The atom is the smallest part of an element that retains the chemical characteristics of the element itself. For our purposes, we can think of the atom as a sphere with a diameter of about 10−10 meters. This is about a million times smaller than the diameter of the period at the end of this sentence. If the atoms in your body were an inch in diameter, you would have to worry about bumping your head on the moon.
Because atoms are so small, there are an exceedingly large number of them in even a small sample of an element. A ½-carat diamond contains about 5 × 1021 atoms of carbon. If these atoms, minute as they are, were arranged in a straight line with each one touching its neighbors, the line would stretch from here to the sun.
If we could look inside the gold atom, we would find that it is composed of three types of particles: protons, neutrons, and electrons. (The physicists will declare that the proton and neutron are themselves composed of simpler particles. Because it is not useful to the chemist to describe atoms in terms of these more fundamental particles, they will not be described here). Every a gold atom in nature, for example, has 79 protons, 79 electrons, and 118 neutrons. Gold is different from phosphorus, because natural phosphorus atoms have 15 protons, 15 electrons, and 16 neutrons.
The particles within the atom are extremely tiny. A penny weighs about 2.5 grams, and a neutron, which is the most massive of the particles in the atom, weighs only 1.6750 × 10-24 grams. The protons have about the same mass as the neutrons, but the electrons have about 2000 times less mass. Because the masses of the particles are so small, a more convenient unit of measurement has been devised for them. An atomic mass unit (also called the unified mass unit) is 1/12 the mass of a carbon atom that has six protons, six neutrons, and six electrons. The modern abbreviation for atomic mass unit is μ, but amu is commonly used.
Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. Charge, a fundamental property of matter, is difficult to describe. Most definitions focus less on what it is than on what it does. For example, we know that objects of opposite charge attract each other, and objects of the same charge repel each other. An electron has a charge that is opposite but equal in magnitude to the charge of a proton. We arbitrarily assign the electron a charge of -1, so the charge of a proton is considered to be +1.
- Portions of this article are exerpted from the chemistry text An Introduction to Chemistry by Mark Bishop.
- Wolfgang Demtröder. 2002. Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics (1st ed.). Springer. ISBN 3-540-20631-0
- Linus Pauling. 1960. The Nature of the Chemical Bond. Cornell University Press. ISBN 0-8014-0333-2.
- Eric R.Scerri. 2007. The periodic table: its story and its significance. Oxford University Press US. ISBN 0-19-530573-6.