Every chemistry student is familiar with the periodic table of the elements. This famous chart arranges the elements-that is, the different varieties of atoms-according to their atomic weights. One of the useful features of this table is that it identifies groups of elements that have similar chemical properties.
The idea of arranging elements within a periodic table was proposed in 1869 by a Russian chemist named Dimitri Mendeleyev. For many years, chemists had understood that matter consisted of many different kinds of basic particles called atoms, and that these basic elements could combine into many different compounds. But even though much information had been learned about these elements, scientists did not yet understand how the many different properties of the elements were related to each other, and they could not predict what kinds of elements would be discovered in the future. Mendeleyev realized that the elements could be arranged meaningfully in terms of their atomic weight. For example, hydrogen is the element with the lightest atomic weight, so hydrogen is given the atomic number one. The element with the next-lightest atomic weight is helium, so helium is given the atomic number two. When Mendeleyev arranged the elements in this way, he discovered that elements located close together usually had different properties, but that elements with similar properties could be found at regular intervals further along the table.
This discovery of the "periodic" repetition of the properties of elements allowed Mendeleyev to predict the properties of elements that had not yet been discovered. Based on the atomic number of a hypothetical element, Mendeleyev could predict its properties, and later discoveries showed that Mendeleyev's predictions were largely correct. The periodic table has allowed the identification of several important groups of elements, which are discussed briefly below. The alkali metals, such as sodium and potassium, are soft metals that readily conduct heat and electricity. They can explode when exposed to water. The halogens, such as fluorine and chlorine, are non-metallic elements that combine with metallic elements to form salts.
The transition metals, such as mercury and gold, are very numerous. They conduct heat and electricity, and can be shaped and stretched. They are often found in compounds with oxygen. Three of the transition metals (iron, cobalt, and nickel) can produce a magnetic field.
The noble gases, such as helium and neon, have also been called "inert" gases, because they do not readily form compounds with other elements. The non-metals, such as oxygen and carbon, do not conduct heat or electricity very well, and cannot be easily stretched or shaped. Non-metals are the main elements in organic compounds