- Valency is the measure of the combining capacity of an element. It is typically equal to the number of electrons an atom gains, loses, or shares to achieve a stable electron configuration (usually a full outermost electron shell). - Carbon (Atomic number 6): Electron configuration is 2, 4. It needs 4 electrons to complete its outer shell, so its common valency is 4 (e.g., CH4, CO2). - Oxygen (Atomic number 8): Electron configuration is 2, 6. It needs 2 electrons, so its common valency is 2 (e.g., H2O, O2). - Nitrogen (Atomic number 7): Electron configuration is 2, 5. It needs 3 electrons to complete its outer shell. Thus, its common valency is 3 (e.g., NH3, NCl3). Nitrogen can also exhibit other valencies (e.g., +5 in nitrates), but 3 is a very common valency. - Sodium (Atomic number 11): Electron configuration is 2, 8, 1. It tends to lose 1 electron to achieve a stable configuration. Its valency is 1 (e.g., NaCl, Na2O).