A new vision of the atom

Diagram of the plum-pudding atom, precursor of the modern atom

The plum-pudding atom, precursor of the modern atom
Before the discovery of the atom nucleus by Rutherford, a popular representation was that of a plum-pudding atom. Electrons carrying negative electric charges had been discovered in 1896, and it was speculated that these charged particles were moving inside a kind of magma of positive charges whose nature and layout were ignored. No one was imagining the modern atom proposed by Niels Bohr in 1913, essentially made of vacuum with all positive charges concentrated within a tiny nucleus.
© IN2P3

In 1911, Rutherford, Marsden and Geiger discovered the dense atomic nucleus by bombarding a thin gold sheet with the alpha particles emitted by radium. Rutherford and his students then counted the number of sparks produced by these alpha particles on a zinc sulphate screen. From this observation, they concluded that almost all the atomic matter was concentrated in a tiny volume situated at the atom center, the atomic nucleus.

The discovery of the nucleus led Niels Bohr to make the first theoretical representation of the atom. The ‘quantum mechanical’ revolution, culminating in the later developments made by Erwin Schrodinger, laid the foundations for our understanding of the infinitely small.

The study of the chemical properties of the elements in the uranium-radium family showed that radium D, radium B and radium G have the same chemical properties as lead. These elements, now known to be lead isotopes, took their names (which have since been forgotten) from their position in the radioactive lineage of radium which in the 1910’s had only recently been isolated and was used as radioactive sources for experiments. In 1913, Frederick Soddy concluding that these three elements belong to the same square of Mendeleyev’s periodic table, introduced the concept of ‘isotopes’, and later won the Nobel Prize for Chemistry in 1921.

This video takes us in 1911, in Manchester, on a journey through the history of particle physics., when Ernest Rutherford conducted a historical experiment that revealed that most of the mass of an atom is concentrated in a tiny nucleus made of protons and neutrons.

Alpha particles were the only nuclear projectiles available for physicists to use. Armed with radioactive sources, physicists were unwittingly examining the nuclei of atoms; so that by 1919 Rutherford was able to generate the first atomic transformation: a nuclear reaction. By firing alpha particles at nitrogen atoms, he was able to transform them into oxygen.

Over the following years, a large variety of nuclear reactions were observed and studied. The most important of these was the one which led to the 1932 observation of neutral radiation composed of particles with a mass approaching that of the proton. This was James Chadwick’s (1891-1973) discovery of the neutron. Soon after, Werner Heisenberg was the first to propose the theory that the nucleus was made up of protons and neutrons.