Valency vs Oxidation State
Though the valency and oxidation state of some atoms and groups are similar in some occasions, it is important to known the differences in these terms.
According to the IUPAC definition valency is the “”maximum number of univalent atoms that may combine with the atom.” That means valency is given by the number of bonds which can be formed by an atom. The number of valence electrons that an atom has determines the valency of that atom. Valence electrons are the electrons in an atom which participate in the chemical bond formation. When chemicals bonds are forming, either atom can gain electrons, donate electrons or share electrons. The ability to donate, gain, or share depends on the number of valence electrons they have. For example, when a H2 molecule is forming one hydrogen atom gives one electron to the covalent bond. Thus, two atoms share two electrons. So the valency of a hydrogen atom is one. Univalent atoms or groups like hydrogen and hydroxyl have a valency of one whereas divalent atoms or groups have a valency of two, etc.
According to the IUPAC definition, oxidation state is “a measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have.” Oxidation state is an integer value, and it can be either positive, negative or zero. Oxidation state of an atom is subjected to change upon chemical reaction. If the oxidation state is increasing, then the atom is said to be oxidized, and if it is decreasing, then the atom has undergone reduction. In the oxidation and reduction reaction, electrons are transferring. In pure elements, oxidation state is zero. There are few rules we can use to determine the oxidation state of an atom in a molecule.
• Pure elements have a zero oxidation state.
• For monatomic ions, oxidation state is same as their charge.
• In a polyatomic ion, the charge is equal to the sum of oxidation states in all the atoms. So the oxidation state of an unknown atom can be found if the oxidation state of other atoms is known.
• For a neutral molecule, the sum of all the oxidation states of atoms is zero.
Other than the above method, oxidation state can also be calculated using the Lewis structure of a molecule. Oxidation state of an atom is given by the difference between the number of valence electron an atom has if the atom is neutral and the number of electrons belongs to the atom in the Lewis structure. For example, the methyl carbon in the acetic acid has a -3 oxidation state. In the Lewis structure, carbon is bonded to three hydrogen atoms. Since the carbon is more electronegative, the six electrons in the bonds belong to carbon. Carbon makes the other bond with another carbon; therefore, they split the two bond electrons equally. So all together in the Lewis structure carbon has seven electrons. When carbon is in the neutral state, it has 4 valence electrons. So the difference between them makes the oxidation number of carbon to be -3.
What is the difference between Valency and Oxidation State?
• Valency is given by the number of bonds that a species can form.
• Oxidation state is the charge an atom or group can have.