Chemistry: The Atom

Front 1

John Dalton Atomic theory

Back 1

• All matter is composed of tiny particles called atoms

• All atoms are indivisible I.e they cannot be broken down into simpler particles

• Atoms cannot be created or destroyed

Front 2

William Crookes

Back 2

• Discovery of the Electron

• Investigated what happens when an electric current is passed through a glass tube containing air at a low pressure (vacuum tube). The glass tube was seen to fluoresce, suggesting that it was being struck by some form of radiation

• realised that this radiation being emitted from the cathode because of a shadow of the Maltese cross was formed at the opposite end of the tube to the cathode.

• A second experiment carried out by Crookes shows that the cathode rays have enough energy to turn the paddle wheel

Front 3

JJ Thomson

Back 3

• Showed that cathode rays consisted of negatively charged particles called electrons

• Passed a narrow beam of cathode rays through a small hole in the positive electrode (anode) of a vacuum tube. The beam passed through two parallel plates and struck a fluorescent screen at the far end of the tube, causing Gn the fluorescent screen to glow

• When a positive charge was placed on the top plate, the cathode rays were attracted towards the positively charged plate, showing that the rays must consist of negatively charged particles

• Thomson was able to use a magnetic field to conceal out the effect of the electric field, bringing the electon beam to its original, undeflected position, allowing him to calculate the ratio e/m, where e= size of the charge on the electron and m= mass of the electron

Front 4

Cathode rays

Back 4

Streams of negatively charged particles called electrons. Travel in straight lines, form cathode to abide, are deflected by electronic and magnetic fields

Front 5

Robert Millikan

Back 5

• Carried out an experiment to determine the value of e, the size of the charge on the electron

• Sprayed tiny droplets of oil between two charged metal plates. Used X-rays to knock electrons out of air molecules. Some electrons picked up by oil droplets, causing them to become negatively charged

• Used an electric field to pull an oil droplet upwards against the force of gravity

Front 6

Thomson Plum pudding model of the atom

Back 6

• Suggested that the atom consisted of a sphere of positive charge in which electrons were embedded at random

Front 7

Discovery of the Nucleus

Back 7

• Ernest Rutherford and Co. studied the scattering of positively charged alpha particles by a thin sheet of gold foil

• Plum-pudding model of the atom suggests that the alpha particles would pass though the atom with little or no deflection caused by the positive charge, spread throughout the atom

• However, experiment showed that a very small number of particles bounced straight back towards the alpha particle source, suggesting that they had collided with a small dense core of matter (nucleus)

• Vast majority of particles passed undeflected though the gold foil, showing the atom is mostly empty space

• Small number of particles were deflected at large angles, suggesting that they had come in close contact with the nucleus and were repelled, showing the nucleus is positively charged

Front 8

Observation: Nucleus

Back 8

• Most alpha particles passed straight through the gold foil

• Some alpha particles are deflected at large angles

• A small number of alpha particles were reflected back along their own paths

Front 9

Conclusion: Nucleus

Back 9

• Most of the atom is empty space

• The alpha particles are repelled when they pass near the small positive nucleus

• A small number of alpha particles collide head on with the nucleus

Front 10

James Chadwick

Back 10

• Used alpha particles to break beryllium nuclei. The alpha particle knocked neutrons out of the nuclei of the beryllium atoms