Important Points and Formulas: Atoms and Molecules | Science Class 9 PDF Download

 Page 1


 
 
ATOMS AND MOLECULES  
 
 
 
 Atom 
Ancient thoughts about ‘atom’: The idea about the divisibility of matter was given by Indian 
and Greek philosophers around 500 BC. The smallest indivisible particle was called ‘parmanu’ 
or ‘atom’ (in Greek).  
 Laws of chemical combination (based on experiments) 
 (a) Law of conservation of mass. Mass can neither be created nor destroyed or in 
any chemical reaction, the total mass of the reactants is equal to the total mass of the products. 
 For example, for the reaction, 
  AgNO3 (aq)+ NaCl (aq) ¾® AgCl (s) + NaNO3 (aq), 
  Total mass of AgNO3 (aq) + NaCl (aq) = Total mass of AgCl (s) + NaNO3 (aq). 
 (b) Law of constant proportions. A chemical compound is always made up of two 
or more atoms of same or different elements combined together in the same fixed proportion 
by mass. For example, H2O always contains H and O in the fixed ratio of 1 : 8 by mass. 
Dalton’s Atomic Theory. This theory was put forward by Dalton (1808) to explain the laws of 
chemical combination.  
  
 Properties of Atoms 
• Atom: An atom is the smallest particle of an element, which may or may not be able 
to exist freely. It is, however, the smallest particle that takes part in a chemical reaction. 
• Size of atoms: Atoms are so small in size that they cannot be seen even under a 
microscope. Their radii are of the order of . 
• How do atoms exist? Atoms of most of the elements exist in the combined state (as 
H2, O2, etc. or H2O, NH3 etc.) or as ions in the aqueous solution (as H
+
, Cu
2+
, Ag
+
 etc.). 
• Symbols of atoms: Short hand method of representing full name of an element is 
called its symbol. It is first capital letter or first capital letter and another small letter from the 
full name. For example, carbon (C), cobalt (Co), chlorine (Cl), copper (Cu, from Latin name, 
cuprum). 
• Atomic mass unit (amu) or unified mass (u): It is 1/12
th
 of the mass of an atom of 
carbon-12 isotope. 
• Atomic mass: Atomic mass of an element is the average relative mass of its atoms 
as compared with mass of carbon-12 isotope taken as 12 units. 
• Molecule: A molecule is the smallest particle of an element or a compound, which is 
capable of free existence. 
• Atomicity: The number of atoms present in one molecule of the substance is called 
its atomicity. Thus, we have monoatomic, diatomic, triatomic, tetratomic or polyatomic 
molecules.  
• Some molecules of elements: Monoatomic = He, Ne, Na, Al, Fe etc. Diatomic = H2, 
O2, N2 etc. Triatomic = O3. Tetratomic = P4. 
• Some molecules of compounds: Diatomic = HCl, CO etc. Triatomic = H2O, SO2, CO2 
m) 10 nm (1 m 10
9 10 - -
=
Page 2


 
 
ATOMS AND MOLECULES  
 
 
 
 Atom 
Ancient thoughts about ‘atom’: The idea about the divisibility of matter was given by Indian 
and Greek philosophers around 500 BC. The smallest indivisible particle was called ‘parmanu’ 
or ‘atom’ (in Greek).  
 Laws of chemical combination (based on experiments) 
 (a) Law of conservation of mass. Mass can neither be created nor destroyed or in 
any chemical reaction, the total mass of the reactants is equal to the total mass of the products. 
 For example, for the reaction, 
  AgNO3 (aq)+ NaCl (aq) ¾® AgCl (s) + NaNO3 (aq), 
  Total mass of AgNO3 (aq) + NaCl (aq) = Total mass of AgCl (s) + NaNO3 (aq). 
 (b) Law of constant proportions. A chemical compound is always made up of two 
or more atoms of same or different elements combined together in the same fixed proportion 
by mass. For example, H2O always contains H and O in the fixed ratio of 1 : 8 by mass. 
Dalton’s Atomic Theory. This theory was put forward by Dalton (1808) to explain the laws of 
chemical combination.  
  
