different types of atoms.The fourth part of the theory states that a chemical reaction is a rearrangement of atoms.Parts of the theory had to be modified based on the discovery of subatomic particles and isotopes.Thomson’s Atomic Model: Rutherford’s Atomic Model:Rutherford atomic model, also called nuclear atom or planetary model of the atom, description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford. The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.Niels Bohr’s Atomic Model:Bohr atomic model, description of the structure of atoms, especially that of hydrogen, proposed (1913) by the Danish physicist Niels Bohr. The Bohr model of the atom, a radical departure from earlier, classical descriptions, was the first that incorporated quantum theory and was the predecessor of wholly quantum-mechanical models. The Bohr model and all of its successors describe the properties of atomic electrons in terms of a set of allowed (possible) values. Atoms absorb or emit radiation only when the electrons abruptly jump between allowed, or stationary, states. Direct experimental evidence for the existence of such discrete states was obtained (1914) by the German-born physicists James Franck and Gustav Hertz.f) Modern Electron Cloud Atomic ModelA German scientist named Werner Heisenberg proposed the idea of the electron cloud model in 1927. It is the most recent model of the atom. Notice that instead of showing the paths of the orbit of the electrons it just shows a “cloud” around the nucleus. The cloud represents where any one electron could be at any time. This electron cloud model is based on quantum mechanics.