Niels Bohr’s greatest contribution to chemistry was, arguably, to be the first to develop a physics-based model of the atom. The “Bohr Model” as it came be to known, incorporated the new ideas of quantum theory, and was the first step in chemistry’s progression from an exclusively experiment-based science to a theoretical science.
The Bohr Model is incredibly complicated, principally because it relies heavily on quantum mechanical ideas. Although Bohr did not get everything right in it (it has been tweaked, but never overhauled), it was certainly groundbreaking work, and it earned Bohr the 1922 Physics Nobel Prize.
The critical success Bohr achieved was in explaining how spectral series emerge. When an atom is heated up, it begins to emit energy, but only at particular frequencies, which depend on the type of atom it is. The problem with the classical model of the atom was that it looked like the solar system: it dictated that an electron could be found orbiting the nucleus at any distance, provided that the (outward) centripetal force and the (inward) electric force were balanced.
This would suggest that an electron dropping from a high energy state to a low energy state could produce a photon with however much energy was lost. The amount lost could be anything. Bohr’s model radically changed this idea.
When Bohr developed his model, he incorporated the notion of quantization. Quantum theory earned its name because it assumes that everything has a smallest value, a quantum. Time, space, energy, none of these are infinitely divisible. What Bohr postulated was that the atom’s electrons could only orbit the nucleus at a particular energy level. Then he took it a step further, and calculated what those energy levels would be.
His calculations demonstrated that the amount of energy an electron can have must always be an integer multiple of “h” (Planck’s constant), which means that only values of 1h, 2h, 3h, etc. are acceptable. When he ran the numbers, he discovered that this perfectly explained spectral series. Spectral series, it turns out, are the photons with energies that correspond perfectly to jumps between various values of h.
Of course, Bohr’s model was revolutionary in many other ways, too. But many of his other ideas would undergo major revisions – particularly when Louis de Broglie introduce a new model of particle behavior, not one year after Bohr earned his Nobel Prize. De Broglie’s discovery was that particles actually behave as waves. Bohr might have predicted this, given a few more years. His model of the atom strongly suggested it.