09Science Chapter 03 Atoms and molecules
GenextStudents 1
Chapter 3
Atoms and Molecules
Lows of Chemical Combination
Given by Antoine L. Lavoisier
1) Law of conservation of mass:
It states that mass can neither be created not be destroyed in a chemical reaction.
Or
Total mass of reactant is equal to total mass of product in a reaction.
2) Law of constant proportions:
It states that in a given compound always contains exactly the same proportion of
elements by mass.
For ex.
2
Mass of
2
= 2
Mass of O = 16
Ration of their mass is always 1:8 irrespective of source from which it is obtained.
DALTONS ATOMIC THEORY:
According to him matter is made up of smallest particle called as ATOM
POSTULATES:
1 All matter is made up of smallest particle to atom
2. Atoms are indivisible particle & can neither be created nor be destroyed in chemical
reaction.
3. Atoms of a given elements are identical in mass & chemical properties.
4. Atoms of different element have different mass & different chemical property.
5. Atoms combine in a ration of small whole numbers to form compound.
6. The relative number and kinds of atoms are constant in a given compound.
09Science Chapter 03 Atoms and molecules
GenextStudents 2
ATOM:
Building block of matter is called as atom
Atomic radius is measured in nm.
1m = 10
9
1nm =
1
10
9
or 10
9
SYMBOLS FOR ELEMENTS:
Aluminum Al Copper Cu Nitrogen N
Argon Ar Fluorine F Oxygen O
Barium Ba Gold Au Potassium K
Boron B Hydrogen H Silicon Si
Bromine Br Iodine I Silver Ag
Calcium Ca Iron Fe Sodium Na
Carbon C Lead Pb Sulphur S
Chlorine Cl Magnesium Mg Uranium U
Cobalt Co Neon Ne Zinc Zn
ITOMIC MASS
It is measured relative to
12
C isotope.
One atomic mass unit (amu) now referred to as unified mass () is defined mass unit
equal to exactly (1/12
th
) the mass of one atom of carbon 12.
Earlier oxygen -16 was taken as reference due to following reasons.
i) Oxygen reacted with a large number of elements and formed compounds.
ii) This atomic mass unit gives masses of most of elements as whole number.
MOLECULE:
Molecule is defined as group of two or more atoms that are held together by attractive
forces. It is smallest particle that is capable of independent existence & shows all
properties of that substance.
Atoms of same different elements can combine to form molecule.
09Science Chapter 03 Atoms and molecules
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Molecules of Elements:
Molecules of elements are made up of same type of atom. The number of atoms
constituting an molecule is known as its atomicity.
Ex.
2
Atomicity 2 diatomic
3
Ozone Atomicity 3 triatomic
8
Sulphur Atomicity = 8
Molecules of Compounds:
Composed of atoms of different elements joined together in fixed proportion
For ex. Combining element Ratio by mass
2
Carbon dioxide C, O 3:8
ION: compounds composed of metal & non-metals contain charged species. These charge
species are called as Ions.
Positively charged species Cation
Charged species Anaion
Ex. Nacl
+
+
+
cation
Anaion
Writing Chemical formulae:
Valency can be used to find out how many atoms can combine with other atoms.
Rules
i) Valency charges on the ion must balance.
ii) When a compound consists of metal & non-metal symbol of metal is written first.
Metals are written on left side & non-metals on right side.
Ex. Ca
In compounds have polyatomic ion. Ions are enclosed in bracket before writing the no.
to indicate ration. Ex.()
2
FORMULAE OF SIMPLE COMPOUNDS:
09Science Chapter 03 Atoms and molecules
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Compounds made up of 2 different elements are called as binary compound.
Examples
Symbol H Cl formula HCl
1 1
Symbol C Cl formula
4
4 1
Symbol Ca OH formula ()
2
2 1
MOLECULAR MASS AND MOLE CONCEPT:
Molecular mass of a substance is sum of atomic masses of all atoms in a molecule of the
substances. Mass of molecule is expressed in atomic mass units ()
Ex. Mass of
2
Mass of Hydrogen = 1
Mass of Oxygen = 16
= 2 1 + 16
= 18
FORMULA UNIT MASS:
Formula unit mass of a substance is sum of atomic masses of all atoms in a formula
mass unit calculate in same way as molecular mass by formula mass unit name is used in
case of ions.
MOLE CONCEPT:
One mole is amount of a substance that contains as many particles or entities as there
are atoms in exactly 12g of the
12
C isotope
Number of entities in 1 mol is known as Avogadro constant & denoted by
1 mol = 6.022 10
23
entities
