Section 2: Atomic Discoveries

Chapter 4: Atoms and Elements

Learning GoalsExplain how the experiments of Thomson, Rutherford, and Chadwick led to the development of the nuclear theory of the atom.

Describe early atomic models.

Solid SphereSolid Sphere Model:

Democritus

Dalton

J. J. ThomsonJ. J. Thomson (1856–1940) In 1897, Thomson performed his famous cathode-ray tube experiments.

J. J. ThomsonElectricity was passed through a

vacuum sealed tube.When charged plates were brought

close to the tube, the beam of electricity reacted. Moved towards the positively charged plate

Moved away from the negatively charged plate

J. J. ThomsonBased on his experiments, Thomson

discovered the following: Electrons are negatively charged.

Electrons are much smaller and lighter than atoms.

J. J. Thomson Electrons are uniformly present in

many different kinds of substances.

He proposed that atoms must contain positive charge that balances the negative charge of electrons.

J. J. ThomsonPlum Pudding Model

J.J. Thomson Negatively charged

electrons were held in a sphere of positive charge

Ernest RutherfordErnest Rutherford

(1871-1937) In 1909, Rutherford

performed his famous gold foil experiment.

Ernest RutherfordTiny particles called alpha-particles were directed at a thin sheet of gold foil. Most particles passed straight through the foil, but some were deflected at large angles.

Ernest Rutherford Expected result of

Rutherford’s gold foil experiment:

If the plum pudding model were correct, the alpha-particles would pass right through the gold foil with minimal deflection.

Ernest RutherfordActual result of

Rutherford’s gold foil experiment:

A small number of alpha-particles were deflected or bounced back.

Ernest RutherfordBased on his experiments, Rutherford discovered the following: Most of the atom’s mass and all of its

positive charge are contained in a small core called the nucleus.

Ernest Rutherford Most of the volume of the atom is empty

space through which the tiny, negatively charged electrons are dispersed.

The number of negatively charged electrons outside the nucleus is equal to the number of positively charged particles (protons) inside the nucleus, so that the atom is electrically neutral.

Ernest RutherfordPlanetary Model

Ernest Rutherford

A small, dense positively charged nucleus is orbited by electrons.

James ChadwickJames Chadwick (1891-

1974) In 1919, Chadwick and his

mentor, Rutherford, kept finding that the atomic mass of an atom was larger than the atomic number of that atom.

James ChadwickChadwick suggested that there could be an additional particle in the nucleus that had mass, but no charge. These particles were called neutrons

James ChadwickNuclear Model

Ernest Rutherford

James Chadwick

▪ 99.9% of the atoms mass is concentrated in a small dense nucleus that contains protons and neutrons.

James Chadwick The dense nucleus makes

up more than 99.9% of the mass of the atom but the nucleus occupies

only a small fraction of an atoms volume

The electrons are distributed through a much larger region but don’t have much mass.

James ChadwickA single grain of sand composed of solid atomic nuclei would have a mass of 5 million kg. Astronomers believe that black holes and neutron stars are composed of this kind of incredibly dense matter.