Chamicals Periodic Table

In the mid 19^th century, scientists were confronted with a mountain of seemingly unconnected chemical data - a situation similar to the particle physics in mid 20^thcentury. In 1869 the Russian chemist Mendeleyev successfully organized the various chemical elements into a Periodic Table. Similar elements are arranged in vertical columns and the properties of the elements change

		progressively across the row. The Periodic Table in Figure 13-01a is the modern version; while Figure 12-19 depicts the simpler one. The atomic number is the number of positive charges in the atomic nucleus. Atomic masses refer to the masses of neutral atoms, including the masses of the nucleus, the electrons and the mass equivalent of their binding energies. It is expressed in mass units such that the mass of the most abundant type of carbon is exactly 12.00 u (1 u = 1.66x10-24 gm).
Figure 13-01a Periodic Table, Modern [view large image, 1 MB]	Figure 13-01b Periodic Table, Unconventional [view large image]	Also see "Extension of the Periodic Table".

It was discovered later that not all of the atoms of a particular element have the same mass. The different varieties (different number of neutrons, same number of protons) of the same element are called its isotopes. The atomic masses now appear in the Periodic Table is the average atomic mass weighted by the abundance of each isotope. Unfortunately, the abundance depends on location where the sample is taken (ultimately depends on the process that created, transported or aggregated the material). The International Union of Pure and Applied Chemistry (IUPAC) has decided in 2011 to list atomic weight in lower and upper bounds, e.g., (1.00784;1.00811) for hydrogen. The affected elements include H, Li, B, C, S, and N. Elements with only one stable isotope such as F, Al, Na, Au and 17 others, are exempted from this ongoing change. And some highly radioactive elements exist too fleetingly for their atomic weights even to be defined.