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Atoms,molecules,ions,atomic masses and molecular masses

Atom:
All substances are made up of tiny particles.Elements are made up of atoms,compounds are made up of molecules or ions. The smallest particle of an element which may or may not be capable of independent existence is called an atom.
Molecule:The smallest particle of a substance(element or compound)which is capable of independent existence is called a molecule.
molecules can be classified into two types:
(1) Homoatomic molecules:   These molecules are made up of atom of the same element.
(2) Heteroatomic molecules: These molecules are made up of the atoms of different elements.
lons:Atoms after gaining or losing electrons become charged species.Such charged species are called ions.lons are of two types:
(1)Cations: The positively charged ions are called cations.e.g.,na+,cu2+and nh4+ ions etc.
(2)Anions:The negatively charged ions are called anions.e.g.,cl- ions etc.
Radicals:An atom or a group of atoms which forms a part of an inorganic compound is called a radical.
For example:
(1) Sodium chloride is made up of two parts:
(a) Sodium radical (na+) and (b) chloride radical(cl-)
The two radicals combine to form sodium chloride molecule.
Valency:The valency of an element is the number of hydrogen atoms or twice the number of oxygen atoms which combine with one atom of the element.
(1) 1bromine atom combines with 1 hydrogen atom to form hbr.
Valencies of radicals:
The number of charges over the radical is called valency of the radial.
Radicals with positive charge are called electropositive or basic radicals whereas radicals with negative charge are called electronegative or acid radicals.
Atomic mass:
The atomic mass of an element is the number of times an atom of that element is heavier than an atom of carbon taken as 12.
                                                or
The atomic mass of an element is the average relative mass of its atoms as compared with an atom of carbon taken as 12.

Atomic mass of chlorine= (35*3+37*1)/4 =355 amu.
Atomic mass unit: Atomic mass unit is defined as:
The quantity of matter whose mass is equal to 1/12th of the mass of an atom of carbon-12 isotope.
Gram atomic mass or gram atom:
 The atomic mass of an element expressed in grams is called gram atomic mass.
The amount of the element is also called one gram atom.For example,atomic mass of sodium is 23 amu and,therefore,its gram atomic mass is 23 g.
Molecular mass:
The molecular mass of a substance is the average relative mass of its molecule as compared with an atom of carbon-12 isotope.
Molecular mass of co2= atomic mass of carbon+2*atomic mass of oxygen
                                           = 12+2*16=44 amu.
Gram molecule mass or gram molecule:
The molecular mass of a substance expressed in grams is called gram molecular mass.
It is also known as one gram molecule. for example, the molecular mass of carbon dioxide is 44 and,therefore,its gram molecular mass is 44 g.

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