Atoms
A basic atom is a positively charged nucleus surrounded by negatively charged electrons.  These electrons orbit the nucleus in concentric spherical shells.  The outermost shell is called the "valence shell" and the electrons there are called "valence electrons".


Valence
Atoms accept or donate valence electrons to bind with other atoms and form new compounds.  The "valence number" represents the number of electrons that an atom donates or receives.  This number is positive for an atom that donates electrons and negative for an atom that accepts electrons.

Example: In the chemical equation for the formation of water two hydrogen molecules accept valence electrons from one oxygen molecule to form two water molecules:
                      2H₂ + O₂ = 2H₂O
                                      
In water Oxygen will have a valence number of minus 2 and Hydrogen will have a valence number of plus 2.  This is also expressed as
                O^-2  or O (-II) and H⁺² or H (II)

When added together the valence numbers of a molecule should equal zero.

Both Oxygen and Hydrogen would be considered bivalent.  This means each received or donated 2 electrons.  The other two most common valence states are

• Trivalent (III) involving 3 valence electrons
• Hexavalent (VI) involving 6 valence electrons.

Transition Metals
There are a number of elements known as "transition metals".  Most elements can only use electrons from their outer most or "valence" shell to bond with other elements. Transition metals can use the two outermost shells to bond with other elements. It's a chemical trait that allows them to easily bond with many elements.

Chromium (Cr)
Chromium is a transition metal.  It is found in nature as part of Chromite Ore (FeCr₂O₄).  This ore is processed to produce pure chromium metal, Cr (0).  As a pure metal Chromium has a valence level of zero.

Chromium is alloyed with other metals, such as Iron (Fe), Nickel (Ni), Carbon (C), to create stainless steel.  Chromium can also be part of various alloys designed to withstand very high temperatures. These are often referred to as high-performance or high-temperature alloys.

Hexavalent Chromium
Compounds containing hexavalent chromium, Cr (VI), are used in electroplating and the production of pigment in paints, corrosion inhibitors and many other products.  Workers can be exposed to hexavalent chromium in the fumes or spray from operations using these compounds. Some of the more common hexavalent chromium compounds are,

• Chromium Trioxide (H2Cr04)
• Sodium Chromate (Na2CrO4)
• Lead Chromate (PbCrO4)
• Strontium Chromate (SrCrO4)
• Potassium Chromate (K2CrO4)
• Sodium Dichromate (Na2Cr2O7)
• Potassium Dichromate (K2Cr2O7)
• Ammonium Dichromate (NH4)2Cr2O7
• Chlorochromic Anhydride (Cr(OCl)2)

Hexavalent chromium fumes can be created during procedures such as torch cutting and welding
(see Welding/Cutting).

Hexavalent chromium is found in deep aquifers, and may also enter drinking water, soil, and air through the discharges of dye, paint pigments, wood preservatives, chrome plating liquid wastes, or through leaching from hazardous waste sites (see EPA Report)