One hundred years ago Niels Bohr discovered the inner workings of atoms that finally convinced the scientific community of their existence after more than a century of discovery, denial and brilliant intuition.

In 1803 John Dalton, a chemistry teacher from Manchester, England, became aware that chemicals always reacted in whole-number ratios by weight. Dalton conjectured that the essence of a chemical was carried in a quantized form. He invoked the ancient Greek "atomos," meaning "indivisible," to refer to this smallest unit of chemical identity.

In Dalton’s theory each invisible "atom" carried a fixed amount of mass, and chemical reactions were due to physical regroupings of atoms.

In the mid-19th century James Joule, a pupil of Dalton and the father of energy, deduced that a gas was made of invisible particles of a certain mass. In Joule’s kinetic theory, these particles moved at tremendous speeds, bouncing off one another and off the container that housed the gas.

In 1865 James Clerk Maxwell — whose famous equations canonized the relationship between electricity, magnetism and light — developed statistical mechanics to describe aspects of the kinetic theory that showed how the speeds of gas particles were distributed.

Around the turn of the 19th century, several discoveries combined to spark the explosion that would become the quantum revolution of the 20th century.

The electron was discovered in 1897, followed the next year by radiation. It was quickly determined that beta particles were electrons. These particles radiated from within matter, and a likely source would be atoms. The mystery was where in the atom these electrons resided.

In 1900 Max Planck, a professor of theoretical physics in Berlin, discovered to his chagrin that when you heat an object until it glows, you can measure the energy only in discrete packets, which he called "quanta."

In 1911 Ernest Rutherford, a New Zealand scientist working in England, directed alpha radiation at a thin sheet of gold foil expecting the alpha particles to bounce off dense gold atoms. To his surprise only a tiny fraction did; the rest passed through without being deflected. Rutherford concluded that the atom was mostly empty space with a positive nucleus and a shell of electrons in orbit around it.

This conclusion clashed with Maxwell’s electromagnetism, which required any accelerated electric charge to radiate electromagnetic energy and quickly spiral into the nucleus, destroying the atom.

Two years later in 1913, Niels Bohr, a brilliant Danish physicist, proposed a stunning alternative model of the atom.

The Bohr atom, as the model is called, has a nucleus like Rutherford’s but allows electrons to exist in stable orbits around the nucleus. Electrons absorb or emit energy quanta as they move between higher and lower energy levels or "shells."

This simplified shell model served as the framework for our current understanding of the atom. Quantum theory today views electrons as clouds of probability around the nucleus with details of their movement inherently unknowable.

Modifications and improvements to the Bohr atom in the past 100 years have not lessened the importance of Bohr’s creative and agile mind in one of the most significant and underrated discoveries of all time.

———
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.