Reviewing Molecular Weight and Valence

Molecular Weight and Valence

By Dr. Donald G. McGahan


An element consists of two basic parts, the nucleus and the electrons which orbit the nucleus. The nucleus primarily contains uncharged neutrons and positive charged protons, while the negative charged electrons orbit the nucleus. In the uncombined or free state, an element has the same number of protons as it has electrons, hence it had no overall electrical charge. Examples of this are metallic iron and copper, and flowers of sulfur.

While elements in an uncombined state are uncharged, most elements have a tendency to gain or to lose electrons. Elements tend to gain or lose electrons until they obtain a stable or noble gas electron configuration (s2p6). The number of electrons that an element gains or loses is its valence. The valence of an element can be thought of as the number of hydrogen ions (H+) that it will replace or combine with in a chemical reaction. You may also think of the valence as the charge on element when it gains or losses electron(s).

A. Major nutrients

Major nutrients valence examples
Major Nutrient Valence Example
Calcium +2 CaCO3
Magnesium +2 MgCO3
Potassium +1 KCl
Phosphorous* +5, +3, -3 H3PO4
Sulfur* -2, +4, +6, +2 H2S, SO2, SO3
Nitrogen* -3, +3, +5, +4, +2 NH3, HNO2, HNO3

B. Minor nutrients

Minor nutrients valences
Minor Nutrient Valance Example
Manganese* +2, +4, +7, +6, +3 MnSO4, MnO2, KMnO
Iron* +2, +3 FeSO4, Fe2O3
Copper* +1, +2 Cu2O, CuO4
Zinc +2 ZnSO4
Boron +3 H3BO3
Molybdenum +3 MoO3

C. Nutrients mainly from water or the atmosphere

Nutrients mainly from water or the atmosphere
Nutrients from water or atmosphere Valance Example
Carbon* -4, 0, +4, +2 CH4, CH12O6, CO2
Hydrogen* (usualy in a soils course) +1 H2O
Oxygen* (usually -2 in a soils course) -1, -2 H2O2, H2O

D. Other miscellaneous elements

Other miscellaneous elements
Element Valance Example
Sodium +1 NaCl
Silicon +4 SiO2 (Quartz)
Aluminum +3 Al2O3
Chlorine -1 NaCl
Fluorine -1 CaF
Silver +1 AgCl

*These elements commonly show variable valences and the valence will usually be calculated from the other elements in the compound.