Atomic Structure

ATOMIC STRUCTURE GROUP MEMBERS Qulb-e-Abbas Faqiha Rizvi

ATOM MEAN

• Democritus a fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called atoms (Greek for uncuttable).

John Dalton • From his experiments and observations, he suggested that atoms were like tiny, hard balls. Each chemical An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances. element had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways. In many ways, Dalton's ideas are still useful today. For example, they help us to understand elements, compounds, and molecules name it as Billiard Ball Model

J.J. Thomson • At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any • An atom is the smallest particle of an element that can still be defined as that element. atom. When he discovered the • Electrons are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells). electron, Thomson was experimenting by applying high voltages to gases at low pressure.

Thomson proposed model for the atom (Plumb Pudding Model ) • He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).

Thomson Model of the Atom

Ernest Rutherford • The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that

• In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (centre of the atom )

Niels Bohr • The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the • The nucleus is the centre of an atom, containing protons and neutrons. nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

Bohr`s Atomic Model

The Refined Bohr Model

Electron Cloud Model (1920's)• an atom consists of a dense nucleus composed of protons and neutrons surrounded by electrons that exist in different clouds at the various energy levels. Erwin Schrodinger and Werner Heisenburg developed probability functions to determine the regions or clouds in which electrons would most likely be found.