How is the valency of an element defined?

The valency of an element is defined as the number of bonds an atom forms when it reacts. This concept is crucial in chemistry as it helps to understand how elements interact with each other to form compounds. An atom's ability to bond is directly influenced by the number of electrons in its outer shell, which are available for bonding with other atoms.

For instance, if an atom has four electrons in its outer shell, it can form four bonds with other atoms, indicating a valency of four. This ability to bond is what determines how many connections an atom can make, guiding the formation of molecules and the structure of compounds. Therefore, option B accurately captures this definition of valency.

The other options do not align with the correct definition: losing electrons when forming an ion pertains to ionic charge rather than bonding ability, whereas isotopes refer to variations of elements with different numbers of neutrons, and atomic mass units relate to the mass of atoms rather than their bonding capabilities.