The turn of the century marked a profound revolution in the development of science and our understanding of the fundamental principles of the natural world.
During the nineteenth century classical physics - the laws of motion, electromagnetic fields, and thermodynamics - had reached an advanced state of development. Chemistry had also reached a considerable degree of sophistication but on a largely empirical basis, the fundamental basis of chemistry remained mysterious. Much had been learned about the Earth and solar system as well. Estimates of the age of the Earth had risen from about 6000 years in the late eighteenth century to tens or hundreds of millions of years; and the view that life, the Earth, and the rest of the solar system had arisen in a single great upheaval in recent times had been replaced by the idea of gradual change over eons.
To some it seemed that science, especially physics, was reaching such a state of maturity that few fundamental principles remained to be discovered. But there were problems. Essentially nothing was known about the fundamental structure of matter that gave rise to the Periodic Law and other chemical behaviors - the very existence of atoms was largely conjectural. Geology and astronomy seemed in serious conflict since the apparent age of the geologic record could not be reconciled with the only power source for the Sun then conceivable, gravitational contraction, which would exhaust itself in mere millions of years. An important part of classical thermodynamics was stubbornly resisting resolution - the properties of blackbody radiation. In fact by the end of 1900s it had become clear that within the existing framework of physics no solution of the blackbody problem was possible (the untenable prediction made by existing physics was termed the "ultraviolet catastrophe"). Something important was missing.
Advancing experimental technique in the seemingly well understood field of electricity and magnetism gave the first clues to the new universe. In 1895 Wilhelm Konrad Roentgen at the University of Wurzburg discovered X-rays. He had been conducting experiments involving high voltage currents in an evacuated tubes. The penetrating radiation he discovered was wholly new and unexpected.
The following year (1896), by serendipitous accident while investigating X-rays, Henri Becquerel (at the Museum of Natural History in Paris) discovered radioactivity in a piece of uranium salt. This discovery provided for the first time direct evidence of the fundamental structure of matter, and also revealed the existence a totally new source of energy independent of the Sun's rays, or of chemical fuels, and vastly more concentrated than either.
Discoveries followed rapidly. Marie Skladowska Curie and her husband Pierre Curie immediately began isolating sources of radiation from uranium ore. This led to the discovery of polonium in 1896, and radium in 1897. Different types of radioactive emissions were soon identified: in 1899 Becquerel found that at least some of the radiation emissions were electrically charged, and Ernest Rutherford further distinguished two types of charged emissions - alpha and beta rays, Paul Villard identified neutral gamma rays.
During this time another key discovery was in the making - the development of Quantum Theory. Two threads led to the foundation of the theory, one theoretical and one experimental. The theoretical development was by Max Planck at the University of Berlin. In pursuing the perplexing problem of blackbody radiation, he developed a theory announced in 1900 that successfully predicted the observed blackbody spectrum. This theory postulated that matter could only absorb or emit energy in arbitrary units or "quanta". In 1898 J.J. Thomson detected the emission of electrons when a metal surface is illuminated by ultraviolet light - the photoelectric effect. The properties of this phenomenon could not be explained, particularly a metal-dependent frequency threshold for the emissions. Albert Einstein united these threads with his theory of the photoelectric effect in 1905 which proposed the existence of the photon - quantized light (for which he received the Nobel Prize). Also in 1905 Einstein formulated his Special Theory of Relativity, one aspect of which (the equivalence of mass and energy) began to give some insight into the origin of the atomic energy that had been revealed by the discovery of radioactive decay.
These developments had also greatly extended the understanding of the Earth and Sun. In 1905 Rutherford and Boltwood used the ratio between radioactive isotopes and their decay products to data a rock to 500 million years old. This great age sharpened the conflict with classical theories of solar development, but radioactivity also offered a resolution. Perhaps some atomic transformation process, not then understood, was the source of the Sun's brilliance and longevity.
With the hints given by these new discoveries, and the powerful new probes of matter offered by the newly discovered ionizing radiations, more discoveries followed swiftly.
Rutherford soon demonstrated that alpha particles were in fact helium atoms, minus their electrons.
In 1906 Rutherford began a series of experiments at McGill University where he was now professor, and continued at the University of Manchester. In these he experiments he studied how alpha rays were scattered by thin layers of mica and gold.
The age of the Earth jumped again in 1907 when Boltwood identified a piece of uraninite as being 1.64 billion years old.
In 1911 Rutherford published his conclusions drawn from the alpha scattering experiments - that nearly all of the mass of the atom is concentrated in a tiny positively charged region in the center called the nucleus.
J.J. Thomson discovers isotopes of neon in 1912, showing that the atoms of the same element could have different masses.
Although it was realized late in the nineteenth century that the identities of chemical elements were related to the number of electrons that each atom contained (the atomic number), it was difficult to determine this number accurately for most elements. In 1913 H. G. J. Moseley demonstrated that by studying X-ray emissions, the atomic number could be easily measured.
It was now possible to study the relationship between the atomic charge (the atomic number) and the atomic mass. Evidence began to accumulate that there were two principal contributors to the mass of the atom and the nucleus, one that was positively charged (later called the proton), and one that was neutral (the neutron).
Also in 1913, Niels Bohr made a key theoretical breakthrough. He devised the "Bohr atom" - a planetary model of the hydrogen atom with the electron orbiting the positively charged nucleus - that explained studying the spectrum of light emitted by hydrogen atom. This model was based on the quantum theory, and was consistent with the atomic structure observed by Rutherford.
Although physics and science continued to advance (Einstein completed the General Theory of Relativity during this period for example) there was a temporary doldrum in key discoveries about the structure of matter lasting for several years. This is partly explainable by the calamity of the First World War that disrupted all of Europe. Some of the destructive effects of the war on science were quite direct - the young genius Moseley perished in the trenches of Gallipolli.
On June 3, 1920 Ernest Rutherford gave his second Bakerian Lecture in London, and in the course of this lecture he speculated on the possible existence and properties of the neutron. This is apparently the earliest public proposal of the idea of positive and neutral particles composing the atomic nucleus.
In 1921 the American chemist H.D. Harkins coined the term "neutron" in a proposal of nuclear structure. Rutherford published further work on the idea in this same year. Little progress was made on developing the idea, or proving its existence for the next several years.
In 1930 two German physicists, W. Bothe and H. Becker, observed unusually penetrating radiation being emitted from beryllium metal when it was bombarded by alpha particles. On 28 December 1931 Irene Joliot-Curie (Marie and Pierre's daughter) reported on these same emissions, but like Bothe and Becker, believed them to be energetic gamma rays. Joliot-Curie discovered that these emissions produced large numbers of protons when they passed through paraffin, or other hydrogen containing materials, something never observed (and apparently impossible to explain) with gamma rays.
Over a ten day period, from February 7 to 17, 1932 James Chadwick conducted a series of experiments that conclusively demonstrated that these unusual emissions were actually neutrons. Using this new potent new tool, rapid progress on the structure of matter began to be made.
Although radioactive decay releases an enormous amount of energy compared to chemical processes, this energy release is gradual and cannot be modified to any significant degree. The possibility of "atomic energy" as a source of human controlled power thus came into existence as a concept, but without any known means of bringing it about - even in theory. On September 12, 1933 this changed.
On that day the brilliant Hungarian physicist Leo Szilard conceived the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy. He also considered the possibility of using this chain reaction to make bombs. These insights predate the discovery of an actual chain reaction process - fission - by more than six years.