The Periodic Table helps predict the chemical properties of the elements. Elements in the same column will have similar chemical properties since they have the same number of electrons in their outer shell available for chemical bonding.
Besides the atomic number (number of protons) the atomic mass is also listed on the table. This is the number of protons and electrons in the element. Since elements all have more then one isotope (atoms with a varying number of neutrons) the atomic mass is not a whole number but usually is very close.
Elements can exist in one of three states: solid, liquid, or gas. Of the 118 known elements, only two are liquid at Standard Temperature and Pressure (STP), Bromine and Mercury. Standard temperature is defined as 0 degrees Centigrade or 32 degrees Fahrenheit (the freezing point of water) and 14.5 psi which is standard atmospheric pressure at sea level. All other elements are solid or gas.
Elements with an atomic number 40 (zirconium) and under are considered stable (do not decompose to smaller elements). Those from 41 to 82 (Lead) are apparently stable (can decompose but it takes billions of years). All above 82 are unstable and decompose at a known rate.
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