Chemical Elements
At first glance, nature appears to consist of countless numbers of very different materials. The world thus appears to be an extremely complex mixture of many different materials. But philosophers and scientists have long held another view of the world. They have found it difficult to believe that nature is really as complex as it appears to be. Instead, they have assumed that the many different materials we see result from the combination of a small number of fundamental substances called elements. Elements refer to a basic substance that cannot be broken down into anything simpler by ordinary chemical of physical means. The idea of an element probably originated with the ancient Greeks. At first, philosophers imagined that only one basic substance exists. They believed that everything was made of some variation of that substance. Thales taught that everything is made from water. By condensing, evaporating, and changing its outward form, water could take the appearance of all other materials, he said. Anaximenes held a similar view, but called air the one single element. For Heraclitus, fire was the elementary material from which everything else was formed.
ATOMIC STRUCTURE AND PERIODIC PROPERTIES The stable superheavy elements are believed to have a special combination of neutrons and protons in each nucleus. The combination allows the force that binds neutrons and protons to temporarily prevent the enormous electrical repulsion between protons from breaking up the nucleus. Both scientist listed the elements according to their atomic weights, beginning with the lightest element, hydrogen. When the elements were arranged in this way, Mendeleev and Meyer found that a pattern began to emerge. Sodium (Na) is chemically similar to lithium (Li), magnesium (Mg) has properties like those of beryllium (Be), aluminum (Al) has properties similar those of boron (B), and so on. The Mendeleev-Meyer discovery is summarized thus in the periodic law: "When the elements are arranged in order according to their atomic weights, their properties are repeated in a periodic way". Credit for the discovery of the periodic law is usually given primarily to Mendeleev for two reasons. First, he used a more complete set of data in working out his version of the law. Second, and more important, he showed how the law could be used to predict the existence of new elements. Most chemists quickly accepted Mendeleev's periodic law. However, it still contained a few problems. In Mendeleev's periodic table, he guessed that the chemists that had measured the weight of the tellurium and iodine had made an error in calculating it. For that reason his placement of the two elements was incorrect. But this time, Mendeleev was wrong. The correct explanation for this apparent confusion did not appear for nearly 50 years. Then the English physicist H.G. J. Moseley unraveled the puzzle. Moseley found that the elements could also be arranged according to their atomic number. Thus arrangement is almost the same as the atomic weight sequence but not exactly. However, when atomic numbers are used instead of atomic weights in building the periodic table, all problems remaining from Mendeleev's original work disappear. The result is the table known to us today.
Some common words found in the essay are:
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Approximate Word count = 1535
Approximate Pages = 6 (250 words per page double spaced)
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