According to the definition given by the International Union of Pure and Applied Chemistry (IUPAC), valency is the “maximum number of univalent atoms that may combine with the atom”. In simple words, the valency of an atom is represented by the number of bonds it can form. The valency of an element is determined by the number of electrons present in its valence shell. During the chemical bond formation, the atom either loses these valence electrons or adds more electrons to the valence shell to stabilize it by completing the outermost shell. Valency is a whole number value that can be positive (when losing electrons) or negative (when gaining electrons).
Image courtesy: chelationtherapyonline.com
According to the IUPAC definition, oxidation state is “a measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have”. The oxidation state of an atom is subject to the chemical reaction it is undergoing. If there is an increase in the oxidation state of an atom it is said to be oxidized, and in the opposite case, the atom is said to be reduced.
Image courtesy: intechopen.com