I. Valence and oxidation state
According to the atomic-molecular theory, valence is the property of atoms of given chemical element to combine with a definite number of atoms of another chemical element.
The electronic formulae of hydrogen, oxygen, nitrogen, water and methane show that the atoms combine through shared electron pairs (one, two, three or four).
When forming shared electron pairs the atoms participate with a definite number of electrons, which in the case of hydrogen is one, oxygen – two, nitrogen – three, carbon – four. According to the theory of the atomic structure and the theory of the chemical bonds, the valence of the elements is determined by the number of electrons with which a given atom participates in the formation of chemical bonds; hydrogen is monovalent, oxygen is bivalent, etc.
The property of atoms of chemical elements to participate in formation of chemical bonds with definite number of electrons is called valence; or the number of electrons participating in formation of chemical bonds determines the valence state of the element.
Somechemicalelementshaveonlyonevalencestatewhileotherscanappearinseveralvalencestates.Elements,whoseatomsparticipatewiththesamenumberofelectronsintheformationofchemicalbonds,aresaidtopossessconstantvalence, whereas atoms that can form several numbers of bonds have variable valence.
Constantvalenceischaracteristicforatomswithsmallnumberofouterelectrons, e.g. ingroupsIA,IIA(Na,Ca).
VariablevalenceischaracteristicfortheelementsofIVA,VA,VIA,VIIAandVIIIAgroupsaswellastheB-groupselements.Insomecasesvariablevalencecanbeexplainedbysplittingofelectronpairsinanouterelectronshell,thusincreasingthenumberofthesingleelectronsthereandenablingtheformationofmoresharedelectronpairs.Forinstance,carbonatomhastwosingleelectronsandonepairinitsoutershell.It can formtwo,orinmanyorganiccompounds,foursharedelectronpairs.Therefore, carbon has valence 2or 4.
Oxidation state(oxidation number)
Theexactnumberofchemicalbondsinagreatnumberofcompoundsisratherdifficulttoestablish,whichcreatesdifficultiesindeterminingthevalenceoftheparticipatingelements.Insomecasesthevalencedoesnotcoincidewiththenumberofbonds.Forexample,theammoniumion,NH₄⁺,hasfourbondsbutthevalenceofnitrogen is three.Inordertocharacterizethestateofthechemicalelementsinthecompoundstheterm oxidationstate(oxidationnumber)hasbeenintroduced.
Theoxidationstateisthehypotheticalchargethatanatomwouldhaveifallbondstoatomsofdifferentelementswerecompletelyionic.Thisisneverexactlytrue for realbonds.
In covalent compounds, this hypothetical charge is the electric charge an atom would have if the bonding electrons were assigned to the more electronegative element.
The bond H + → Cl - in the molecule of HCl is covalent polar because H Cl. Assuming that the shared electron pair completely shifts to the chlorine atom, the bond will transform into an ionic one. The charges of the obtained in these manner ions are equal to the oxidation state of the atoms. In HCl the oxidation number for the hydrogen would be +1 and that of the Cl would be -1; the oxidation number of carbon in methane would be -4, etc.
+1 -1 -4+1 +4-2 HCl CH₄ NO₂
The atom with higher negativity (typically a nonmetallic element) is assigned negative oxidation number while the otheratom (often a metallic element) is given positive oxidation number.
The oxidation state of theelements in ionic compounds is equal to the number of charges of the ions.
The oxidation state of sodium in the compound sodium chloride NaCl is + 1, while that of chlorine is – 1.
+1 -2 +1-1 +2 -1 Na2S KBr CaCl₂