CHEMISTRY   
       
  FORMATION OF MOLECULES:

As we know, ions are atoms with a charge on them. The atoms possessing a positive or negative charge (by losing or gaining electrons) on the whole are called ions. Ions may consist of a single atom having a positive or negative charge or a group of atoms possessing a net charge. Atoms or ions bond together chemically or attract each other because of their net charges to form MOLECULES. Where on one hand, the atoms do not usually exist independently (they either form ions or molecules), on the other, the molecules are the smallest particles of an element or compound to exist independently. The molecules made up of atoms of the same element are called HOMO-ATOMIC (O2, Cu, Fe). The molecules made up of atoms of different elements are called HETERO-ATOMIC (H2O).

Classification of molecules on the basis of the number of atoms: 		 
     
 

 

ATOMICITY of one molecule of an element is the no. of atoms that it contains.

CHEMICAL FORMULAE:

It is an easy way of expressing information about the atoms of a particular compound. It gives us the exact composition of the compound and no. of elements that constitute it. It represents the simple ratio of the positive and negative ions present in each compound.

MOLECULAR FORMULAE:

For molecular compounds, the chemical formula is known as the molecular formula. It expresses the number of elements present in each compound and also the no. of atoms of each element present in one discrete molecule of that compound. In short, it describes one molecule of a particular compound.

THE FOLLOWING IS A LIST OF THE CHEMICAL COMPOUNDS AND THEIR FORMULAS AND CAS NUMBER (USEDFOR IDENTIFICATION AND SEARCH CONVINIENCE OF ALL COMPOUNDS):

MOLECULAR MASS:

It indicates how many times the molecule of a particular substance is heavier than 1/12th of a C 12 atom. It is equal to the atomic masses of all the atoms present in one molecule of the substance. The following example shows the calculation of molecular mass of a compound named Capsaicin (used in chilli peppers):

Formula of capsaicin- C18H27NO3

 
 
ELEMENT MOLAR MASS*  X RATIO**  
Carbon(C) 12.011 g/mol X 18 = 216.198
Hydrogen (H) 1.007 g/mol X 27 = 27.189
Nitrogen (N) 14.007 g/mol X 1 = 14.007
Oxygen (O) 15.999 g/mol X 3 = 47.997
        305.391 g/mol
 
 
*to be seen from the periodic table (including molar mass) for elements

**refer to the formula of the compound

RELATIVE MOLECULAR MASS (RMM):
The RMM is the weight of a molecule of a particular element or compound in relation to the 1/12th mass of a carbon atom i.e. how much that molecule is heavier than 1/12th mass of C12 (an atom of carbon).

RMM = mass of one molecule of the substance
1/12 the mass of a carbon atom (C 12) 		 
  GRAM MOLECULAR MASS:
It is the mass of one mole of a molecular substance. It is the relative molecular mass expressed in grams. The difference is that relative mass is a ratio and has no unit whereas the gram mass is expressed in the unit of grams.

MOLE CONCEPT:

The number of atoms present in 12gms of darbon of 12C isotope is 6.023 X 1023 atoms. This is called as the Avogadro number. One mole of a gas is the amount of a substance (of atom, molecule, ion or a group of ions) containing 6.023 X 1023 particles. It is the basic unit of the substance.

NOTE: under standard conditions of temperature and pressure (STP), a sample of gas contains a volume of 22.4 L. Number of moles X 22.4 L = volume of gas at standard pressure and temperature.

MOLAR VOLUME:

Molar volume (Vm) is the volume occupied by one mole of a substance at STP.