History of the discovery of atomic structure Beginning

History of the discovery of atomic structure

Beginning Man has always wanted to know, to understand how nature works. It is in his nature to ask questions and then to seek the answers. One such question is “What is the nature of matter that makes up the physical universe? ”

Start of the Quest for Answers Democritus – a Greek natural philosopher In 400 B. C. he did a mind experiment in an attempt to explain the nature of matter. His question was “If I take an object and repeatedly cut it in half, will there be a point that it cannot be cut any smaller? ”

His Answer YES! He decided there was such a point. He called that final particle “an atom”.

Democritus’s Atomic Theory 1. All matter consists of invisible particles called atoms (from the Greek “Atomos” for indivisible). 2. Atoms are indestructible. 3. Atoms are solid but invisible. 4. Atoms are homogenous. 5. Atoms differ in size, shape, mass, position, and arrangement.

John Dalton Brought Democritus’s Atomic Theory into the realm of scientific knowledge, instead of being relegated to the whims of the human mind. Presented his theory to the world in 1803.

John Dalton described atoms as tiny balls of material. He said the atoms of a particular element are all identical. These are original Dalton model atoms He used small wooden balls to model atoms and showed how they could combine to produce compounds

John Dalton’s Theory 1. All matter is made up of atoms 2. All atoms of the same element are identical. All atoms of different elements are different 3. Atoms can not be created nor destroyed only rearranged. (law of conservation of matter) 4. Atoms combine in fixed ratios to form compounds. (law of definite proportions) 5. Atoms combine in different ratios to form different compounds. (law of multiple proportions)

JJ Thomson In 1897, whilst studying cathode rays, JJ Thomson discovered tiny particles with a negative charge. These negative particles were given out by atoms and were much smaller than atoms. Thomson had discovered the existence of electrons. This discovery contradicted Dalton’s theory that atoms were solid spheres of matter. This led Thomson to propose a new model of the atom.

J. J. Thomson’s Experiment Using a cathode ray tube, he projected a beam of particles through a magnetic field and observed how they were deflected.

The plum pudding model Thomson suggested that an atom is a positively-charged sphere with negative electrons distributed throughout it. This model became known as the plum pudding model, because the electrons in the atom were thought to be like raisins in a plum pudding. Electrons had been proved to exist but there were still doubts about this model. 11 of 23 © Boardworks Ltd 2011

In 1911 Ernest Rutherford, (a New Zealand scientist working in Britain) carried out an experiment that proved that atoms were not solid lumps of material as thought by Thomson but were in fact mostly empty space with a very small solid centre called the nucleus. He came up with a new model which had a solid, positively charged centre and the negative electrons in ‘orbits’ around it. Rutherford’s Atomic Model

Geiger and Marsden’s experiment 13 of 23 © Boardworks Ltd 2011

The results of Geiger and Marsden’s experiment were: 2. Some alpha particles were slightly deflected by the gold foil. 3. A few alpha particles were bounced back from the gold foil. 1. Most alpha particles went straight through the gold foil, without any deflection. The experiment was carried out in a vacuum, so deflection of the alpha particles must have been due to the gold foil. How can these results be explained in terms of atoms? 14 of 23 © Boardworks Ltd 2011

It was almost as incredible as if you fired a 15 -inch shell at a piece of tissue paper and it came back and hit you.

Rutherford’s explanation 16 of 23 © Boardworks Ltd 2011

www. youtube. com/watch? v=XBq. Hkraf 8 i. E

Niels Bohr In 1913, Niels Bohr proposed an atom was like a tiny solar system (the nucleus was like the sun and the electrons like the planets).

Niels Bohr Electrons travel in discrete orbits defined by their angular momentum. This made it possible to calculate the energy levels of electrons and the frequency of the light emitted when electrons returned to the home state.

In 1920 Rutherford came up with the idea that atoms must contain a third particle. He thought this because the masses of atoms that were being measured were heavier than you would get from just the masses of protons and electrons added together. He said that this particle would have no charge and that this would make it very difficult to detect. Following work in Germany and France the Neutron was finally identified by James Chadwick working in Cambridge. He realised that an unusual radiation that had been observed in some experiments was in fact a neutral, sub-atomic particle.

James Chadwick Along with other scientists, Chadwick realized the atomic mass of an element, was often twice what it should be. This led to Chadwick conducting an experiment that proved the existence of neutrons.

James Chadwick In 1932, he proved the existence of the neutron, a particle having the same mass as a proton and no charge.