Chemistry

Structure of the Atom

Valaence Electrons (Or Valancy Electrons)

The outermost electron shell of an atom is known as valence shell.The electrons present in the outermost shell of an atom are known as valence electrons (or valency electrons) because they decide the valency (combining capacity) of the atom.Only the valence electrons of an atom take part in chemical reactions because they have more energy than all the inner electrons of the atom. Let us take the example of sodium atom to understand the meaning of valence electrons clearly.
The atomic number of sodium is 11, which means that one sodium atom has 11 electrons in it. So, the electronic configuration of sodium atom will be

A = \begin{matrix} k \\ 2 \end{matrix}

A = \begin{matrix} L \\ 8 \end{matrix}

A = \begin{matrix} M \\ 1 \end{matrix}

.In the sodium atom,M shell is the outermost shell or valence shell (see Figure 24).There is 1 electron in the outermost shell of sodium atom, therefore, sodium atom has 1 valence electron.
When a sodium atom combines with other atoms, only its one valence electron (in the M shell) takes part in chemical reactions. The electrons of two inner shells (K shell and L shell) of the sodium atom never take part in chemical reactions.
We can now say that those electrons of an atom which take part in chemical reactions are called valence electrons.Valence electrons are located in the outermost shell of an atom.In a chemical reaction,valence electrons of an atom are either transferred to the valence electrons of another atom, or shared with the valence electrons of another atom.
(i) The elements Hydrogen, Lithium, Sodium and Potassium have 1 valence electron each in their atoms.
(ii) The elements Helium,Beryllium, Magnesium and Calcium have 2 valence electrons each in their atoms.
(iii) The elements Boron and Aluminium have 3 valence electrons each in their atoms.
(iv) The elements Carbon and Silicon have 4 valence electrons each in their atoms.
(v) The elements Nitrogen and Phosphorus have 5 valence electrons each in their atoms.
(vi) The elements Oxygen and Sulphur have 6 valence electrons each in their atoms.
(vii) The elements Fluorine and Chlorine have 7 valence electrons each in their atoms.
(viii) The elements Neon and Argon have 8 valence electrons each in their atoms.
In order to find out the number of valence electrons in an atom of the element, we should write down the electronic configuration of the element by using its atomic number. The outermost shell will be the valence shell and the number of electrons present in it will give us the number of valence electrons.This point will become more clear from the following example.
Sample Problem. What is the number of valence electrons in the atom of an element X having atomic number 17 ? Name the valence shell of this atom.
Solution. In order to find out the number of valence electrons, we should write down the electronic configuration of element X. The atomic number of element X is 17, so one atom of X contains 17 electrons.The electronic configuration will be :

A = \begin{matrix} K \\ 2 \end{matrix}

A = \begin{matrix} L \\ 8 \end{matrix}

A = \begin{matrix} M \\ 7 \end{matrix}

Here M shell is the outermost shell or valence shell of the atom and it has 7 electrons in it.Thus,there are 7 valence electrons in the atom of element X.
Before we can understand valency (or combining capacity) of elements,it is necessary to know why the elements called noble gases (or inert gases) are chemically unreactive and why atoms of elements combine with one another.This is discussed below.

Notes

Structure of the Atom - Notes

1. Drawback of Rutherford’s Model of the Atom