HKNIC - More About the Atom
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More About the Atom

An atom is very small. The size of a table tennis ball to the earth is very approximately equivalent to the size of an atom to the table tennis ball. For those interested in the details, the diameter of the earth is 12.7 x 106m, that of a table tennis ball is 3 x 10-2m, and that of an atom is 10-10m. (106 means 10 multiplied by itself five times, or 1,000,000. 10-2 means 1/102, or one divided by 10 twice or 0.01.)

The nucleus of an atom is typically 10-14m in diameter, with its surrounding cloud of electrons to make a total atomic diameter of 10-10m. Hence an atom is mostly "empty" space. The ratio of an atom to its nucleus is about the same as the width of the Hong Kong harbour from Tsimshatsui to the Central (around 1.2km) to the size of a rubber duck floating in the harbour.

Both the mass and density of atomic particles are also of a vastly different scale to our everyday world. The mass of a proton is 1.673 x 10-23 kg. A neutron is slightly more massive at 1.675 x 10-23 kg. However, since these subatomic particles are so small, they are very dense. The density of protons, neutrons or the atomic nuclei is typically about 2.3 x 1017 kg/m3. This is about 100 million million times denser than liquid water, which is at about 1 x 103 kg/m3 or one tonne per cubic metre.

The mass of an electron is 9.11 x 10-31kg – about a hundred million times lighter than a proton or neutron. Its size is so small that is it presently not yet measurable.

A positron is the antimatter counterpart of an electron. In other words, it has a positive electric charge equal and opposite to the negative electric charge of an electron, and has the same mass. A low-energy positron when colliding with a low-energy electron will annihilate each other and produces two high-energy photons – particles of light. The subsequent discovery of positron and other sub-atomic particles in the mid 20th century has upset our classical, somewhat simple view of matter developed only some 50 years before. In contrast, one of our earlier scientific concepts, the belief that the sun revolves round the earth, lasted unchallenged from ancient Greece for about two thousand years.