We all know that the chemical formula for water is H2O. How many times have we wondered why it is written as “H2O” and not something else? What is the reason for this particular formula? The answer to the above question is “Valency”. Let us know more about Valency and how it helps in determining a formula!
What is Valency?
Valency is the measure of the combining capacity of atoms or molecules. Therefore, it is the capacity of an atom of a single element to react and combine with particular numbers of atoms of another element.
The Concept of Valency Explained
Electrons in an atom are arranged in different orbitals (shells) represented as K, L, M, N, and so on. The electrons present in the outermost shell/orbit of an atom are called valence electrons. The valence electrons take part in any chemical reaction because the outermost orbit usually contains more energy than the electrons present in other orbits.
According to the Bohr-bury scheme, the outermost orbit of an atom will have a maximum of 8 electrons. However, if the outermost orbit is completely filled then very little to no chemical activity is observed in the particular element. Their combining capacity becomes negligible or zero.
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This is why noble gases are least reactive because their outermost orbit is completely filled. However, the reactivity of other elements depends upon their capacity to gain noble gas configuration. It will also help to determine the valency of an atom.
Achieving Complete Octet
If the outermost shell of an atom has a total of 8 electrons then the atom is said to have attained a complete octet. An atom has to gain, lose or share a particular number of electrons from its outermost orbit to obtain complete octet. Therefore, a capacity of an atom is the total number of electrons gained, lost, or shared to complete its octet arrangement in the outermost atom. This capacity of an atom will also determine the valency of an atom.
For instance, hydrogen has 1 electron in its outermost orbit so it needs to lose 1 electron to attain stability or octet. Thus, the valency of hydrogen is 1. Similarly, magnesium has 2 electrons in its outermost orbit and it needs to lose them to attain octet and obtain stability. Therefore, the valency of magnesium is 2.
Stability is also determined by the ability of atoms to gain electrons. For instance, Fluorine has 7 electrons in its outermost orbit. It is difficult to lose 7 electrons but it is easy to gain one electron. Thus, it will gain one electron to obtain octet so the valency of fluorine is 1.
Examples of Valency
Valency of Sodium
The atomic number of sodium is 11 (Z=11). The electronic configuration of sodium can be written as 2, 8, 1. 2, 8, 1 electrons are distributed in the shells K, L, M respectively. Therefore, valence electron in sodium is 1 and it needs to lose 1 electron from the outermost orbit to attain octet. Hence, the valency of sodium is 1.
Valency of Chlorine
The atomic number of chlorine is 17 (Z=17). The electronic configuration of chlorine can be written as 2, 8, 7. 2, 8, 7 electrons are distributed in the shells K, L, M respectively. Therefore, valence electron in chlorine is 7 and it needs to gain 1 electron from the outermost orbit to attain octet. Hence, the valency of chlorine is 1.
Examples of Valency on the basis of Chemical Formula
We know valency is the capacity of an atom to combine with a particular number of atoms of another element. In the case of ammonia, one nitrogen atom combines with 3 hydrogen atoms. The atomic number of hydrogen is 7. The electronic configuration is 2, 5. 2, 5 electrons are distributed in the orbits K, L. Therefore, a nitrogen atom needs to gain 3 electrons in its outermost orbit to complete octet.
The nitrogen atom combines with 3 hydrogen atom in the case of NH3. Therefore, its valency is 3. However, the hydrogen atoms present in ammonia combines with one nitrogen atom. Therefore, the valency of hydrogen is one. This is how chemical formulae of compounds are formed by swapping the valencies.
Uses of Valency
- It helps to determine a chemical formula.
- It helps to determine how many atoms of an element will combine with another element to form any chemical formula.
Methods of Determining Valency
The valency of the same group of the element present in the periodic table is the same. If we consider group 8 in the periodic table, all the elements of group 8 have completely filled outermost orbit and have attained octet arrangement. So, the elements of group 8 have zero valencies. The valency of any element can be determined primarily by 3 different methods:
1) The Octet Rule
If we cannot use the periodic table to determine valency then the octet rule is followed. This rule states that atoms of an element or chemicals have a tendency to obtain 8 electrons in their outermost orbit either by gaining or losing electrons in whatever form of compound it is present. An atom can have a maximum of 8 electrons in its outermost orbit. The presence of 8 electrons in the outermost shell indicates stability of an atom.
An atom tends to lose electron if it has one to four electrons in its outermost orbit. When an atom donates these free electrons it has positive valency. An atom will gain electrons if it has four to seven electrons in its outermost orbit. In such cases, it is easier to accept electron rather than donating it. Therefore, we determine the valency by subtracting the numbers of electrons from 8. All noble gases have 8 electrons in its outermost orbit except helium. Helium has 2 electrons in its outermost orbit.
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2) Using the Periodic Table
In this method, valency is calculated by referring to the periodic table chart. For example, all the metals, be it hydrogen, lithium, sodium and so on, present in column 1 have valency +1. Similarly, all the elements present in column 17 have valency -1 such as fluorine, chlorine, and so on. All the noble gases are arranged in column 18. These elements are inert and have valency 0.
However, there is an exception to this method of valency determination. Certain elements like copper, iron, and gold have multiple active shells. This exception is usually noticed in transitional metals from column 3 through 10. It is also observed in heavier elements from column 11 through 14, lanthanides (57-71), and actinides (89-103).
3) On the Basis of the Chemical Formulae
This method is based on the octet rule. The valencies of many transitional elements or radicals can be determined in a particular compound by observing how it chemically unites with elements of known valency. In this case, the octet rule is followed where the elements and radicals combine and try to attain eight electrons in the outermost shell in order to become stable.
For instance, consider the compound NaCl. We know that the valency of sodium (Na) is +1 and Chlorine (Cl) is -1. Both sodium and chlorine have to gain one electron and lose one electron respectively to achieve stable outermost orbit. Therefore, sodium donates an electron and chlorine accept the same electron. This is how the valency is determined. It is the classic example of ionic reaction as well.
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Difference between Valency and Oxidation Number
The Combining capacity of an atom is called valency. Thus, it is the number of valence electron an atom has to gain or lose from its outermost orbit. The oxidation number is the charge an atom can carry.
For instance, nitrogen has valency 3 but its oxidation number can range from -3 to +5. Oxidation number is an assumed charge of a particular atom in a molecule or ion. It helps to determine the capacity of an atom to gain or lose electrons within a particular species.
The Valency of First 20 Elements
A Solved Question for You
Q: Determine the valencies of neon, phosphorus, sulfur.
Answer: Neon=0, Phosphorus= 3, Sulphur= 2. Explanation:
- Atomic Number of Neon=10
Electronic configuration of Neon= 2, 8
Therefore, Valency =0 (It is already in its octate arrangement or stable state)
- Atomic Number of Phosphorus =15
Electronic configuration of phosphorus= 2, 8, 5
Therefore, Valency= 8-5=3
- Atomic Number of Sulphur =16
Electronic configuration of Sulphur = 2, 8, 6
Therefore, Valency= 8-6=2