The total mass is always conserved in a chemical reaction because mass cannot be destroyed nor created. This is expressed in the law of conservation of mass. Atoms in a reaction cannot form another type of atoms which has different properties than the original element. It can only exchange electrons which is responsible for bonding.
Explanation;
Through any chemical reaction no particles are generated or destroyed: the atoms are simply prepared from the reactants to the products. The products may have modified properties to the reactants. Mass is nevermore lost or grown in chemical reactions. We say that mass is always conserved.
Bonds are made up of the atoms and while they do have mass and the products of a rejoinder might really gain or lose electrons, no matter is converted to energy, so the mass difference is irrelevant to nill.
