Atomic models
- 1. Atom Models From Democritus to Bohr’s
- 2. •Introduction 460-370 B.C He asked if matter could be divided into smaller and smaller pieces forever Ernest Rutherford (1871–1937) He described the structure of atom Neils Bohr (1885–1962) He explained how atomic spectra worked DOMOCRITUS John Dalton (1766-1844) He proposed the Atomic theory of matter based on his experimental observationsDALTON BOHR THOMSON RUTHERFORD John Thomson (1856-1940) He proposed the plum pudding model of atom
- 3. •Timeline of Atomic Models Timeline of Atomic Structure 460 – 370 BC 1766 – 1844 Democritus proposes the 1st atomic theory John Dalton proposes his atomic theory in 1803 1856 – 1940 J.J. Thomson discovers the electron and proposes the Plum Pudding Model in 1897 1871 – 1937 Ernest Rutherford performs the Gold Foil Experiment in 1909 1885 – 1962 Niels Bohr proposes the Bohr Model in 1913
- 4. • The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago. He asked: • Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? • His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. •DEMOCRITUS'S ATOMIC THEORY
- 6. He proposed the Atomic theory of matter based on his experimental observations. The main postulates of Dalton’s atomic theory are as follows. To Dalton : All matter is made of atoms. Atoms are indivisible and => indestructible. All atoms of a given element are identical in mass and properties. Compounds are formed by a combination of two or more different kinds of atoms A chemical reaction is a rearrangement of atoms. Atoms can be neither created nor destroyed. He also calculated the atomic weight of various elements •DALTON'S ATOMIC THEORY
- 7. Thomson's atomic theory proposed a model of atom which is known as plum pudding model • In the year 1897 He totally changed the view of an atom by discovering electron. Thomson’s atomic theory suggested that the atom is not indivisible as it was of smaller pieces – electrons and protons. • An atom consists of a sphere of positive charge with negatively charged electron embedded in it. • The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge. •THOMSOM'S ATOMIC THEORY
- 9. • Ernest Rutherford presented a paper , accurately describing the structure of the atom. Based on an experiment he had performed - with totally unexpected results - he realized that the atom must have almost all of its mass concentrated at its center, in a nucleus, with the vast majority of the atom consisting chiefly of empty space. Two years earlier, in 1909, he had conducted an experiment with two other scientists, in which they studied the deflection angles of "alpha particles" that they shot through a microscopically thin layer of gold. Alpha particles are just helium atoms stripped of their 2 electrons. • He showed that while the nucleus contains virtually all of the mass of the atom, it only takes up one-billionth of the volume of the atom, an inconceivably tiny amount. Much smaller particles - electrons - orbit the nucleus at a great distance, relatively speaking. •RATHERFORD'S ATOMIC THEORY
- 11. •BOHR'S ATOMIC THEORY Neils Bohr refined Rutherford's model in 1913 by proposing that electrons: • Orbit the nucleus without losing energy; • Could move only in fixed orbits of specific energies. • Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus.
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