 Properties of Atoms 
• Atom: An atom is the smallest particle of an element, which may or may not be able 
to exist freely. It is, however, the smallest particle that takes part in a chemical reaction. 
• Size of atoms: Atoms are so small in size that they cannot be seen even under a 
microscope. Their radii are of the order of . 
• How do atoms exist? Atoms of most of the elements exist in the combined state (as 
H2, O2, etc. or H2O, NH3 etc.) or as ions in the aqueous solution (as H
+
, Cu
2+
, Ag
+
 etc.). 
• Symbols of atoms: Short hand method of representing full name of an element is 
called its symbol. It is first capital letter or first capital letter and another small letter from the 
full name. For example, carbon (C), cobalt (Co), chlorine (Cl), copper (Cu, from Latin name, 
cuprum). 
• Atomic mass unit (amu) or unified mass (u): It is 1/12
th
 of the mass of an atom of 
carbon-12 isotope. 
• Atomic mass: Atomic mass of an element is the average relative mass of its atoms 
as compared with mass of carbon-12 isotope taken as 12 units. 
• Molecule: A molecule is the smallest particle of an element or a compound, which is 
capable of free existence. 
• Atomicity: The number of atoms present in one molecule of the substance is called 
its atomicity. Thus, we have monoatomic, diatomic, triatomic, tetratomic or polyatomic 
molecules.  
• Some molecules of elements: Monoatomic = He, Ne, Na, Al, Fe etc. Diatomic = H2, 
O2, N2 etc. Triatomic = O3. Tetratomic = P4. 
• Some molecules of compounds: Diatomic = HCl, CO etc. Triatomic = H2O, SO2, CO2 
m) 10 nm (1 m 10
9 10 - -
=
2 
 
etc. Tetratomic = NH3, H2O2 etc., Polyatomic = H2SO4, P2O5 etc. 
• Molecular mass: It is the average relative mass of its molecules as compared with 
that of an atom of C-12 isotope taken as 12. It is calculated by adding atomic masses of all 
the atoms present in one molecule of the substance. 
• Ion: An atom or a group of atoms which carries positive or negative charge is called 
an “ion”, e.g., Na
+
, K
+
, Ca
2+
, Mg
2+
 etc.(cation) or Cl
–
, Br
–
, I
–
, , ,  etc. (anion) 
 
Names and symbols of some ions 
Valency Metallic 
element 
Symbol Non-metallic 
element 
Symbol Polyatomic ions Symbol 
1. Sodium Na
+
 Hydrogen H
+
 Ammonium NH
4+
 
Potassium K
+
 Hydride H
-
 Hydroxide OH
-
 
Silver Ag
+
 Chloride Cl
-
 Nitrate 
NO 
Copper (I)* Cu
+
 Bromide Br
-
 Hydrogen 
carbonate 
HCO 
Iodide I
-
 
