You can search for a picture of an atom on the internet and you'll find one, even though nobody's actually seen an atom before. But we've got an estimation of what a single atom looks like because of the work of a bunch of different scientists like Danish physicist Niels Bohr.
Atoms are the building blocks of matter — a single atom of any individual element is the most basic entity in nature that still abides by the rules of physics we can observe in everyday life (the subatomic particles that make up atoms have their own special rules). Scientists suspected atoms existed for a long time before they could conceptualize their structure — even the ancient Greeks figured the matter of the universe was made up of components so small they couldn't be broken down into anything smaller, and they called these fundamental units atomos, which means "undivided." By the end of the 19th century, it was understood that chemical substances could be broken down into atoms, which were very small and atoms of different elements had a predictable weight.
But then, in 1897, British physicist J.J. Thomson discovered electrons — negatively-charged particles inside the atoms everyone had spent the better part of a century believing were entirely indivisible — as the smallest things that existed. Thomson just hypothesized that electrons existed, but he couldn't work out exactly how electrons fit into an atom. His best guess was the "plum pudding model," which depicted the atom as a positively-charged pie studded with negatively-charged areas scattered throughout like fruit in an old-timey dessert.
"Electrons were found to be negative electric, and all with the same mass and very small compared with atoms," says Dudley Herschbach, a Harvard chemist who shared the Nobel Prize in Chemistry in 1986 for his "contributions concerning the dynamics of chemical elementary processes," in an email. "Ernest Rutherford discovered the nucleus in 1911. Nuclei were positive electric, with various masses but much larger than electrons, yet very small in size."
A Giant Leap Forward
Niels Bohr was Rutherford's student who gamely took over his mentor's project of deciphering the structure of the atom in 1912. It took him only a year to come up with a working model of a hydrogen atom.
"Bohr's model of 1913 for the hydrogen atom had circular electron orbits about the proton — like Earth orbits around the sun," says Herschbach. "Bohr had made use of a simple and regular pattern for the spectrum of the hydrogen atom, which had been found by Johann Balmer in 1885. He also made use of the idea of the quantum idea, found by Max Planck in 1900."
In 1913, the Bohr's model was a giant leap forward because it incorporated features of the newborn quantum mechanics into the description of atoms and molecules. That year, he published three papers on the constitution of atoms and molecules: The first and most famous was devoted to the hydrogen atom and the other two described some elements with more electrons, using his model as a framework. The model he proposed for the hydrogen atom had electrons moving around the nucleus, but only on special tracks with different energy levels. Bohr hypothesized that light was emitted when an electron jumped from a higher energy track to a lower energy track — that's what made hydrogen glow in a glass tube. He got hydrogen right, but his model was a little glitchy.
"The model failed to predict the right value of the ground-state energies of many-electron atoms and binding energies of the molecules — even for the simplest 2-electron systems, such as the helium atom or a hydrogen molecule," says Anatoly Svidzinsky, a professor in the Institute for Quantum Science and Engineering at Texas A&M, in an email interview. "So, already in 1913, it was clear that Bohr's model is not quite correct. Even for the hydrogen atom, the Bohr's model incorrectly predicts that atom's ground state possesses nonzero orbital angular momentum."
The 1922 Nobel Prize
Which, of course, might not make a lot of sense to you if you're not a quantum physicist. However, Bohr's model was fast-tracked to receive a Nobel Prize in physics in 1922. But even as Bohr was cementing his reputation in the world of physics, scientists were improving upon his model:
"Bohr's model for the hydrogen atom was improved by Arnold Sommerfeld in 1916," says Herschbach. "He found elliptical orbits which accounted for spectra lines nearby those that had come from circular orbits. The Bohr-Sommerfeld model for the hydrogen atom is basic, but quantum and relativity became major aspects."
Between 1925 and 1928, Werner Heisenberg, Max Born, Wolfgang Pauli, Erwin Schrodinger and Paul Dirac developed these aspects far beyond Bohr's atomic model, but his is by far the most recognized model of an atom. The atomic models quantum physics have given us look less like a sun surrounded by electron planets and more like modern art. It's likely we still use the Bohr model because it's a good introduction to the concept of an atom.
"In 1913, Bohr's model demonstrated that quantization is a right way to go in the description of the micro-world," says Svidzinsky. "Thus, Bohr's model showed scientists a direction to search and stimulated further development of quantum mechanics. If you know the path, then sooner or later you will find the right solution to the problem. One can think of the Bohr's model as one of the direction signs along a hiking trail into the quantum world."