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There are seven base quantities used in the International System of Units. The seven base quantities and their corresponding units are:  • length (metre) • mass (kilogram) • time (second) • electric current (ampere) • thermodynamic temperature (kelvin) • amount of substance (mole) • luminous intensity (candela) Supplementary units: It includes plane angle in radian and solid angle in steradian.

To express the measurement of any physical quantity two things are considered: (i) Its unit, (ii) The numerical value. Magnitude of a physical quantity = numerical value * unit Units are of two types: (i) Basic units (ii) Derived units (i) The basic or fundamental units are those of length (m), ass (kg), time (s), electric current (A), thermodynamic temperature (K), amount of substance (mol) and luminous intensity (cd). (ii) Derived units are basically derived from the fundamental units, e.g., unit of density is derived from units of mass and volume. The systems used for describing measurements of various physical quantities are (a) CGS system: It is based on centimetre, gram and second as the units of length, mass and time respectively. (b) FPS system: A British system which used foot(ft). pound (lb) and second (s) as the fundamental units of length, mass and time. (c) MRS system: Uses metre (m), kilogram (kg) and second (s) respectively for length, mass and time; ampere (A) was added...

Atom is the smallest particle of an element which can take part in a chemical reaction. It mayor may not be capable of independent existence. Molecule is the simplest particle of matter that has independent existence. It may be homoatomic e.g., H₂, CI₂, N₂ (diatomic), O3(triatomic) or heteroatomic, e.g., HCI, NH₃, CH₃ etc