A Level Secondary Science

Chemistry in 1913: part 2 – Niels Bohr and electron orbits

In 1913 scientists made some important discoveries and announced theories that are still important to the chemistry that we learn today. In part 1 we found out how Henry Moseley discovered that the atomic number of an element decided where it came in the Periodic Table. 1913 was also the year in which Niels Bohr published his theory of electron orbits.

In 1898 at the Cavendish Laboratory of Cambridge University, J.J.Thomson did an experiment that showed that atoms were not the hard little balls that many people thought they were. He found that they contained tiny negatively charged particles that came to be called electrons. Thomson suggested that atoms were a pudding of positive charge with the electrons buried inside like currants.

Just over ten years later Hans Geiger and Ernest Marsden were working with Ernest Rutherford at Manchester University. They shot tiny alpha particles at gold leaf and found that most passed through while just a few were deflected. Rutherford concluded that most of the mass and all of the positive charge of atoms was concentrated at their centre with the electrons in the space around it. In 1912 Rutherford named the dense centre of the atom, the nucleus.

There was however a problem with Rutherford’s “nuclear atom”. Newton’s laws of motion and Maxwell’s theories of electromagnetism predicted that, however fast they moved, the electrons would spiral into the nucleus in less than a second. It appeared that Rutherford’s atoms couldn’t exist. It was time for a new idea and Niels Bohr was the man on the spot.

Niels Bohr was born in 1885 in Copenhagen, Denmark, where his father was a professor of physiology at the university and his mother was also well-educated.

Niels did well at school and went on to Copenhagen University to study physics. Before he completed his degree in 1909 he had made a name for himself by winning a prize from the Danish Academy of Sciences for a study of liquids.

Once he had completed his PhD in 1911 he travelled to Cambridge to work with Thomson. By now Bohr was more interested in theoretical work than in doing experiments and he didn’t get on with Thomson. After getting married in 1912 he joined Rutherford in Manchester and began to tackle the problems of the nuclear atom.

Bohr knew that as observations and experiment had shown that atoms had a nucleus and electrons there must be an explanation for how they can exist. He thought that there must be some special orbits that the electrons could stay in without falling into the nucleus. This idea went against the “classical” theories of Newton and Maxwell.

To explain his new theory Bohr drew on the discoveries of Max Planck. In 1900, Planck had shown that the light given off by hot objects could only have certain energies. He showed that the energy and frequency of light was related by a simple equation. He named the differences between two light energies a “quantum” and he said that the energy of the light was “quantised”. It became known as Planck’s Quantum Theory of Light.

Bohr applied the quantum theory to the particles that make up atoms. He said that electrons in an atom could only have certain energies. These energies determined how far from the nucleus the electron orbit was. The orbits were also called shells. Bohr said that electrons could move from one shell to another by absorbing or emitting a certain quantum of energy as light given by Planck’s equation.

Bohr used his theory to calculate the frequencies of light given out by hydrogen atoms when heated. His predictions matched the observed hydrogen spectrum. The theory could explain why the spectrum of every element is made up of a series of coloured lines. Bohr also suggested that the number of electrons in each shell would determine the chemical properties of elements.

Bohr’s theory was just the start of the science of quantum theory. In the decades afterwards many other scientists such as Schrodinger and Heisenberg added to the theory. The arrangement of electrons in the orbits of all the elements in the Periodic Table were worked out. Diagrams of electrons orbiting around a nucleus became iconic images of the twentieth century.

Niels Bohr returned to Copenhagen University in 1913 and in 1916 was appointed as Professor of Theoretical Physics. In 1920 he was made the head of a new Institute of Theoretical Physics at the university and he was awarded a Nobel Prize in 1922. During the Second World War, Bohr had to flee from Denmark because his mother was Jewish. He went to the USA and worked on the atom bomb project. After the war Bohr returned to the Institute and remained there until hius death in 1962. He pursued the peaceful use of nuclear power and helped to set up CERN that now has the Large Hadron Collider to investigate the particles that make up atoms.


1. In what ways was Bohr’s idea different to Rutherford’s first model of the nuclear atom?
2. Bohr is called a theoretical physicist because he didn’t do experiments himself, while Rutherford was very much an experimental scientist. Discuss whether or not experiments are needed to produce theories.
3. Why did Bohr have to reject the Classical theories (that is the theories of Newton and Maxwell) of science?
4. Planck thought his quantum theory was just a useful mathematical tool which allowed him to solve a particular problem. Why do some scientists think it the most important theory of twentieth century science?
5. Bohr’s theory led to the understanding of how electron arrangements are related to the position of elements in the Periodic Table. Explain
(a) what is meant by an electron arrangement;
(b) how electron arrangements determine the position of an element in the Periodic Table;
(c) how the electron arrangement of an element is related to its chemical properties.
6. Bohr lived through one of the most exciting periods of science and a time of great upheaval in Europe. Find out more about his life and work.
7. Find out about some of the other scientists who worked on quantum theory.

Peter Ellis

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