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.”
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.
The smallest particle of an element which cannot exist independently and take part in a chemical
reaction is known as Atom.
Hexogen(H), Carbon (C), Sodium (Na), Gold (Au) etc.
The particle of a substance (Element or Compound) which can exist independently and show all the
properties of that substance is called molecule.
Atoms of the same or different elements react with each other and form molecule.
Atoms of some elements can exist independently, since they have property of molecule so they are
called mono atomic molecule.
Examples of Molecules of the elements are Hydrogen (H2). Nitrogen (N2), Sulphur (S8) etc.
Molecules of different elements are called compounds. For example HCl, H2O, CH4 etc.
Valency The combining capacity of all elements with other elements is called valency.
H = 1
C = 4
Al = 3
Mg = 2
Na = 1
“A brief name used for full chemical name at a compound is called Chemical Formula.”
A chemical formula is used to represent an element or a compound in terms of symbols. It also
represents the number and type of atoms of elements present in the smallest unit of that substance.
The chemical formula of hydrogen sulphide is H2S. It shows two types of elements (H and S) and number
of atoms of element (2H and 1S). Similarly the formula of NaCl show number and type of different atoms
present in its smallest unit.
“The formula which shows the minimum (simple) ratio between atoms present in a compound is known
as Empirical Formula.”
For example the empirical formula of hydrogen peroxide is HO that of water is H2O and benzene is CH.
The formula of an element or a compound which represents the actual number of atoms present in the
molecule of these substances is called molecular formula.
Water, Hydrogen Peroxide, Ethylene Benzene and Sulphur have molecular formula H2O, H2O2, C2H4,
C6H6 and S8 respectively.
Molecular mass of an element or a compound is defined as the mass of its molecule relative to 1/12th of
the mass of C-12. It is the sum of the atomic masses of all the atoms presents in its molecular formula. Example
Molecular mass of water (H2O) = 2 + 16 = 18 a.m.u
Mass of hydroge