Chapter 3 Atoms and Molecules Class 9 Notes | EduRev

Class 9 : Chapter 3 Atoms and Molecules Class 9 Notes | EduRev

 Page 1


Chapter 3 Atoms and Molecules 
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O). 
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their 
masses. E.g. Carbon dioxide (CO 2). 
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N  
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen 
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and 
nitrogen i.e. N 2. 
Laws of Chemical Combination 
1. Law of Conservation of mass 
Mass can neither be created nor be destroyed. It can only be converted from one form to another. 
2. Law of Constant Proportion 
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound. 
Example 
?  Carbon (C) + Oxygen (O 2)                  Carbon dioxide (CO 2) 
 3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.  
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen. 
Dalton’s Atomic Theory 
1. Matter is made of tiny particles called atoms. 
2. Atoms can neither be created or be destroyed. 
3. All the atoms of an element have same mass and chemical properties. 
4. Atoms of different element have different atomic mass and chemical properties. 
5. Atoms combine in the ratio of whole numbers to form compounds. 
6. The relative number and kinds of atoms are constant in a given compound. 
 
Atoms 
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N. 
? However, an atom is made up of protons, neutrons and electrons. 
? Atoms are very small. 
? Radius of hydrogen atom = 10
-10
m 
? Radius of a water molecule = 10
-9
m 
Symbols of elements 
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various 
reactions. 
? Dalton was the first scientist to use symbols  
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements. 
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number. 
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or 
u). 
Page 2


Chapter 3 Atoms and Molecules 
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O). 
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their 
masses. E.g. Carbon dioxide (CO 2). 
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N  
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen 
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and 
nitrogen i.e. N 2. 
Laws of Chemical Combination 
1. Law of Conservation of mass 
Mass can neither be created nor be destroyed. It can only be converted from one form to another. 
2. Law of Constant Proportion 
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound. 
Example 
?  Carbon (C) + Oxygen (O 2)                  Carbon dioxide (CO 2) 
 3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.  
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen. 
Dalton’s Atomic Theory 
1. Matter is made of tiny particles called atoms. 
2. Atoms can neither be created or be destroyed. 
3. All the atoms of an element have same mass and chemical properties. 
4. Atoms of different element have different atomic mass and chemical properties. 
5. Atoms combine in the ratio of whole numbers to form compounds. 
6. The relative number and kinds of atoms are constant in a given compound. 
 
Atoms 
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N. 
? However, an atom is made up of protons, neutrons and electrons. 
? Atoms are very small. 
? Radius of hydrogen atom = 10
-10
m 
? Radius of a water molecule = 10
-9
m 
Symbols of elements 
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various 
reactions. 
? Dalton was the first scientist to use symbols  
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements. 
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number. 
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or 
u). 
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom. 
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of 
one carbon-12 atom. 
Some common elements and their atomic number and masses: 
 
 
 
 
 
 
 
 
 
 
 
 
 
Molecule 
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as 
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.  
Molecules of elements 
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable 
substance.  
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own 
kind to form a molecule. 
Atomicity – number of atoms in a molecule. 
 Monoatomic – A molecule that contain only one atom 
 Diatomic – A molecule that contain two atoms. 
 Triatomic – A molecule that contain 3 atoms. 
 Tetra-atomic -  molecule that contain four atoms.  
 
 
Element Symbol Atomic number Atomic mass Way to 
remember  1 to 
20 elements 
Hydrogen H 1 1 High 
Helium He 2 4 Heels 
Lithium Li 3 7 Liye 
Beryllium Be 4 9 Bena 
Boron B 5 11 Bhi 
Carbon C 6 12 Cal  
Nitrogen N 7 14 Nani 
Oxygen O 8 16 Or 
Fluorine F 9 18 Fufa  
Neon Ne 10 20 Nachenge 
 
Sodium Na 11 23 Na  
Magnesium Mg 12 24 Mang 
Aluminium Al 13 27 Allah 
Silicon Si 14 28 Se 
Phosphorus P 15 31 Pepsi 
Sulphur S 16 32 Soda 
Chlorine Cl 17 35 Cola 
Argon Ar 18 40 Aur 
Potassium K 19 39 Kaju 
Calcium Ca 20 40 Cake 
Page 3


