Even in ancient times, people began to suspect that matter, despite its appearance of being continuous, possesses a definite structure on a microscopic level beyond the direct reach of our senses. Democritus came up with the term "atom" to define these little structures.
While the scientists of the late nineteenth century accepted the idea that elements consisted of atoms, they knew almost nothing about the atoms themselves. The discovery of the electron in 1887 and the realization that all atoms contain electrons provided the first important insight into atomic structure. Since electrons carry a negative charge and the atom as a whole is neutral, positively charged matter of some kind must be present in atoms.
One suggestion, made by British physicist J. J. Thomson in 1898, was that atoms are simply positively charged lumps of matter with electrons embedded in them. His model was like that of raisins in a fruitcake. Because Thomson was well-respected scientist of his time, his idea was taken very seriously. But the actual atomic structure turned out to be quite different.
In 1911, at the suggestion of Ernest Rutherford, alpha particles (helium nuclei) were emitted behind a screen with a small hole in it, so that a narrow beam of alpha particles was produced. This beam was then directed at a thin gold foil. A zinc sulfide screen, which gives off a visible flash of light when struck by an alpha particle, was set on the side of the foil.
It was expected that the alpha particles would go right through the foil with hardly any deflection because in the Thomson model, an electric charge inside an atom is assumed to be uniformly spread through its volume. With only weak electric forces exerted on them, alpha particles that pass through a thin foil ought to be deflected only slightly, less than a degree.
What the scientists actually found was that although most of the alpha particles indeed were not deviated by much, a few were deflected in very large angles. Some were even deflected in the backward direction. As Rutherford remarked, "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Our depiction of this statement is below.
Since alpha particles are relatively heavy (over 7000 electron masses) and those used in this experiment had high speeds, it was clear that strong forces had to be exerted upon them to cause such marked deflections. The only way to explain the results, Rutherford thought, was to picture an atom as being composed of a tiny nucleus, in which its positive charge and nearly all its mass are concentrated, with electrons some distance away. So, an atom would largely be empty space. With an atom having this characteristic, it is easy to see why most alpha particles go right through a thin foil. However, when an alpha particle comes near a nucleus, the intense electric field there causes it to be scattered through a large angle. So, everything made a little more sense.
In 1913, just two years after English physicist Ernest Rutherford's theory, the great Danish physicist Niels Bohr proposed a model for the hydrogen atom that not only accounted for the presence of the spectral lines but predicted their wavelengths to an accuracy of about 0.02%. Although Bohr's theory was successful for hydrogen, it proved less unseful for more complex atoms. We now regard Bohr's theory as an inspired first step toward the more comprehensive quantum theory that followed it.
Bohr, realizing that classical physics could not explain the structure of the hydrogen atom, put forward two bold postulates. Both turned out to be enduring features that carry over in full force to modern quantum physics. Moreover, both turned out to be quite general, applying not only to the hydrogen atom but to atomic, molecular, and nuclear systems of all kinds. These postulates are the following:
It was physically and mathematically determined that an electron does not simply orbit the nucleus due to stability considerations. The electron in this atom is obliged to whirl around the nucleus to keep from being pulled into it and yet must radiate electromagnetic energy continuously.
The standard model is the model of all known fundamental particles and particle interactions. It is basically a chart which encompasses all known particles and their characteristics (such as mass, spin, charge, etc.) as well as the interactions between them. It describes the quantum theory that includes the theory of strong interactions and the unified theory of weak and electromagnetic interactions (electroweak). Gravity is not part of the standard model.
An alternative to the standard model which employs the use of a new type of particle called the tachyon which can travel faster than the speed of light.
These theories interpret pointlike particles, such as electrons, as being unimaginable tiny, closed loops. Strangely, extra dimensions beyond the familiar four dimensions of spacetime appear to be required.