Atomic Theory and The Periodic Table Page:
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Chemists are concerned with only three subatomic particles - the proton, neutron, and electron - because they explain the basic chemical behavior of atoms.
Normal chemical reactions only involve valence electrons, those farthest from the nucleus. Chemical reactions produce a small fraction of the energy produced by nuclear reactions.
Atoms have extremely small masses. Because of this, the mass of atoms is generally expressed in atomic mass units, not grams or kilograms.
- Atomic mass units may be abbreviated amu.
- The formal SI abbreviation for the atomic mass unit is u.
- 1 u = 1.661 X 10−24 g
- A proton has a mass of 1.0073 u.
- A neutron has a mass of 1.0087 u.
- An electron has a mass of 5.486 X 10−4 u. It would take 1836 electrons to equal the mass of 1 proton.
Most atoms have diameters between 100-500 picometers (pm). Although not an SI unit of length, the angstrom,  , is often used to express atomic dimensions.
One angstrom equals 10−10 (1 ten-billionth) meter. In everyday terms, a sheet of paper is approximately 1,000,000 angstroms thick.
The diameter of a hydrogen atom is about 1 angstrom.
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