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Define Valency.

Answer Verified Verified
Hint: Valency of an element is defined as the number of hydrogen atoms that can combine with or replace one atom of the element, either directly or indirectly. In other words, it is the combining power of an atom of the element, the number of electrons an atom of the element uses to combine with atoms of other elements.

Complete step by step answer:
-In an atom, the valence electrons are the electrons that participate in combination with other atoms - these are the electrons in the orbitals of the outermost shell which is also called valence shell.
- Alternatively, valency can also be defined as the number of electron(s) an element must lose in order to gain noble gas configuration. In order for us to define valency from this perspective we will first have to learn about Octet rule.
 -Octet Rule states that an atom is most stable when the valence shell is completely filled with eight electrons. If the atom has less than eight electrons in its valence shell then it tends to be unstable and tries to gain or lose electrons in order to have a completely filled valence electron. After the valence shell is completely filled, then only the atom becomes stable. Noble gases are the most stable as they already have a completely filled valence shell while all the other elements try to lose, gain or share electrons to achieve the noble gas configuration, i.e., fully filled valence shell.
 For example,
-Magnesium has atomic number 12. The electronic configuration of silicon is 2, 8, 2. The number of valence electrons is 2. To attain the nearest noble gas configuration of Neon, having electronic configuration of 2,8, it will lose 2 electrons in its valence shell. So, the valency is 2.
-Aluminum has atomic number 13. The electronic configuration of silicon is 2, 8, 3. The number of valence electrons is 3. To attain the nearest noble gas configuration of Neon, having electronic configuration of 2,8, it will lose 3 electrons in its valence shell. So, the valency is 3.
-Carbon has atomic number 14. The electronic configuration of silicon is 2, 8, 4. The number of valence electrons is 4. To attain the nearest noble gas configuration of Neon, having electronic configuration of 2,8, it will lose 4 electrons in its valence shell. So, the valency is 4.
-Phosphorus has atomic number 15. The electronic configuration of silicon is 2, 8, 5. The number of valence electrons is 5. To attain the noble gas configuration of Neon, having electronic configuration of 2,8, it will either lose 5 electrons in its valence shell or to attain the noble gas configuration of Argon, having electronic configuration of 2,8,8, it will gain 3 electrons. So, it shows valency of both 3 and 5.
-Oxygen has atomic number 8. The electronic configuration of silicon is 2, 6. The number of valence electrons is 6. To attain the nearest noble gas configuration of Neon, having electronic configuration of 2,8, it will gain 2 which will complete its octet. So, the valency is -Chlorine has atomic number 17. The electronic configuration of silicon is 2, 8, 7. The number of valence electrons is 7. To attain the nearest noble gas configuration of Argon, having electronic configuration of 2,8,8, it will gain 1 electron. So, the valency is 1.

Note:
When two elements combine to form compounds, the molecular formula of the product is simply the interchange of their valencies to obtain the ratio of their combining atoms or groups, written along with their symbol