We owe our modern understanding of the atom and its structure to the Danish physicist Niels Bohr. The model he presented in the early years of the 20th Century challenged some of the most fundamental concepts that had gone unquestioned since the days of the Ancient Greeks. For centuries, the atom had been seen as an indivisible unit of matter; the basic building block of everything around us.
This school of thought was eventually discredited, in large part thanks to the discovery of radioactivity, Thomson's discovery of the electron, and Rutherford's experiments on the atomic nucleus. These three findings not only dispelled the concept of the unsplittable atom, but undermined the laws of physics deemed to apply to the infinitely small.
According to classical mechanics, electrons orbiting a central nucleus would radiate away energy and eventually collapse into it. Bohr resolved this difficulty in 1913, by postulating the existence of energy levels – to which correspond a limited number of orbits mathematically allowed for the electrons around the nuclei. This model of Bohr's, later to be refined by Schrödinger's equation, lays at the heart of all future developments in atomic physics.
The natural image of the atom is to imagine it as a complete sphere; totally full of matter. Incredibly, the atom is in reality almost entirely comprised of empty space. A popular analogy at the time was that the nucleus was like a 'fly in a cathedral' – with the electrons at the distance of the walls.
Though slightly inaccurate mathematically, made decades before the real subatomic distances were known, the analogy remains a helpful one in trying to imagine the atom. The tiny electrons, travelling at very high speeds, are able to do thousands of rotations of the atom in a very short period of time. As a result, the atom often appears to be full, though it is, in reality, largely empty.
The nucleus, a subdivision of the unbreakable atom, is itself comprised of even smaller particles: nucleons. There are two different types of nucleons; protons and neutrons, given the collective name for obvious reasons. Protons and neutrons are identical in size, and are each almost two thousand times as heavy as an electron.
The complicated interactions between protons and neutrons, packed together inside the nucleus, occasionally make the resultant nucleus unstable, and cause it to collapse. It is this 'nuclear instability' that is responsible for the phenomenon of radioactivity.
When considering any property of atoms or their constituents, we must keep two things in mind. First, their infinitesimal size, and second, their numbers are tremendous. Radioactivity, though it affects us on our own, large, scales, is ultimately a microscopic phenomenon.
A world with its own laws...
The world of the atom is not only very empty and very small, but has its own physical laws. For instance, the layered structure of the electron orbits is not only very precisely defined, but continues to astound even the most jaded physicists. To properly understand the way the electrons revolve around the nucleus, as well as the complicated energy levels existing within the nucleus itself, we have to abandon our much-loved 'classical' physics, for a system that focuses on the world of the very small: Quantum Physics.
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