One of the key concepts that allows us to describe the oxidation and reduction properties of molecules is the oxidation state. This is only an auxiliary quantity: it does not describe the true charge on each of the atoms in the molecule, but helps to get an idea how the giving up and acceptance of electrons takes place in oxidation and reduction reactions. There is a certain method that helps us to calculate correctly the oxidation states for each atom.
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How to determine the oxidation state in a simple substance
Substances are called simple which consist of atoms of the same kind. For example, simple substances include oxygen (O₂), hydrogen (H₂), sodium (Na), beryllium (Be), iodine (I₂), ozone (O₃) and others.
Each of these substances has a zero oxidation state. This can be explained by the fact that the electrons in molecules of this type do not shift anywhere. In diatomic molecules consisting of atoms of one element, a covalent non-polar bond is realized (Cl-Cl; H-H): as bonded atoms are equal, the displacement of electron density to any other atom is not observed, and so the movement of electrons does not take place. In monoatomic molecules (for example helium He, Argon Ar), the oxidation state is also zero.
How to determine oxidation states in complex substances
Substances are called complex which consist of two or more types of atoms.
For example, table salt NaCl is a complex (or binary, i.e. consisting of atoms of two types) compound, as it contains atoms of different electron configurations that are chemically connected to each other. In these compounds you can place the non-zero oxidation states, as a movement of electron density is observed to the most electrically negative element. In sodium chloride, the electrical negativity is higher in chlorine (this non-metal is a strong oxidizer, and so its electrical negativity is much higher than sodium, which is a reducer). The oxidation state of sodium is +1, and the oxidation state of chlorine is -1.
To establish the correct oxidation state on an atom in a compound, we may use the following rules.
1. The oxidation state of oxygen in compounds is usually -2 (an exception is peroxide (for example Na₂O₂) and superoxides (KO₂), where the oxidation state of oxygen is -1 and -1/2 respectively; in ozonides such as KO₃ the oxidation state of oxygen is -1/3; oxygen only has the positive oxidation state of +2 in the compound with fluorine OF₂).
2. The oxidation state of fluorine is always -1.
By Giovani Rech – Own work, CC BY-SA 4.0, Link
Animation showing the crystal structure of beta-fluorine
3. The maximum oxidation state of an element is frequently equal to the number of the group it is located in; exceptions are oxygen (+2), fluorine (-1), iron (+6), the subgroup of nickel (+3, more rarely +4), and noble gases.
4. The minimum negative oxidation state is calculated according to the formula: the number of the group minus 8 (in calculating the valence, the formula is calculated vice versa – the number of the group is subtracted from 8).
5. Oxidation states of simple monoatomic ions are equal to their charges (for example, Na(+) has both a charge of 1+ and an oxidation state of +1; a similar situation exists with Mg(2+), F(-) etc.).
6. In non-ionic compounds, the oxidation degree of hydrogen is +1 (an exception is compounds with silicon and arsenic SiH₄ и AsH₃; in hydrogen hydrides hydrogen also has a negative oxidation state: in NaH sodium has an oxidation state of +1, while hydrogen has an oxidation state of -1).
7. In compounds of non-metals, which do not contain hydrogen or oxygen, the atom with the negative oxidation state is the one with a higher electrical negativity (it can be seen in the corresponding reference table): the value of the oxidation state in these compounds for a more electrically negative non-metal corresponds to the charge of its most widespread ion (in carbon sulfide CS₂ carbon has the oxidation state of +4, while sulfur is a more electrically negative atom, and its most common ion has the charge of -2.
Carbon sulfide CS₂
According to these rules, we can calculate the oxidation states of atoms for any molecule.
Calculating oxidation states in complex molecules
The summary oxidation of a molecule should be zero, as the molecule is neutral.
Calculating values for elements which can have several oxidation states
In calculating summary oxidation states, attention is always paid to indices: in the perchloric acid molecule HClO₄ oxygen has the oxidation state of -2. As there are 4 oxygen atoms in the molecule, its oxidation state is multiplied by 4: -2*4 = -8.
This plays a role in determining oxidation states in elements in which this value may vary. Chlorine has many possible oxidation states, so the value for HClO₄ may be calculated mathematically, with the equation:
+1 + х + (-2)*4 = 0
х = +7
The oxidation state of chlorine in perchloric acid is +7, as each of the 4 oxygen atoms have an oxidation state of -2, this value is +1 for hydrogen, and the molecule must have a zero oxidation state in this sum).
Equation of oxidation states of elements in magnesium and beryllium hydroxides
In magnesium hydroxide Mg(OH)₂ there are two hydrogen atoms with an oxidation state of +1 and two oxygen atoms with oxidation states of -2. If these oxidation states are added taking the indices into account, we may receive the value of -2: (+1)*2+(-2)*2= -2.
The oxidation state of magnesium in the compound is +2 (as magnesium is a member of the second group of the periodic table).
When we add the values, we get zero: +2+(-2)=0.
This means that the oxidation states have been calculated correctly: for magnesium the value is +2, for oxygen -2 and for hydrogen +1.
All atoms in magnesium hydroxide Mg(OH)₂ have fixed values of oxidation states, so this compound is a rather simple case for determining conditional charges in atoms.
The situation with beryllium hydroxide Be(OH)₂ is similar: the oxidation state of beryllium always corresponds to its charge and is +2, the oxidation state of oxygen of compounds is -2, and of hydrogen +1. If these values are added taking into account the indices, we get zero:
+2 + (-2 + (+1))*2 = 0.
How the oxidation state differs from valence and charge
The oxidation state, valence and charge of an element are often identical in value. Nevertheless, these concepts have a different meaning. The oxidation state is the conditional charge on each atom in the compound (it is written above each atom, and first its algebraic sign must be indicated, and then the numerical value). The ion charge is written differently: for simple ions it is also written above the element symbol, but first its value is indicated, and then the algebraic sign (for example, 2+). For complex ions (such as the sulfate ion SO₄²⁻), the charge is not indicated above the specific element, as the oxidation state, but above the entire complex ion. Click here to find out more about oxidation states.
The charge is connected with its oxidation states: for example in Mg(OH)₂ two hydroxyl groups are present. The charge of the OH group is always (1-). According to the rules, the sum of the oxidation states of atoms in this group should be equal to its charge (for the OH group, which consists of oxygen and hydrogen, this rule is observed, as -2+1=-1).
Given that there are two OH groups in magnesium hydroxide, we may say that their summary charge is (2-). The oxidation state of magnesium (+2) coincides with its charge (2+).
Valence is the ability of atoms to form a certain number of chemical bonds. It can only have a positive value. Often valence coincides with the oxidation level in its numerical value, but there are also certain exceptions – in nitric acid HNO₃ the valence of nitrogen is IV, but the oxidation state is +5.
In molecular nitrogen a triple bond is realized between atoms (so valence is III), but the oxidation state is 0. Valence may be determined by the structural formula of the substance.
The oxidation state plays a key role in recording the oxidation-reduction processes by the method of electron balances. The electron balance is the simplest method of recording the movement of electrons in a reaction, in which not real particles are examined, which exist in a solution (for example ions), but only atoms in compounds, which change their oxidation states, giving and taking electrons.