Picture an atom, and you may imagine spherical electrons orbiting a nucleus packed with particles like neutrons. Only certain orbits - quantum levels - are possible.
It's a simplistic model, yet provides insights into atoms and chemical properties, and this year marks 100 years since the model was first proposed by Danish physicist Niels Bohr.
The idea that matter comprises indivisible units dates back to Indian and Greek philosophers.
But it was not until around 1803 that the first useful atomic theory of matter was introduced, by British chemist-physicist John Dalton. He proposed that all matter is composed of atoms, which differ between elements and cannot be made or destroyed though can create compounds in chemical reactions.
Almost a century later, another Briton - physicist Joseph Thomson - discovered the electron through experiments with cathode rays. This led him to propose that atoms were made of electrons, or corpuscles as he called them, embedded in a sphere of uniform positive charge. His idea became known as the "plum-pudding model", with the electrons likened to plums within a popular pudding.
But just five years after Thomson published his model in 1904, it was disproved by an experiment directed by a New Zealand-born scientist who became known as the father of nuclear physics, Ernest Rutherford.
The experiment involved firing positively charged alpha particles at gold foil, and seeing where they went. According to the plum-pudding model, most should have passed in essence straight through, deflected by a few degrees at most. But in practice, a very small percentage were deflected at high angles, occasionally almost bouncing back in the direction they had come from.
To Rutherford, this result "was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." He concluded that each atom was mostly empty space, with a tiny but dense "central charge", since called the nucleus.
Niels Bohr received a doctorate in physics from Copenhagen University and was awarded a fellowship for overseas study. As he had focused on electrons in metals, he joined Thomson's research group in Cambridge. Bohr looked forward to meeting Thomson and discussing some apparent errors with his theories, yet reportedly found him little interested in such discussions. Soon, Bohr instead spent a brief spell with Rutherford in Manchester, England.
Based on the experiments involving gold foil, Rutherford had proposed that an atom was like a miniature solar system, with electrons circling the nucleus. But this did not fit classical theories, according to which the charged electrons should continually radiate energy and eventually spiral into the nucleus.
Returning to Denmark, Bohr explored whether the structure of the atom could be explained by the recently introduced quantum theory. This had arisen through German physicist Max Planck looking at materials radiating heat, and finding that the energy was always in discrete units he called quanta.
Early in 1913, a colleague drew Bohr's attention to the Balmer series, a well-known but unexplained formula that described a series of wavelengths of light in the spectrum emitted by hydrogen atoms. To Bohr, it was immediately clear that this formula dovetailed with his new theory of the atom.
Bohr postulated that an electron could follow certain stable orbits around a hydrogen nucleus, and while in these it did not give off energy. It could also fall from higher to lower levels, and energy was released during the transitions. He produced a formula that neatly agreed with the Balmer series.
During 1913, Bohr published three papers that included his new model of the atom. He had grander aims too, noting that "outlines are given of a theory of the constitution of the atoms of the elements and of the formation of molecules of chemical combinations", yet even his atom proved hard for anyone to accept. Rutherford considered it was "very difficult to form a physical idea" of the theory, and suggested a grave difficulty was that it implied an electron knew where it was going as it passed from one stationary state to another.
But there was no ignoring the fact that Bohr's theory worked for the Balmer series, and even explained that a mysterious set of accompanying lines arose from helium spectra. Plum-pudding notions had gone; quantum theory had become integral to the study of the atom.
Just 27 years old when he published his trilogy, Bohr became an eminent scientist, being awarded the Nobel Prize in Physics and with his name prominent among the renowned individuals at the forefront of developing quantum theory.
At one conference, Albert Einstein remarked, "God does not play dice," as he disliked the uncertainties now inherent within physics.
Bohr responded: "Einstein, stop telling God what to do."
But Bohr and Einstein were mutual admirers, and in his autobiography Einstein recalled that Bohr's atomic theory "appeared to me like a miracle".
Atomic models swiftly evolved, with Bohr recognising that the idea of circular orbits was incorrect, and complex equations later describing probabilities of electron distributions - which only roughly correspond with orbits. These models have made it far, far harder to form a physical picture of the atom.
If you find such notions more than a little baffling, especially on a Sunday, you might take comfort in a quote from Bohr: "If anybody says he can think about quantum theory without getting giddy, it merely shows that he hasn't understood the first thing about it!"
Martin Williams is a Hong Kong-based writer specialising in conservation and the environment, with a PhD in physical chemistry from Cambridge University.