The Changing Model of the Atom

The Changing Model of the Atom From Democritus to Bohr

http://www.youtube.com/watch?v=MdDjnVlYKpU • http://www.youtube.com/watch?NR=1&feature=endscreen&v=vUzTQWn-wfE

Democritus 370 – 460 BCE

Democitus 460 – 370 BCE • Democritus described atoms as being indestructible. • Democritus thought that there are many different kinds of atoms, each distinct in shape and size and that all atoms move around in space.

claimed that there was no smallest part of matter different substances were made up of different proportions of fire, air, earth, and water. The four element theory Aristotle 400 BCE

John Dalton

John Dalton 1850 • Dalton stated that all matter is made of indivisible and indestructible atoms, which differ from element to element. • The billiard ball theory

J. J. Thompson Cathode ray tube

J.J. Thompson 1897 • For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen • Thomson found that the glowing stream would bend toward a positively charged electric plate. • Thomson theorized, that the stream was made up of small particles, pieces of atoms that carried a negative charge. • These particles were later named electrons.

Thompson’s Plum Pudding TheoryOr Rasin Bun Theory

Ernest Rutherford

Ernest Rutherford • Rutherford fired tiny alpha particles at solid objects such as gold foil. • He found that most of the alpha particles passed right through the gold foil • a small number of alpha particles passed through at an angle (as if they had bumped up against something) • and some bounced straight back like a tennis ball hitting a wall.

Rutherford’s gold foil experiment

Rutherford’s Planetary model • In 1911, Rutherford proposed a revolutionary view of the atom. • He suggested that the atom consisted of a small, dense core of positively charged particles in the center (or nucleus) of the atom and mostly empty space • Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the center and the electrons revolving around the nucleus.

Rutherford’s planetary model

James Chadwick 1932 • Chadwick discovered a third type of subatomic particle, which he named the neutron. • Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. • Neutrons always reside in the nucleus of atoms and they are about the same size as protons. • Neutrons do not have any electrical charge; they are electrically neutral.

James Chadwick

Neils Bohr

Bohr’s Model 1913 • The Bohr model of the atom says that electrons travel in discrete orbits around the atom's nucleus. • His theory was radical and unacceptable to most physicists at the time

Bohr’s model

The electron cloud model uses the basic idea of Bohr’s model except that the electrons are not found in distinct orbits but their position can be thought of as in a cloud that has a particular energy The Electron Cloud Model

http://videos.howstuffworks.com/science/atoms-videos-playlist.htm#video-29292http://videos.howstuffworks.com/science/atoms-videos-playlist.htm#video-29292 • http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

The Changing Model of the Atom