Chemical valencies of elements

 

Chemical valencies of elements

 

Valence (or valency) is the capability of an atom to combine in particular proportion with another atom.

 

Cations: Generally metallic elements (Na, K, Cu, Mg, Ca, Fe, Mn, Ni, Co, Pb)

Anions: Generally non-metallic elements (O^-2, S^-2, F^-, Cl^-, Br^-, I^-)

 

Valency	Other atoms	Examples
Monovalent	1	H, Li, Na, K, Cu1
Divalent	2	Mg, Ca, Co, Cu2, Fe2, Mg, Mn2, Ni, O, Pb2, Zn).
Trivalent	3	Bi, Cr3, Al, Fe3, Sb3
Tetravalent	4	C, Si, Ti, Zr, Mn4, Pb4
Pentavalent	5	P, As, Sb5
Hexavalent	6	Cr6

 

Many atoms such as Fe, Mn, Cu, Cr, Pb, and Sb have multiple valencies: they are capable of combining with other atoms in different proportions.

Metals and oxides

 

                    Metals                                Oxides and names

                    Al (Aluminium)                Al₂O₃ (Alumina)

                    C (Carbon)                        CO (Carbon monoxide)

                                                              CO₂ (Carbon dioxide)

                    Ca (Calcium)                    CaO (Calcium oxide)

                    Co (Cobalt)                       CoO (Cobalt oxide)

                    Cr (Chromium)                 Cr₂O₃ (Chrome)

                    Fe (Iron)                            FeO (Ferrous oxide) ⎯ Iron protoxide

              Fe₂O₃ (Ferric oxide) ⎯ Iron sesquioxide

                    H (Hydrogen)                   H₂O (Water)

                    K (Potassium)                   K₂O (Potassium oxide)

                    Mg (Magnesium)              MgO (Magnesia)

                    Mn (Manganese)               MnO (Mangannous oxide)

              MnO₂ (Mangannic oxide) ⎯ Pyrolusite

                    Na (Sodium)                      Na₂O (Sodium oxide)

                    Ni (Nickel)                        NiO (Nickel oxide)

                    P (Phosphorous)                P₂O₅ (Phosphorous pentaoxide)

                    S (Sulphur)                        SO₂ (Sulphur dioxide)

                    Si (Silicon)                        SiO₂ (Silica)

               

Important oxides in mineralogical studies

Univalent metal:         H₂O = Water

K₂O = Potassium oxide

Na₂O = Sodium oxide

Cu₂O = Cuprous oxide 

 

Divalent metal:           MgO = Magnesia

MnO = Manganous oxide

NiO = Nickel oxide

CoO = Cobalt oxide

CaO = Calcium oxide

CuO = Cupric oxide

FeO = Iron protoxide (ferrous iron)

ZnO = Zinc oxide 

 

Trivalent metal:          Cr₂O₃ = Chrome

Al₂O₃ = Alumina

Fe₂O₃ = Iron sesquioxide (ferric iron)

 

Tetravalent metal:       SiO₂ = Silica

CO₂ = Carbon dioxide

MnO₂ = Mangannic oxide (Pyrolusite)

SO₂ = Sulphur dioxide

TiO₂ = Titanium oxide 

 

Pentavalent metal:      P₂O₅ = Phosphorous oxide

 

Representation of mineralogical formulae in terms of oxides

 

Most rock-forming minerals can be represented in terms of oxides:

 

Mineral name                  Formula                            Representation by oxides

Forsterite olivine             Mg₂SiO₄                            = 2MgO.SiO₂

Fayalite olivine                Fe₂SiO₄                             = 2FeO.SiO₂

Enstatite/Bronzite            MgSiO₃                             = MgO.SiO₂

Serpentine                        H₄Mg₃Si₂O₉                     = 3MgO.₂SiO₂.2H₂O

Talc                                  H₂Mg₃(SiO₃)₄                   = 3MgO.4SiO₂.H₂O

Magnetite                         Fe₃O₄                                = FeO.Fe₂O₃

Hematite                          Fe₂O₃                                 = Fe₂O₃

Goethite                           H₂Fe₂O₄                            = Fe₂O₃.H₂O

Limonite                          H₆Fe₄O₉                            = 2Fe₂O₃.3H₂O

Chromite                          FeCr₂O₄                            = FeO.Cr₂O₃

Atomic weights

Atomic weights of elements are relative weights in comparison to Hydrogen as 1.0 or Oxygen as 16.0.

 

H            O           Mg         Al           Si           Ca          Cr        Mn        Fe              Ni            Co

1.0         16.0       24.3       26.9       28.0       40.0       52.0       54.9       55.8           8.6           58.9

 

Ionic radii of Common rock-forming elements

 

Ions are measured in terms of their radii. The unit of radius is an angstrom. One angstrom is one ten millionth of a millimetre (1 ºA = 10-7 mm or 10-10 m):

 

Si           Al           Cr₆         Mn₄       Co₃        Cr₃         Fe₃         Mg         Ti           Ni               Co

0.42       0.51       0.52       0.60       0.63       0.63       0.64       0.66       0.68           0.69       0.72

 

Fe₂         Zr           Mn₂       Na         Ca          K            O           S^-2

0.74         0.79         0.80          0.97         0.99         1.33          1.40         1.84