Valency refers to the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration, typically resembling that of a noble gas. It is not the same as the number of electrons in the last orbit or the number of electrons needed to fill the last orbit.
The last orbit or energy level of an atom is called the valence shell, and the electrons present in this shell are known as valence electrons. Valence electrons are the ones involved in chemical bonding and reactions. The number of valence electrons determines the atom's chemical properties and its tendency to form bonds with other atoms.
The valency of an atom is determined by the number of valence electrons it possesses. To achieve a stable electron configuration, atoms tend to gain, lose, or share electrons in a way that fills or empties their valence shell.
For example, atoms with one to four valence electrons typically lose those electrons to achieve a stable configuration, and their valency corresponds to the number of electrons they lose. On the other hand, atoms with five to seven valence electrons tend to gain electrons, and their valency is determined by the number of electrons needed to complete their valence shell.
The concept of valency helps in understanding an atom's bonding behavior and its ability to form compounds with other atoms. By knowing the valency of an element, one can predict how it may interact with other elements to form chemical bonds.