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In 1803, Dalton published a new system of chemical philosophy in which the following statements comprise the atomic theory of matter:
1. Matter consists of indivisible atoms.
2. All the atoms of a given chemical element are identical in mass and in all other properties.
3. Different chemical elements have different kinds of atoms, and, in particular, such atoms have different masses.
4. Atoms are indestructible and retain their identity in chemical reactions.
5. The formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole number ratio.

Atomic Mass

One atomic mass unit (amu), equals exactly one twelfth of mass of a carbon-12 atom, Atomic mass unit (amu) is now a days written as unified mass unit and is denoted by the letter ‘u’. The relative atomic mass of an element expressed in atomic mass unit is called its atomic weight. Now a days, we are using atomic mass in place of atomic weight.

        Atomic mass = mass of protons + mass of neutrons

Total number of protons and neutrons is called mass number (A). By convention, atomic number is written at the bottom of left comer of the symbol of the atom of a particular element, and mass number is written at the top left corner. For example, symbol 12 6C indicates that there is a total of 12 particles (nucleons) in the nucleus of a carbon atom, 6 of which are protons. Thus, there must be 12-6 = 6 neutrons. Similarly, 16 80 indicates 8 protons and 16 nucleons (8 protons + 8 neutrons). Since atom is electrically neutral, oxygen has 8 protons and 8 electrons in it. Further, atomic number (Z) differentiates the atom of one element from the atoms of the other elements.

In 1961, C-12 (or 12 6C) atomic mass scale was adopted. This scale depends on measurement of atomic mass by an instrument called mass spectrometer.

Isotopes and Atomic Mass

Atoms of an element that have the same atomic number (Z) but different mass number (A) are called isotopes.


A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement. An atom is the smallest particle of a substance but can not exist freely. Contrary to this, molecules can be considered as the smallest particle of an element or of a compound which can exist alone or freely under ordinary conditions.

Molecular Mass

Molecular formula of a compound is normally used for determining the molecular mass of that substance.

If a substance is composed of molecule, for example: CO2, H2O, or NH3, it is easy to calculate the molecular mass. Molecular mass is the sum of atomic masses of all the atoms present in that molecule. Thus, the molecular mass is the sum of atomic masses of all the atoms present in that molecule.


A mole is the amount of substance that contains as many elementary entities (atoms, molecules, formula unit, or other fundamental particles) as there are atoms in exactly 0.012 kg of carbon-12 isotope.

Molar Mass

Mass of one mole of a substance is called its molar mass. A substance may be an element or a compound. Mass of one mole atoms of oxygen means mass of 6.02 x 1023 atoms of oxygen.

Atomic number (Z)

The number of proton or electron in an atom of the element is called atomic number. It is denoted by Z.
      Z = e = p
where, e = no. of electrons and p = no. of protons.

Mass number (A)

The sum of number or protons and neutrons in an atom of the element is called mass number. It is denoted by A.

      A = p + n
where, p — no. of protons and n = no. of neutrons


These are atoms of the elements having the same atomic number but different mass number.



These are atoms of the elements having the same mass number but different atomic numbers



These are atoms of different elements having the same number of neutrons.



These are atoms / molecules / ions containing the same number of electrons.
O2- F-, Ne, Na+, Mg2+
CN- , N2, O22+ etc.

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