Atoms and Molecules

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The smallest particle of an element, which may or may not have independent existence is called an atom, while the smallest particle of a substance which is capable of independent existence is called a molecule. Molecules are classified as homoatomic and heteroatomic. Homoatomic molecules are made up of the atoms of the same element and have different atomicity (number of atoms in a molecule of an element) like monoatomic, diatomic, triatomic and polyatomic.

Atomic Mass Unit

An atom is a very minute particle and so the actual mass of an atom is extremely small. It is physically not possible to measure such small masses easily. In 1811, Avogadro suggested that atomic and molecular masses can be expressed on an atomic mass scale relative to that of a standard reference atom. The standard reference atom chosen by IUPAC in 1961 is carbon-12 isotope designated as ¹²C₆ or simply as¹²C. The ¹²C isotope has been assigned an atomic mass of 12.000000 atomic mass unit. Hence, one atomic mass unit (amu) may be defined as follows: The mass equal to 1/12^th of the mass of a ¹²C atom. It is abbreviated as amu or m_au

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Absolute mass of ¹²C atom is 1.9924 x 10^-23g