Discovery and Similarity:
        The modern science of chemistry began during the eighteenth century when several brilliant "natural philosophers" classified the products of decomposition into a small number of fundamental substances. One famous instance is the discovery - in 1774 by the Englishman Joseph Priestley - that when the red powder called mercuric oxide is heated, it decomposes to liquid metal mercury and a colorless gas capable of supporting combustion. (This gas was later named oxygen.) Most substances can similarly be decomposed into several simpler substances either by heat or an electrical current; however, the most fundamental substances could not be broken down further, even with extraordinary temperature or electric voltage. Those basic building blocks of all other substances came to be called the chemical elements.
        When the French chemist Antoine Lavoisier published his famous list of elements in 1789, there were only 33 of them, and several of those were erroneous. By 1930, the diligent labors of thousands of chemists had increased the tally of naturally occurring chemical elements of 90. More recently, physicists in high-energy laboratories have been able to create about 20 highly radioactive, unstable elements that do not exist naturally on Earth, although they are probably produced in the hot cores of some stares.
        The number of chemical elements has now reached 109. Fortunately for students, only about 40 are of importance to basic chemistry. Please take a reconnaissance glance at the Periodic Table of the Chemical Elements.

The Periodic Table:
        In 1869, the Russian chemist Dmitri Mendeleev published his great systematization called the periodic table. He arranged all known chemical elements in order of their atomic weights and found that similar physical and chemical properties recurred every 7 elements for the lighter elements and every 17 elements for the heavier ones. (The inert gases had not been discovered at that time; the correct values for similar properties are 8 and 18.) The periodic table is based on atomic weights and similar properties. In each row, the atomic weights increase toward the right. Each column contains a group of elements with similar chemical behavior.
        The modern periodic table contains four datas - the atomic number, element symbol, element name, and atomic weight.

The Atomic Number:
        The atomic number usually appears above each element symbol, represents the meaningful order in the periodic table. When ever an element is referred tro by an integer, that means the atomic number, not the atomic weight. Thus, element 27 is cobalt (atomic number is 27), not aluminum (atomic weight is 27). The atomic number not only the order of the elements in the periodic table; it is also the number of protons in an element.

The Element Symbol:
        The element symbol was the abbreviation of the element's name. It was the same throughout the world. Most countries have different names for the same element, and it must be corrected or otherwise people would have to spent a lot of time is translating the names of those elements. The symbol of elements were very simple which is easy to learn and easy to express different compounds.

The Atomic Weights:
        By the early nineteenth century, chemists were striving to organize their rudimentary knowledge of the chemical elements. It was known that differing weights of element reacted to form compounds. As an example, they found that 3 grams of magnesium metal reacted with precisely 2 grams of oxygen to form magnesium oxide. The following chart summarizes those relative combining weights. In normal condition, people uses hydrogen (H) as the standard of atomic weight - 1. Some other people uses carbon (C) as the standard of atomic weight - 12.