Chapter 3 Atoms and Molecules 
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O). 
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their 
masses. E.g. Carbon dioxide (CO 2). 
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N  
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen 
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and 
nitrogen i.e. N 2. 
Laws of Chemical Combination 
1. Law of Conservation of mass 
Mass can neither be created nor be destroyed. It can only be converted from one form to another. 
2. Law of Constant Proportion 
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound. 
Example 
?  Carbon (C) + Oxygen (O 2)                  Carbon dioxide (CO 2) 
 3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.  
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen. 
Dalton’s Atomic Theory 
1. Matter is made of tiny particles called atoms. 
2. Atoms can neither be created or be destroyed. 
3. All the atoms of an element have same mass and chemical properties. 
4. Atoms of different element have different atomic mass and chemical properties. 
5. Atoms combine in the ratio of whole numbers to form compounds. 
6. The relative number and kinds of atoms are constant in a given compound. 
 
Atoms 
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N. 
? However, an atom is made up of protons, neutrons and electrons. 
? Atoms are very small. 
? Radius of hydrogen atom = 10
-10
m 
? Radius of a water molecule = 10
-9
m 
Symbols of elements 
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various 
reactions. 
? Dalton was the first scientist to use symbols  
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements. 
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number. 
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or 
u). 
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom. 
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of 
one carbon-12 atom. 
Some common elements and their atomic number and masses: 
 
 
 
 
 
 
 
 
 
 
 
 
 
Molecule 
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as 
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.  
Molecules of elements 
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable 
substance.  
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own 
kind to form a molecule. 
Atomicity – number of atoms in a molecule. 
 Monoatomic – A molecule that contain only one atom 
 Diatomic – A molecule that contain two atoms. 
 Triatomic – A molecule that contain 3 atoms. 
 Tetra-atomic -  molecule that contain four atoms.  
 
 
Element Symbol Atomic number Atomic mass Way to 
remember  1 to 
20 elements 
Hydrogen H 1 1 High 
Helium He 2 4 Heels 
Lithium Li 3 7 Liye 
Beryllium Be 4 9 Bena 
Boron B 5 11 Bhi 
Carbon C 6 12 Cal  
Nitrogen N 7 14 Nani 
Oxygen O 8 16 Or 
Fluorine F 9 18 Fufa  
Neon Ne 10 20 Nachenge 
 
Sodium Na 11 23 Na  
Magnesium Mg 12 24 Mang 
Aluminium Al 13 27 Allah 
Silicon Si 14 28 Se 
Phosphorus P 15 31 Pepsi 
Sulphur S 16 32 Soda 
Chlorine Cl 17 35 Cola 
Argon Ar 18 40 Aur 
Potassium K 19 39 Kaju 
Calcium Ca 20 40 Cake 
Name Symbol of molecule Atomicity 
Argon Ar Monoatomic 
Helium He Monoatomic 
Oxygen O 2 Diatomic 
Chlorine Cl Monoatomic 
Nitrogen N 2 Diatomic 
Sodium Na Monoatomic 
Aluminium Al Monoatomic 
Copper Cu Monoatomic 
Phosphorus P 4 Tetra-atomic 
Ozone O 3 Tri-atomic 
 
2. Molecules of Compounds  
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses. 
E.g. Carbon dioxide (CO 2). 
 
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8                   
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2) 
 
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8 
Ion 
Charged species are called ions. 
? Atoms are made up of electrons, protons and neutrons. 
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge. 
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus. 
 
                Fig. An atom of carbon 
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an 
uncharged atom of sodium contain 11 electrons as well as 11 protons. 
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged. 
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of 
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative 
sodium becomes positively charged. 
That can be written as 
    Na        Na
+
 + e
- 
Sodium produces sodium ion on losing one electron. 
Types of ions 
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses 
electron. Metals usually form cations. 
Page 4


Chapter 3 Atoms and Molecules 
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O). 
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their 
masses. E.g. Carbon dioxide (CO 2). 
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N  
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen 
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and 
nitrogen i.e. N 2. 
Laws of Chemical Combination 
1. Law of Conservation of mass 
Mass can neither be created nor be destroyed. It can only be converted from one form to another. 
2. Law of Constant Proportion 
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound. 
Example 
?  Carbon (C) + Oxygen (O 2)                  Carbon dioxide (CO 2) 
 3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.  
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen. 
Dalton’s Atomic Theory 
1. Matter is made of tiny particles called atoms. 
2. Atoms can neither be created or be destroyed. 
3. All the atoms of an element have same mass and chemical properties. 
4. Atoms of different element have different atomic mass and chemical properties. 
5. Atoms combine in the ratio of whole numbers to form compounds. 
6. The relative number and kinds of atoms are constant in a given compound. 
 
