Define valency by taking examples of silicon and oxygen.

Answer : The valency may be defined as the combining capacity    of the atoms of  elements,  or their tendency to  form molecules with atoms of the same or different elements. The electrons in an atom are arranged in different shells/orbits. The electrons present in the outermost shell of an atom are known as the valence electrons. The outermost shell of an atom can accommodate a maximum of 8 electrons. All atoms of various elements have tendency to have a fully-filled outermost shell with 8 electrons or octet . This is achieved through reaction with atoms of other elements,  by sharing, gaining or losing electrons. 

• In case of  Silicon, it has 4 electrons in the outer most shell, thus it is deficient by 4 electrons to have a fully filled outer most shell. Hence Its valency is 4.
• In case of  Oxygen, it has 6 electrons in the outer most shell, thus it is deficient by 2 electrons to have a fully filled outer most shell. Hence Its valency is 2.

This is an answered question from Chapter 4. Structure Of The Atom of CBSE Class 9th Science

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