元素周期表历史.docVIP

Organization of the Elements:The Periodic Table, as we know it, was not something that was “stumbled upon.” It has been developed, modified, and improved for the last 200 years.By the eighteenth century, chemists realized that each element had a characteristic mass. Thus, they used mass as their basis for organization of the elements.In 1817, Johann Dbereiner of Germany was one of the first to formally organize the elements. He grouped elements in threes based on their similar differences in atomic mass. He called the groups of three triads. These triads had similar chemical and physical properties.Ex -ElementMass D MassProperties16.1lithium 6.9Soft, 16.1sodium23.0very reactivepotassium39.1metals.44.4Ex -chlorine 35.5Coloured,47.0bromine 79.9very reactiveiodine126.9 non-metals.However, Dbereiners “Law of Triads” was found to be merely a coincidence. By 1829, 80% of the elements known did not fit into a triad!The next major development in the Periodic Table began in 1864 with the British chemist, John Newlands. He arranged the known elements in order of increasing atomic mass. Newlands found that every eighth element had similar chemical and physical properties. Since the pattern was in eights, Newlandss discovery was called the “Law of Octaves.”Unfortunately, this method of organization only worked for the first 20 elements. As more elements were discovered, the Law of Octaves failed because the properties became inconsistent.For example, the reactive metals Li, Na, K, Rb, and Cs were grouped with the non-reactive metals Cu, Ag, and Au.As a result of this failure, improvements came about very rapidly. In 1869, a Russian chemist Dmitri Mendelev proposed the following hypothesis, known as Mendelevs Periodic Law:“The properties of the elements are periodic functions of their relative atomic masses.”Although he had this law, Mendelev did not restrict himself to patterns of eight; he included sub-groups. Mendelev also had the foresight to understand that it was possible some elements had not yet been discovered. So if he decided that the next heaviest element did not have properties that allowed it to fit within the next group, but had properties that made it fit better in a following group, he left “blanks.” He decided that elements that would fill the blanks would be discovered later HE WAS RIGHT!For example, arsenic (As, #33) did not fit in a group with aluminum (Al #13) or silicon (Si, #14) the properties did not match. So he left slots #31 and #32 blank and placed As in #33.In 1875 gallium (Ga) was discovered. It was lighter than arsenic and had properties similar to aluminum; therefore, it was placed in its group at #31.And in 1886 germanium (Ge) was discovered. It had a lower mass than arsenic and its properties matched those of silicon; therefore, it was placed at #32.All was nearly well; however, there were still some inconsistencies. For example, the element tellurium (Te, #52) preceded iodine (I, #53) based on properties, this, however, does not follow Mendelevs Law because Te (127.6 amu) has a greater atomic mass than I (126.9 amu).WHAT WERE WE GOING TO DO?The British chemist, Henry Mosley, modified Mendelevs theory in 1912. He used X-rays to determine the positive charge on an elements nucleus; that is the atomic number.By using atomic number, rather than atomic mass, to organize the periodic table - all problems with Mendelev were eliminated.The Periodic Law (from