A molecule is the smallest particle of a substance that is an element or a compound, and possesses properties of that particular substance.
They are electrically neutral group of two or more atoms chemically bonded together.
A molecule can exist in Free State and are either made of elements or compounds.
MOLECULAR MASS:
It is the amount of mass associated with a molecule, also called as molecular weight.
Molecular mass of elements depends upon the constituent atoms of the molecule.
It can be calculated by adding the mass of each atom multiplied by the number of atoms of the element present in the molecule.
Consider water which is made up of 2 hydrogen atoms and 1 oxygen atom.
Mass of water molecule is equal to the average atomic mass of hydrogen multiplied by two plus the atomic mass of oxygen.
The molecular weight of molecules can be determined by following methods:
Mass spectrometry: This method is generally used in determining the mass of small molecules. This is reported as monoisotropic mass.
Hydrodynamic method: The weight is determined as per Mark-Houwink relations. This method requires calibration hence it is also described as relative molecular weight determination method.
Static Light Scattering: Molecular weight is determined from the amount of light scattered using the Zimm method.
MOLECULES OF ELEMENTS:
Two or more similar atoms chemically combine together to form molecule of an element.
For example: Hydrogen gas consists of H2 molecules and not of single H atoms
Atomicity: The number of atoms present in one molecule of an element is called its atomicity.
Noble gases (helium, neon, argon, krypton, etc.) have one atom each in their molecules such as He, Ne, Ar and Kr. And hence, the atomicity of noble gases is 1.
The atomicity of metal elements like sodium (Na), magnesium (Mg), aluminium (Al), copper (Cu) and iron (Fe), etc., is also taken to be 1.
Hydrogen (H2), nitrogen (Nsub>2), all have 2 atoms each in their molecules. So, the automaticity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.
Ozone (O3) has 3 atoms in its molecule, so the atomicity of ozone is 3.
Phosphorus (P4) has 4 atoms in its molecule, so the atomicity of phosphorus is 4.
Solid sulphur (S8) has 8 atoms in its molecule, so the atomicity of sulphur is 8.
MOLECULES OF COMPOUNDS:
Two or more different types of atoms, combine together chemically to form a molecule of a compound.
For example, the molecule of hydrogen chloride (HCl) contains two different types of atoms i.e. hydrogen atom (H) and chlorine atom (Cl)
CHEMICAL FORMULAE:
A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.
