Although our world is a wonderfully complex place, people have long suspected that this complexity arises from the interactions of a rather limited number of building blocks. The Greeks, for example, believed that there were four elements — fire, earth, air, and water — from which all else was made. Nowadays, we know that there are 92 naturally occurring chemical elements; they range from the lightest element, hydrogen, to the heaviest element, uranium. All the material that we see around us in our everyday world consists of one or more of those 92 elements in varying combinations. Eventually, scientists discovered that elements differ because the atoms that constitute them are different; a hydrogen atom is different from a uranium atom. The source of this difference became clear once scientists realized that atoms, rather than being indivisible, possess a structure. An atom consists of one or more electrons orbiting a central nucleus, and the atomic nucleus itself possesses structure: a nucleus consists of one or more protons and a variable number of neutrons. (The property Building Blocks of the Universe that identifies an atom is the number of protons in its nucleus; different isotopes of an element possess the same number of protons in the atomic nucleus, but differing numbers of neutrons. The simplest hydrogen atom consists of one electron orbiting one proton; the most common isotope of uranium has atoms formed of 92 electrons orbiting a nucleus consisting of 92 protons and 146 neutrons.) More recently, scientists discovered that protons and neutrons themselves possess a structure; they consist of three quarks bound tightly together.
KeywordsMicrowave Europe Uranium Expense Posite
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