Atomic Mass unit

Word particle suggest atoms or molecules or ions. In particulars, atoms are very small indivisible unit of substance. Molecule is formed by chemical reaction of two or more atoms. Particularly, ions are charged species carrying either positive and negative charge. Now continuing with atom. Atoms are extremely small, therefore, to measure the mass of atoms earlier scientist took reference of particular atom. At first, oxygen was chosen as reference atom, because of it’s ready to react nature with other elements and easy availability. Mass of atom was defined using term atomic mass unit. Therefore atomic mass unit is defines as,

“Atomic mass unit using oxygen as reference atom is 1/16 weight of oxygen atom.”

But due to gaseous nature of oxygen again it was bit difficult for scientist to calculate mass of atom. There was need of some solid reference element for more convenience to calculate mass of atom. Hence, carbon was chosen as reference atom, due to its solid nature and ability to form compounds with many elements. Therefore, atomic mass unit with carbon as reference element is defined as,

“A mass unit exactly equal 1/ 12th mass of one atom of carbon -12 isotope (¹²C).”

Atomic mass unit is denoted by amu, by recent IUPAC recommendations it is denoted by u.

Relative atomic mass unit

“Average mass of atoms, compared to 1/12th mass of one Carbon -12 atom.”

Molecular mass

Molecular mass is sum of mass of all atoms present in a molecule. SI unit is u. For example:
Molecular mass of water = ( 2 × Atomic mass of hydrogen) + Atomic mass of oxygen
                                        = (2 × 1 u ) + 16 u
                                        = 2 u + 16 u
                                        = 18 u

Formula unit mass

Masses of ions are calculated in terms of formula unit mass. It sum of masses of all atoms in formula units i.e. ions. For example: Compound NaCl contains formula units Na⁺ and Cl^–.
Formula unit mass of NaCl = Mass of atoms of Na⁺ + Mass of atoms of Cl^–
                                             = 1 × 22.9u + 1 × 35.45u
                                             = 22.9u + 35.45u
                                             = 58.35 u

Keywords: Atoms and Molecules, Molecular mass, Formula mass unit.

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