This document discusses molecules, atoms, and the mole concept in chemistry. It defines molecules as combinations of two or more atoms, and describes molecules of elements and compounds. Molecules can be diatomic, triatomic, tetra-atomic, or poly-atomic depending on the number and type of atoms combined. The mole concept is introduced as a convenient way to represent large numbers of particles collectively. One mole equals Avogadro's number of particles, and conveys information about number, mass, and volume.

1. ATOMS AND
MOLECULES
LECTURE-3
CHEMISTRY
BY: MISS RITIMA
( YOU TUBE CHANNEL : SHORT SHOT BIOLOGY
FOLLOW @ SLIDE SHARE APP- MISS
RITIMABIOLOGYEXP)

2. WHAT IS A MOLECULE
• MOLECULES OF ELEMENT ARE FORMED BY
THE COMBINATION OF TWO OR MORE
ATOMS OF SAME ELEMENT.
• THE NUMBER OF ATOM PRESENT IN THE
MOLECULE OF ELEMENT REPRESENT
ATOMICITY.
• MOLECULES ARE OF TWO TYPES:
a. MOLECULES OF ELEMENT
b. MOLECULES OF COMPOUND

3. MOLECULES OF ELEMENTS
• A MOLECULE OF HYDROGEN IS MADE FROM
TWO ATOMS OF THE HYDROGEN . ITS
ATOMICITY IS REPRESENTED BY 2.
• A MOLECULE OF OZONE IS MADE FROM
THREE ATOMS OF OXYGEN . ITS ATOMICITY IS
3.
• A MOLECULE OF PHOSPHORUS IS MADE
FROM FOUR ATOMS OF PHOSPHORUS . ITS
ATOMICITY IS 4.

4. ATOMICITY OF SOME ELEMENTS
NAME OF THE ELEMENT ATOMICITY
ARGON 1 – MONOATOMIC
HELIUM 1- MONOATOMIC
OXYGEN 2- DIATOMIC
HYDROGEN 2 – DIATOMIC
NITROGEN 2- DIATOMIC
CHLORINE 2- DIATOMIC
PHOSPHORUS 4- TETRA – ATOMIC
SULPHUR 8 – POLY-ATOMIC

5. MOLECULES OF COMPOUND
• IN MOLECULES OF COMPOUND, THE ATOMS OF
DIFFERENT ELEMENTS ARE COMBINED
TOGETHER BY CHEMICAL BOND.
• THESE ARE PRESENT IN DEFINITE PROPORTION BY
MASS ACCORDING TO THE LAW OF CONSTANT
PROPORTIONS.
• THE MOLECULES OF THE COMPOUND MAY BE
DIATOMIC , TRIATOMIC , TETRA-ATOMIC AND
POLY-ATOMIC IN NATURE DEPENDING UPON THE
NUMBER OF ATOMS COMBINED BY THE
CHEMICAL BOND.

6. MOLECULES OF SOME COMPOUNDS
COMPOUNDS COMBINING
ELEMENTS
NATURE RATIO BY MASS
HYDROGEN
CHLORIDE
HYDROGEN ,
CHLORINE
DIATOMIC 1: 35.5
WATER HYDROGEN , OXYGEN TRIATOMIC 1:8
AMMONIA HYDROGEN ,
NITROGEN
TETRA- ATOMIC 14:3
CARBON DIOXIDE CARBON , OXYGEN TRI-ATOMIC 3:8
METHANE CARBON , HYDROGEN PENTA- ATOMIC 3:1

7. WHAT IS AN ION?
• COMPOUNDS COMPOSED OF METALS ND
NONMETALS CONTAINS CHARGED SPECIES.
THE CHARGED SPECIES ARE KNOWN AS IONS.
• THE ION WHICH HAS A POSITIVE CHARGE IS
CALLED CATION AND ION WHICH HAS A
NEGATIVE CHARGE IS CALLED AS ANION.
• SO ION MAY BE DEFINED AS AN ATOM OR
GROUP OF ATOMS HAVING POSITIVE OR
NEGATIVE CHARGE.

8. RULES OF WRITING CHEMICAL
FORMULAE OF COMPOUND
• THE VALANCIES OR CHARGE ON THE ION MUST
BALANCE.
• WHEN COMPOUND CONSIST OF METAL AND
NONMETAL, THE NAME OR SYMBOL OF METAL IS
WRITTEN FIRST. EXAMPLE:NaCl , CuO.
• THE COMPOUND FORMED BY THE POLYATOMIC
IONS , THE NUMBER OF IONS PRESENT IN THE
COMPOUND IS INDICATED BY ENCLOSING
FORMULA OFION IN BRACKETS AND WRITING
NUMBER OF IONS OUTSIDE BRACKET. EXAMPLE
Mg(OH)2.

9. FORMULAE OF SIMPLE COMPOUNDS

10. MOLE CONCEPT
• A MOLE DENOTES AVOGADRO’S NUMBER OF
PARTICLES.
• NECESSITY OF MOLE CONCEPT:
WE KNOW THAT ATOMS AND MOLCULES OF ANY
SUBSTANCE ARE VERY SMALL IN SIZE. THIS MEANS IT
IS NOT POSSIBLE TO COUNT THESE ATOMS
INDIVIDUALLY . HOWEVER THEY CAN BE
COLLECTIVELY REPRESENTED AAS ONE MOLE. THIS IS
VERY CONVENIENT METHOD TO REPRESENT
DIFFERENT PARTICLES.
NUMBER OF OXYGEN ATOM IN 3 MOLES:
3X AVOGADRO’S NUMBER

12. FORMULA OF MOLE

13. SIGNIFICANCE OF MOLE
A MOLE CONVEYS FOLLOWING INFORMATION:
• NUMBER OF PARTICLES OF A SUBSTANCE.
• MASS OF SUBSTANCE.
• VOLUME OF GAS.
• EXPRESSING THE NUMBER OF PARTICLES OF
A SUBSTACE IN TERMS OF MOLE.