Typical oxidation states for chemical elements
How to count the electrons' transmission
Before we discuss the oxidation state, let’s recall the main rules of chemistry and physics:
- all substances are formed from molecules, and molecules from atoms;
- any atom is electrically neutral; i.e. it has a total charge of zero;
- the zero charge of an atom is determined by the identical number of positively and negatively charged particles in it;
- negatively charged particles inside the atom are called “electrons” – they move around the atom nucleus. The charge of one electron is -1;
- the total negative charge of all the electrons of the atom is equal to their quantity;
- positive particles of the atom are called “protons” and are located in its nucleus, and the charge of one proton is (+1);
- the total positive charge of the nucleus is equal to the total number of protons in it;
- the precise number of protons and electrons in the atom can be determined by checking its number in the periodic system:
№ of element = number of protons in the atom = number of electrons in the atom.
Let us examine the above statements based on the examples of oxygen (О), hydrogen (Н), calcium (Са) and aluminum (Аl).
Oxygen (О) has the ordinal number of eight in the periodic system, and this means that there are eight protons in its nucleus, and eight electrons move around its nucleus.
Thus, the charge of the nucleus of its atom is equal to (+8), and the total charge of electrons moving around its nucleus is equal to (-8).
The total charge of the atom for the chemical element is determined by adding all the positive and negative charges in the atom:
(+8)+(-8)=0.
Hydrogen (Н) holds first place in the periodic system, and accordingly there is one proton in its nucleus, and one electron moves around the nucleus.
(+1)+(-1)=0.
Calcium (Са) is in 20th place in the periodic system, and so its atom has 20 protons and electrons, the total charges of which are (+20) and (-20) respectively:
(+20)+(-20)=0.
As for aluminum (Аl), its 13th place in the periodic system shows that it has 13 protons and 13 electrons:
(+13)+(-13)=0.
A few words about the oxidation state
As we know, in the earth’s crust chemical elements not only exist in a free state. Their atoms also enter into chemical reactions, forming complex substances. This can be easily demonstrated from the example of the formation of oxides.
For example, oxygen (О) can interact with hydrogen (Н). At the same time, hydrogen gives oxygen its only electron. After this, the hydrogen atom no longer has any free electrons, and accordingly, the positive charge of the atom’s nucleus of (+1) has nothing to neutralize it, and the entire hydrogen atom gains a charge of (+1). Thus, the electrically neutral hydrogen atom turns into a positively charged particle – a proton:
(+1) + (-1) - (-1)= (+1).
The oxygen atom, which in a free state also has a zero charge, can join two electrons to itself at once. This means that it enters into a reaction with two hydrogen atoms at once, each of which gives it its only electron.
Thus, oxygen, which before its reaction with hydrogen has eight protons and electrons, acquires another two electrons in this chemical interaction. And so its total charge becomes:
(+8)+(-8)+(-2)=(-2).
This example illustrates a reaction in which an atom of one chemical element gives its electrons to an atom of another chemical element. These reactions are called reductive-oxidative in chemistry.
It is accepted that the atom that gives its electrons oxidizes, and the atom that receives them is reduced. In this case, hydrogen oxidizes, and oxygen reduces. The charge which both atoms receive as a result of the reaction is written in the top right-hand corner, above the symbols of their chemical elements.
It should also be remembered that oxygen and hydrogen are gases, and so there are two identical atoms in their molecules. Accordingly, the full reaction of the interaction of oxygen with hydrogen looks like this:
2Н₂⁰ + О₂⁰ → 2Н₂⁺¹О⁻²
In this case, this concerns the formation of compounds of the type X₂O, in which in order to receive a molecule of a complex substance, two identical atoms of another element attach to one oxygen atom. The oxidation state (+1) is characteristic for elements of the first group of the periodic system belonging to the main sub-group.
Oxidation state in XO
The second group of the periodic system, namely its main subgroup, contains chemical elements in which each atom can give two electrons to oxygen. This atom, its reductive-oxidative reaction, gains a charge (+2), and oxygen, as always, receives the charge (-2). For example, the reaction of the oxidation of calcium:
2Са⁰ + О₂⁰→2Са⁺²О⁻².
Zinc (Zn), located in the subsidiary subgroup of the second group, shows the same oxidation state as calcium, namely XO:
2Zn⁰ + О₂⁰→2Zn⁺²О⁻²
Oxidation state in X₂O₃
The feature of the elements of the main subgroup of the third group of the periodic system is that each atom can easily give three electrons to the oxygen atom. But one oxygen atom can only accept two electrons.
Accordingly, the ratio of atoms in the molecule of oxides for elements of third group, on the example of aluminum oxide, is the following:
- If one aluminum atom can give three electrons, two aluminum atoms give six electrons – three each;
- one oxygen atom can take only two electrons, and so each oxygen atom takes two electrons from the aluminum atoms:
4Al⁰ + 3O₂⁰ → 2Al₂⁺³O₃⁻²
Thus, in this chemical reaction four aluminum atoms take part, which give 12 electrons to six atoms (or three molecules) of oxygen.
As a result of the reaction, each aluminum atom will lack three electrons for a zero charge, and thus the positive charge of the nucleus will predominate over the negative charge of the electrons:
+13 (the charge of the Al atom has not changed) -10 (electrons left after the reaction)= (+3).
Oxidation states in XO₂
This oxidation state is shown by chemical elements located in the main subgroup of the fourth group of the periodic system. Each atom can give four electrons at once, and as the oxygen molecule is diatomic, each of the oxygen atoms takes two electrons.
Let us examine this reductive-oxidative reaction on the example of the reaction of oxygen with carbon:
С⁰ + О₂⁰ → С⁺⁴О₂⁻²
This is a reaction that illustrates the combustion of a solid, carbon, in the presence of oxygen gas. So the oxygen molecule is diatomic, and the carbon molecule is monoatomic.
Click here for finding out how different metals can be oxidized.
Oxidation state in в X₂O₅ and XO₃
For some elements of the fifth group, in the main subgroup, the oxidation state of (+5) is characteristic, i.e. they can give an oxygen atom five electrons. For example, the reaction of the combustion of phosphorous in the presence of oxygen:
4Р⁰ + 5О₂⁰ → 2Р₂⁺⁵О₅⁻².
Some elements of the sixth group can give six electrons, after which their oxidation state becomes (+6). For example, the reaction of sulfur with oxygen:
2S⁰ + 3O₂⁰ → 2S⁺⁶O₃⁻²
