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 HOW CAN I CALCULATE VALENCY?

 When we move from left to right in periodic table valency increases. But after 4, it decreases also.

The electrons present in the last shell determine the valency of a particular element.

If the number of valence electrons ≤4: valency = number of valence electrons

If the number of valence electrons >4: valency = 8 - number of valence electrons.

For e.g.

Group in periodic table

Number of valency electrons

valency

1

1

1

2

2

2

13

3

3

14

4

4

15

5

3, 5

16

6

2, 6

17

7

1, 7

18

8

0, 8

For your convenience, I am providing two examples to show how valency of any element can be found out.

If you consider the element aluminum and try to write its electronic configuration, then it is 2, 8, 3 the valency is equal to 3.

However, for element oxygen having electronic configuration 2, 6 the valency is 8 - 6 = 2.

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  1. As a general valency of H is considered as +1, Halogen as -1, oxygen as -2 for most calculation of valency .
  2. In neutral molecules, the sum of the valance of all the atoms is zero.
  3. In ions, the sum of the valance of all the atoms is equal to the charge on the ion. For eg..

 

In SO4 2- ion, the sum of valency of all the atoms will be equal to -2.

Now , let us calculate the valency of S in SO4 -2.

Let the Valency of S in SO4 2- is x.

Now X + 4(-2) = -2

Or X = -2+8 = +6

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 Valence shell refers to the outermost shell of any atom or element. Valency is defined as the number of electrons gained or lost by an atom. The electrons present in the last shell determine the valency of a particular element. If,

· Number of valence electrons ≤ 4

Then the valency = number of valence electrons

· Number of valence electrons >4

Then the valency = 8 - number of valence electrons


If you consider the element ‘magnesium’ and write down its electronic configuration, it comes out to be 2, 8, 2. The electrons present in the outermost shell are 2 (which is less than 4), so the valency is equal to 2.

However, for element ‘oxygen’ the electronic configuration is 2, 6. The electrons present in the outermost shell are 6 (which is greater than 4), so the valency is equal to 8 – 6 = 2.

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Calculating the Valency of an Element (or Molecule)
1

Consult the periodic table of the elements to determine the valency of an element. The periodic table is organized by groups in rows and columns, and the elements of groups I-VIII have the same valency as others in their group.

All the elements in group VIII have eight electrons in their outer shells, and thus have a valency of zero (highly stable). Elements in group I just have one valent electron in their outer shells and thus have a valency of one, which means they are very reactive. Group IV/valency 4 elements like carbon are relatively stable. Group VI and VII elements like oxygen are also reactive as they seek electron pairs to complete their outer shell octet.
2

Calculate the valency of an element using the total number of electrons. The valency of an atom is equal to the number of electrons in the outer shell if that number is four or less. Otherwise, the valency is equal to eight minus the number of electrons in the outer shell. The number of electrons in each shell of an atom is regular so if you know the number of electrons in the atom, then you can calculate the valency. All atoms (except hydrogen) have two electrons in the first electron shell, and up to eight electrons in each succeeding electron shell. For example, carbon has six electrons, two in the first shell, and four in the outer shell, giving it a valency of four. Oxygen has eight electrons, two in the first shell and six in the outer shell, giving it a valency of two (8 - 6 = 2).
3

Calculate the valency of multi-element molecules using the same procedure. For example, to determine the valency of the ionic molecule phosphorus tetraoxide (PO4, four atoms of oxygen and one atom phosphorus) you multiply the total valency of the four oxygen atoms (valency 2) and subtract that from the valency of the phosphorus atom (valency 5). That reveals the valency of PO4 is 3.

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