2. Magnesium Mg
2+
 Oxide O
2-
 Carbonate 
CO 
Calcium Ca
2+
 Sulphide S
2-
 Sulphite 
SO 
Zinc Zn
2+
    
Sulphate 
 
SO 
Iron (II)* Fe
2+
 
Copper (II)* Cu
2+
 
3. Aluminium Al
3+
  
Nitride 
 
N
3-
 
 
Phosphate 
 
PO 
Iron (III)* Fe
3+
 
 
• Cations and Anions: The ion carrying positive charge is called “cation” whereas an 
ion carrying a negative charge is called an “anion”. 
• Monoatomic and Polyatomic ions: Ions consisting of only single atom are called 
monoatomic whereas ions consisting of groups of atoms are called polyatomic. 
• Valency: Valency of an element is defined as its combining capacity. It is equal to the 
number of H-atoms or number of Cl-atoms or double the number of O-atoms with which one 
atom of the element combines. In case of ions, valency is defined as the number of units of 
charge present on the ion. Thus, we generally have monovalent, divalent or trivalent ions. 
• Examples of cations: Monovalent = H
+
, Na
+
, K
+
, Ag
+
,  etc. 
 Divalent = Mg
2+
, Ca
2+
, Ba
2+
, Zn
2+
, Cu
2+
 etc. Trivalent = Al
3+
, Fe
3+
 etc. 
• Examples of anions: Monovalent = Cl
–
, Br
–
, I
–
,  etc. Divalent = O
2–
, S
2–
, 
 etc. Trivalent =  etc. 
-
3
NO
- 2
3
CO
- 2
4
SO
-
3
-
3
- 2
3
- 2
3
- 2
4
- 3
4
+
4
NH
-
3
NH
- - 2
4
2
3
SO , CO
- 3
4
PO
Page 3


 
 
ATOMS AND MOLECULES  
 
 
 
 Atom 
Ancient thoughts about ‘atom’: The idea about the divisibility of matter was given by Indian 
and Greek philosophers around 500 BC. The smallest indivisible particle was called ‘parmanu’ 
or ‘atom’ (in Greek).  
 Laws of chemical combination (based on experiments) 
 (a) Law of conservation of mass. Mass can neither be created nor destroyed or in 
any chemical reaction, the total mass of the reactants is equal to the total mass of the products. 
 For example, for the reaction, 
  AgNO3 (aq)+ NaCl (aq) ¾® AgCl (s) + NaNO3 (aq), 
  Total mass of AgNO3 (aq) + NaCl (aq) = Total mass of AgCl (s) + NaNO3 (aq). 
 (b) Law of constant proportions. A chemical compound is always made up of two 
or more atoms of same or different elements combined together in the same fixed proportion 
by mass. For example, H2O always contains H and O in the fixed ratio of 1 : 8 by mass. 
Dalton’s Atomic Theory. This theory was put forward by Dalton (1808) to explain the laws of 
chemical combination.  
  
 Properties of Atoms 
• Atom: An atom is the smallest particle of an element, which may or may not be able 
to exist freely. It is, however, the smallest particle that takes part in a chemical reaction. 
• Size of atoms: Atoms are so small in size that they cannot be seen even under a 
microscope. Their radii are of the order of . 
• How do atoms exist? Atoms of most of the elements exist in the combined state (as 
H2, O2, etc. or H2O, NH3 etc.) or as ions in the aqueous solution (as H
+
, Cu
2+
, Ag
+
 etc.). 
• Symbols of atoms: Short hand method of representing full name of an element is 
called its symbol. It is first capital letter or first capital letter and another small letter from the 
full name. For example, carbon (C), cobalt (Co), chlorine (Cl), copper (Cu, from Latin name, 
cuprum). 
• Atomic mass unit (amu) or unified mass (u): It is 1/12
th
 of the mass of an atom of 
carbon-12 isotope. 
• Atomic mass: Atomic mass of an element is the average relative mass of its atoms 
as compared with mass of carbon-12 isotope taken as 12 units. 
• Molecule: A molecule is the smallest particle of an element or a compound, which is 
capable of free existence. 
• Atomicity: The number of atoms present in one molecule of the substance is called 
its atomicity. Thus, we have monoatomic, diatomic, triatomic, tetratomic or polyatomic 
molecules.  
• Some molecules of elements: Monoatomic = He, Ne, Na, Al, Fe etc. Diatomic = H2, 
O2, N2 etc. Triatomic = O3. Tetratomic = P4. 
• Some molecules of compounds: Diatomic = HCl, CO etc. Triatomic = H2O, SO2, CO2 
m) 10 nm (1 m 10
9 10 - -
=
2 
 
etc. Tetratomic = NH3, H2O2 etc., Polyatomic = H2SO4, P2O5 etc. 
• Molecular mass: It is the average relative mass of its molecules as compared with 
that of an atom of C-12 isotope taken as 12. It is calculated by adding atomic masses of all 
the atoms present in one molecule of the substance. 
• Ion: An atom or a group of atoms which carries positive or negative charge is called 
an “ion”, e.g., Na
+
, K
+
, Ca
2+
, Mg
2+
 etc.(cation) or Cl
–
, Br
–
, I
–
, , ,  etc. (anion) 
 