Atoms 
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N. 
? However, an atom is made up of protons, neutrons and electrons. 
? Atoms are very small. 
? Radius of hydrogen atom = 10
-10
m 
? Radius of a water molecule = 10
-9
m 
Symbols of elements 
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various 
reactions. 
? Dalton was the first scientist to use symbols  
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements. 
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number. 
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or 
u). 
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom. 
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of 
one carbon-12 atom. 
Some common elements and their atomic number and masses: 
 
 
 
 
 
 
 
 
 
 
 
 
 
Molecule 
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as 
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.  
Molecules of elements 
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable 
substance.  
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own 
kind to form a molecule. 
Atomicity – number of atoms in a molecule. 
 Monoatomic – A molecule that contain only one atom 
 Diatomic – A molecule that contain two atoms. 
 Triatomic – A molecule that contain 3 atoms. 
 Tetra-atomic -  molecule that contain four atoms.  
 
 
Element Symbol Atomic number Atomic mass Way to 
remember  1 to 
20 elements 
Hydrogen H 1 1 High 
Helium He 2 4 Heels 
Lithium Li 3 7 Liye 
Beryllium Be 4 9 Bena 
Boron B 5 11 Bhi 
Carbon C 6 12 Cal  
Nitrogen N 7 14 Nani 
Oxygen O 8 16 Or 
Fluorine F 9 18 Fufa  
Neon Ne 10 20 Nachenge 
 
Sodium Na 11 23 Na  
Magnesium Mg 12 24 Mang 
Aluminium Al 13 27 Allah 
Silicon Si 14 28 Se 
Phosphorus P 15 31 Pepsi 
Sulphur S 16 32 Soda 
Chlorine Cl 17 35 Cola 
Argon Ar 18 40 Aur 
Potassium K 19 39 Kaju 
Calcium Ca 20 40 Cake 
Name Symbol of molecule Atomicity 
Argon Ar Monoatomic 
Helium He Monoatomic 
Oxygen O 2 Diatomic 
Chlorine Cl Monoatomic 
Nitrogen N 2 Diatomic 
Sodium Na Monoatomic 
Aluminium Al Monoatomic 
Copper Cu Monoatomic 
Phosphorus P 4 Tetra-atomic 
Ozone O 3 Tri-atomic 
 
2. Molecules of Compounds  
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses. 
E.g. Carbon dioxide (CO 2). 
 
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8                   
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2) 
 
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8 
Ion 
Charged species are called ions. 
? Atoms are made up of electrons, protons and neutrons. 
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge. 
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus. 
 
                Fig. An atom of carbon 
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an 
uncharged atom of sodium contain 11 electrons as well as 11 protons. 
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged. 
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of 
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative 
sodium becomes positively charged. 
That can be written as 
    Na        Na
+
 + e
- 
Sodium produces sodium ion on losing one electron. 
Types of ions 
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses 
electron. Metals usually form cations. 
2. Anions – A negatively charged ion is called anion. Anions are formed when an atom/molecule gains 
electrons. 
 
Chemical Formula 
The symbolic representation of the composition of elements in a compound is called its chemical formula. 
*Valency – The combining capacity of an element of molecule is called its valency. 
*Details are given in the next chapter. 
Valency of some Ions 
 
 
 
 
 
 
 
 
 
 
 
Rules to write a chemical formula: 
? Valencies of the ion should be balanced (the compound should not have any charge as a whole). 
? If a compound consists of a metal and a non-metal, name of symbol of the metal should be written 
first followed by the name or symbol of the non-metal. 
? If the compound is formed from polyatomic ion, the ions should be enclosed in a bracket before 
writing the ratio. 
 
