THE ROAD TO THE

ATOMATOM

Democritus

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

JOHN DALTON (1803)• Dalton viewed the atom as a small

solid sphere. 

• Each element was composed of the same kind of atoms.

• Each compound was composed of different kinds of atoms.  

J.J. THOMSON (1897)• Developed a primitive

electron gun.

• Experimented with beams of particles accelerated in the electron gun.

• Found that beam gets attracted by a positively charged plate.

• Lead to the discovery of the electron, a negatively charged sub-atomic particle.

• New Discovery!!! The atom is NOT the smallest, as it consists of much smaller particles.

• Now scientist knew the atom consists of positive and negative particles.

• But where were they in the atom?

• 1904: Thomson suggested that there is a cloud of positive charge in the atom embedded with negative particles in it. This became known as the Plum Pudding model.

ERNEST RUTHERFORD (1911)

• Rutherford tested Thomson’s model. He experimented with alpha-particles and made a remarkable discovery.

• What are alpha-particles?

• Helium nuclei. They are positively charged.

Rutherford’s Experiment:• He bombarded a thin gold foil leaf

with alpha-particles.• Why gold foil?

• Gold is a very heavy atom, therefore it has a high number of protons and electrons.

• Why thin gold foil?• The thin foil acted as if the alpha-

particles were being fired at a single row of atoms.

Diagram of Gold Foil experiment

Rutherford’s findings:

• Most alpha-particles traveled straight through.

This indicated that most of the atom was empty space.

• Some were deflected by large angles.

• A few were rebounded directly.

This indicated that there was something small and dense inside the atom, and it repelled positive particles.

• Rutherford called the small, dense center the Nucleus.

• He knew the Nucleus was positively charged, as it deflected some alpha-particles.

• If the nucleus only contains positive particles, then the electrons must orbit the nucleus to keep it neutral.

Rutherford’s MODEL of the Atom:

• A dense, positively charged nucleus.

• Negatively charged electrons orbiting the nucleus.

• Later Rutherford measured the mass of a particle from the nucleus and found it much less than that of an alpha-particle.

• These positively charged particles were then named PROTONS.

• Millikan determined the mass of an electron through his oil drop experiment, and it was known that the proton’s mass is about 1835 times that of an electron.

Neils Bohr (1919)

• Electrons orbit the nucleus in fixed positions, according to their energy levels.

The Rutherford-Bohr model of the atom:

James Chadwick (1932)

• Discovered that there was very high radiation coming from particles in the nucleus.

• These were not deflected by electric or magnetic fields, therefore could not be protons or electrons.

• He discovered the neutron; large neutral particles in the nucleus with mass about the same as for protons.

• All three sub-atomic particles as we know them at school level, were now discovered.

• Atom masses and isotope masses could now be determined.

• The atom model for use at school level was now complete.

• Examples:

Further interesting facts that lead from these early scientists:

• Bohr’s analysis of the energy given off when an electron dropped from a higher energy orbit to a lower energy orbit didn't hold up for atoms bigger than hydrogen

• In 1924, a French physicist named Louis de Broglie suggested that, like light, electrons could act as both particles and waves

• Another question quickly followed de Broglie's idea. If an electron traveled as a wave, could you locate the precise position of the electron within the wave? A German physicist, Werner Heisenberg, answered no in what he called the uncertainty principle

• an Austrian physicist named Erwin Schrodinger derived a set of equations or wave functions in 1926 for electrons. According to Schrodinger, electrons confined in their orbits would set up standing waves and you could describe only the probability of where an electron could be. The distributions of these probabilities formed regions of space about the nucleus were called orbitals. Orbitals could be described as electron density clouds

• Atom model with electron clouds:

• Soon after the shape of the electron clouds were determined and a refined model for the atom came about.

• The orbitals were also named : s, p, d and f.

s - orbitals p - orbitals

• And so, until the next model, this is how it happened: