Guess Papers ( Chemistry, 9th Class )

The mass of an atom of the element relative to the mass of some reference or standard element is called atomic mass. Atoms are very small particles. They have very small mass. If the masses of atoms were to be expressed in gram. It is a very big unit for this very tiny object. Then it was decided by the chemists that masses of the atoms were to be found after comparing with mass to some standard form. Hydrogen being the lightest element is taken as standard. The mass of the hydrogen atom taken as one. The atomic mass could be defined as "Atomic mass of an element is the mass of an atom of that element as compared to the mass of an atom of hydrogen taken as one." Example The atomic mass of sodium is 23. It means that an atom of sodium is 23 times heavier than hydrogen atom. Similarly atomic mass of oxygen is 16. It means that an atom of oxygen is 16 times heaviest than that of hydrogen.