Stoichiometry

# Stoichiometry Important Terms Definitions

Stoichiometry is a quantitative chemistry. “Calculations based on law of conservation of mass, law of constant composition and balanced chemical equations are known as stoichiometric calculations.”

## Sub-atomic particles

Fundamental sub-atomic particles of atoms are as follow :

• Electrons
• Protons
• Neutrons

# Types of Ions:

## Formation of cations is endothermic process while formation of anions is exothermic process. Ions are of three types.

(i) simple ion: If a single atom bears +ve or –ve charge then it is called simple ion e.g. H+, O-2. (ii) compound ion: When a group of atoms bear +ve or –ve charge then it is called compound ion e.g. NH4+, SO4-2 etc.

(iii) molecular ion: When a molecule gains or loses electrons, the resulting species is called molecular ion e.g. CH4+, O2+, CO+, N2+ etc. Cationic molecular ions are more common than anionic molecular ions.

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## Atoms having same atomic number but different atomic masses are called isotopes. e.g. Hydrogen has three isotopes protium 1H1, deuterium 1H2, tritium 1H3, Carbon has three isotopes

6C12, 6C13, 6C14, oxygen also has three isotopes 8O16, 8O17, 8O18 etc. More than 300 isotopes occur in nature they include 40 radioactive isotopes. Often, elements with even atomic number have more number of isotopes and are abundant.

## “Number of protons in the nucleus of an atoms” or “Number of electrons in an atom” is called atomic number.

Number of electrons in an atom          = Z

Number of protons in an atom            = Z

Number of neutrons in an atom          = A – Z

## Atomic masses:

Atomic masses are expressed in three ways:

(i)         Mass Number (A):       Sum of protons and neutrons in an atom is called its mass number or nucleon number.

A = Protons + Neutrons

(ii)        Relative atomic mass (Ar)     The atomic mass of an atom as compared to mass of Carbon-12, is called relative atomic mass e.g. relative atomic mass of Hydrogen (protium i.e. 1H1) is 1.0078 amu.

(iii)       Average atomic mass (Ar)     The weight average of isotopic masses of an element is called average atomic mass of the element.

e.g. (i)                          Relative isotopic mass             Relative abundance

Carbon-12                   12.00000 amu                         98.89%

Carbon-13                   13.00335 amu                         1.11%

Average atomic mass =      12.00000 x 98.89   +   13.00335 x 1.11 / 100

=  12.01 amu

## amu: This is a unit to measure atomic masses. It is one-twelfth of the mass of carbon-12.

1 a.m.u = 1.66 x 10-24g            or 1.66 x 10-27kg

Relative molecular masses (Mr): Sum of relative atomic masses of atoms present in a molecule gives relative molecular masses

Chemical Formulae:

The symbolic representation of a compound that shows elements in the compound and also the ratio of their atoms is called chemical formula.

## Types of Chemical formulae

Chemical formulae are of three types:

(i)         Emperical formula or Simple formula:

The formula of a compound that shows elements in the compound and their simple atomic ratio is called empirical or simple formula.

(ii)        Molecular formula:

The formula of a compound that shows elements in the compound and actual number of their atoms is called molecular formula.

(iii)       Structural formula:

The formula of a compound that shows elements in the compound, actual number of their atoms and arrangement of the atoms in the molecule is called structural formula.