Valency is defined as the combining capacity of an atom or a molecule. There are different ways to determine the valency of a given atom and they are:
- Octet rule: According to the octet rule, a compound is said to be stable when the outermost shell of the compound is having eight electrons. If the number of electrons in the outer shell is between one to four, the compound is said to have positive valency. For compounds with electrons four, five, six, or seven, the valency is determined by subtracting the electron from eight. All noble gases except for Helium have eight electrons.
- Periodic table: Periodic table consists of elements that are arranged in such a way that, the valency of a given element can be determined. Metals in column 1 have a valency of +1 while the noble gases in column 18 have a valency of 0 and are inert.
- Valency from the chemical formula: This rule is based on the octet rule and by observing the combining of an element, the valency can be determined.
Therefore, these are the ways or methods that are used for calculating the valency.