Names and symbols of some ions 
Valency Metallic 
element 
Symbol Non-metallic 
element 
Symbol Polyatomic ions Symbol 
1. Sodium Na
+
 Hydrogen H
+
 Ammonium NH
4+
 
Potassium K
+
 Hydride H
-
 Hydroxide OH
-
 
Silver Ag
+
 Chloride Cl
-
 Nitrate 
NO 
Copper (I)* Cu
+
 Bromide Br
-
 Hydrogen 
carbonate 
HCO 
Iodide I
-
 
2. Magnesium Mg
2+
 Oxide O
2-
 Carbonate 
CO 
Calcium Ca
2+
 Sulphide S
2-
 Sulphite 
SO 
Zinc Zn
2+
    
Sulphate 
 
SO 
Iron (II)* Fe
2+
 
Copper (II)* Cu
2+
 
3. Aluminium Al
3+
  
Nitride 
 
N
3-
 
 
Phosphate 
 
PO 
Iron (III)* Fe
3+
 
 
• Cations and Anions: The ion carrying positive charge is called “cation” whereas an 
ion carrying a negative charge is called an “anion”. 
• Monoatomic and Polyatomic ions: Ions consisting of only single atom are called 
monoatomic whereas ions consisting of groups of atoms are called polyatomic. 
• Valency: Valency of an element is defined as its combining capacity. It is equal to the 
number of H-atoms or number of Cl-atoms or double the number of O-atoms with which one 
atom of the element combines. In case of ions, valency is defined as the number of units of 
charge present on the ion. Thus, we generally have monovalent, divalent or trivalent ions. 
• Examples of cations: Monovalent = H
+
, Na
+
, K
+
, Ag
+
,  etc. 
 Divalent = Mg
2+
, Ca
2+
, Ba
2+
, Zn
2+
, Cu
2+
 etc. Trivalent = Al
3+
, Fe
3+
 etc. 
• Examples of anions: Monovalent = Cl
–
, Br
–
, I
–
,  etc. Divalent = O
2–
, S
2–
, 
 etc. Trivalent =  etc. 
-
3
NO
- 2
3
CO
- 2
4
SO
-
3
-
3
- 2
3
- 2
3
- 2
4
- 3
4
+
4
NH
-
3
NH
- - 2
4
2
3
SO , CO
- 3
4
PO
3 
 
• Writing chemical formulae: Write symbols of elements or ions (polyatomic in 
brackets). Write their respective valencies below them. Apply crisscross of valencies. Cancel 
out common factor, if any. For example, for aluminium sulphate, 
    
 Mole Concept 
• Gram atomic mass: Atomic mass of an element expressed in grams is called its gram 
atomic mass. This amount is called ‘one gram atom’. 
• Gram molecular mass: Molecular mass of a substance expressed in grams is called 
gram molecular mass. This amount is called ‘one gram molecule’. 
• Gram formula unit mass: For ionic compounds, the formula unit mass expressed in 
grams is called gram formula unit mass. 
• Avogadro’s number (N0): Avogadro’s number of particles =  particles. 
Particles can be atoms, molecules, ions, grains, sand etc. 
 Mole 
• One mole of atoms has mass equal to gram atomic mass and contains Avogadro’s 
number of atoms. 
• One mole of molecules has mass equal to gram molecular mass and contains 
Avogadro’s number of molecules or has a volume of 22.4 L at STP if the substance is a gas. 
• One mole of an ionic compound has mass equal to gram formula unit mass and 
contains Avogadro’s number of formula units. 
• Molar mass: The mass of one mole of the substance is called its molar mass. Its 
units are g mol
–1
. Thus, it can be equal to gram atomic mass or gram molecular mass or 
gram formula mass depending upon whether the substance is atomic, molecular or ionic 
 
Symbol Al              (SO
4
) 
         Formula = Al
2
(SO
4
)
3
 
Charge 3+ 2– 
23
10 022 . 6 ´