In order to achieve an octet in the outermost shell, atoms react. This is done by gaining, sharing or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives the combining capacity of the element which is called the valency of an atom.
Valency of silicon: The atomic number of silicon is 14. It has 14 electrons in its atom. The electronic configuration of silicon atom will be K shell- 2, L shell- 8 and M shell- 4. Silicon has 4 electrons in its valence shell. Now, Silicon atom can neither lose 4 electrons nor gain 4 electrons to complete its the octet due to energy considerations. Hence, silicon atom will share its 4 electrons with the 4 electrons of atoms of other elements to complete its octet. Since one silicon atom will share 4 electrons to achieve inert gas electron arrangement, therefore, the valency of silicon is 4.
Valency of oxygen: The atomic number of oxygen is 8. It has 8 electrons in its atom. The electronic configuration of oxygen atom will be K shell-2 and L shell- 6. Oxygen has 6 electrons in its outermost shell. Now, an oxygen atom having 6 outermost electrons can gain 2 electrons from some other atom to achieve the nearest noble gas configuration. Since one atom of oxygen can gain 2 electrons to complete its octet. Therefore, the valency of oxygen is 2.
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