Periodic Table of the Elements
Reading the Table:
The periodic table contains an enormous amount of important information:
Atomic number: The number of protons in an atom is referred to as the atomic number of that element. The number of protons defines what element it is and also determines the chemical behavior of the element. For example, carbon atoms have six protons, hydrogen atoms have one, and oxygen atoms have eight.
Atomic symbol: The atomic symbol (or element symbol) is an abbreviation chosen to represent an element ("C" for carbon, "H" for hydrogen and "O" for oxygen, etc.). These symbols are used internationally and are sometimes unexpected. For example, the symbol for tungsten is "W" because another name for that element is wolfram. Also, the atomic symbol for gold if "Au" because the word for gold in Latin is aurum.
Atomic weight: The standard atomic weight of an element is the average mass of the element in atomic mass units (amu). Individual atoms always have an integer number of atomic mass units; however, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass.
Atomic weight for elements 93-118: For naturally occurring elements, the atomic weight is calculated from averaging the weights of the natural abundances of the isotopes of that element. However, for lab-created trans-uranium elements — elements with atomic numbers higher than 92 — there is no "natural" abundance. The convention is to list the atomic weight of the longest-lived isotope in the periodic table. These atomic weights should be considered provisional since a new isotope with a longer half-life could be produced in the future.
