Niels Bohr
Niels Bohr was born in Copenhagen in 1885, and died there in 1962. He was one of the most distinguished physicists of all time. He is best known for the development of the Bohr model of the atom, his theory explaining the existence of spectral lines, and the principle of complementarity. His work won him the 1922 Nobel Prize in Physics.
His interest in science began at an early age because his father was a professor of physiology. Specialising in Mathematics and Physics during his final years of school, Bohr continued these studies at university. He received his Master's degree from the University of Copenhagen in 1909 and his PhD in 1911. Later that same year, he went to England where he worked with Sir Joseph Thomson (who had discovered the electron), and Ernest Rutherford (who had put forward the concept of a nucleus within the atom).
He worked on the structure of the atom using quantum ideas from Max Planck and Albert Einstein. In Bohr's model of the atom there is a nucleus, and electrons move around the nucleus in stable states (also known as orbits or energy levels) without radiating energy. When an electron moves from one state to another, only very specific amounts of energy are lost or gained. If the atom gains energy, the electron jumps to a level further from the nucleus; if it loses energy, it drops to a level closer to the nucleus. Whenever energy is lost or gained, a line in a spectrum is produced. This model is now known as a quantized atom, from the term quantum introduced by Planck to describe small packets of energy.
In 1927, Bohr put forward his principle of complementarity, which refers to effects such as wave-particle duality. Bohr's principle was the most groundbreaking scientific concept of the 20th century. In essence, the principle states that things may have dual or contradictory properties, but we can only experience one property at a time. For example, we can think of an electron as a wave or as a particle, but we cannot think of it as both at the same time, even though it may actually be both at once.
In 1920, Bohr had been appointed director of the Institute of Theoretical Physics and he continued to work there throughout the 1920s and 1930s. However, during the German occupation in World War II, due to his having a Jewish mother, it was necessary for Bohr to avoid arrest by the police. He therefore escaped to Sweden. From there, he travelled to England and then to America, where he became involved in the Atomic Energy Project which aimed to build the first atomic bomb. He made a significant contribution by discovering that only uranium-235 could produce the fission chain reaction required for an atomic explosion. However, he was concerned about the political problems that the development of atomic weapons could cause, and supported the idea of sharing the new technology with other countries, particularly the USSR.
Niels Bohr held many important positions, and was honoured by many important scientific institutions. He was President of the Royal Danish Academy of Sciences, and a member of many other famous Academies. Bohr was awarded honorary doctorates by the world's greatest universities. Interestingly, Bohr's son also became a physicist and won the Nobel Prize for Physics in 1975.