Examples 
1. Formula of sodium chloride 
Name             Sodium  Chloride 
Symbol Na   Cl 
Valency 1     1 
Formula           NaCl  
 
Valency Metallic 
ions 
Symbol Non-metallic 
ions 
Symbol Polyatomic 
ions 
Symbol 
 
 
       1 
Sodium Na
+ 
Hydrogen H
+ 
Ammonium NH4
+ 
Potassium K
+ 
Hydride H
-
 Hydroxide OH
- 
Silver Ag
+ 
Chloride Cl
- 
Nitrate NO3
- 
Copper Cu
+ 
Bromide Br
-
 bicarbonate HCO3
- 
  Iodide I
-
   
       
 
 
        2 
Magnesium Mg
2+ 
Oxide O
2-
 Carbonate CO3
2- 
Calcium Ca
2+ 
Sulphide S
2-
 Sulphite SO3
2- 
Zinc Zn
2+
   Sulphate SO4
2- 
Iron (II) Fe
2+
     
Copper (II) Cu
2+
     
       
        
       3 
Aluminium Al
3+
 Nitride N
3- 
Phosphate PO4
3- 
Iron (III) Fe
3+
     
Page 5


Chapter 3 Atoms and Molecules 
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O). 
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their 
masses. E.g. Carbon dioxide (CO 2). 
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N  
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen 
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and 
nitrogen i.e. N 2. 
Laws of Chemical Combination 
1. Law of Conservation of mass 
Mass can neither be created nor be destroyed. It can only be converted from one form to another. 
2. Law of Constant Proportion 
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound. 
Example 
?  Carbon (C) + Oxygen (O 2)                  Carbon dioxide (CO 2) 
 3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.  
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen. 
Dalton’s Atomic Theory 
1. Matter is made of tiny particles called atoms. 
2. Atoms can neither be created or be destroyed. 
3. All the atoms of an element have same mass and chemical properties. 
4. Atoms of different element have different atomic mass and chemical properties. 
5. Atoms combine in the ratio of whole numbers to form compounds. 
6. The relative number and kinds of atoms are constant in a given compound. 
 
Atoms 
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O, 
nitrogen N. 
? However, an atom is made up of protons, neutrons and electrons. 
? Atoms are very small. 
? Radius of hydrogen atom = 10
-10
m 
? Radius of a water molecule = 10
-9
m 
Symbols of elements 
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various 
reactions. 
? Dalton was the first scientist to use symbols  
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements. 
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number. 
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or 
u). 
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom. 
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of 
one carbon-12 atom. 
Some common elements and their atomic number and masses: 
 
 
 
 
 
 
 
 
 
 
 
 
 
Molecule 
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as 
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.  
Molecules of elements 
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable 
substance.  
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own 
kind to form a molecule. 
Atomicity – number of atoms in a molecule. 
 Monoatomic – A molecule that contain only one atom 
 Diatomic – A molecule that contain two atoms. 
 Triatomic – A molecule that contain 3 atoms. 
 Tetra-atomic -  molecule that contain four atoms.  
 
 
Element Symbol Atomic number Atomic mass Way to 
remember  1 to 
20 elements 
Hydrogen H 1 1 High 
Helium He 2 4 Heels 
Lithium Li 3 7 Liye 
Beryllium Be 4 9 Bena 
Boron B 5 11 Bhi 
Carbon C 6 12 Cal  
Nitrogen N 7 14 Nani 
Oxygen O 8 16 Or 
Fluorine F 9 18 Fufa  
Neon Ne 10 20 Nachenge 
 
Sodium Na 11 23 Na  
Magnesium Mg 12 24 Mang 
Aluminium Al 13 27 Allah 
Silicon Si 14 28 Se 
Phosphorus P 15 31 Pepsi 
Sulphur S 16 32 Soda 
Chlorine Cl 17 35 Cola 
Argon Ar 18 40 Aur 
Potassium K 19 39 Kaju 
Calcium Ca 20 40 Cake 
Name Symbol of molecule Atomicity 
Argon Ar Monoatomic 
Helium He Monoatomic 
Oxygen O 2 Diatomic 
Chlorine Cl Monoatomic 
Nitrogen N 2 Diatomic 
Sodium Na Monoatomic 
Aluminium Al Monoatomic 
Copper Cu Monoatomic 
Phosphorus P 4 Tetra-atomic 
Ozone O 3 Tri-atomic 
 
2. Molecules of Compounds  
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses. 
E.g. Carbon dioxide (CO 2). 
 
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8                   
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2) 
 
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8 
Ion 
Charged species are called ions. 
? Atoms are made up of electrons, protons and neutrons. 
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge. 
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus. 
 
                Fig. An atom of carbon 
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an 
uncharged atom of sodium contain 11 electrons as well as 11 protons. 
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged. 
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of 
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative 
sodium becomes positively charged. 
That can be written as 
    Na        Na
+
 + e
- 
Sodium produces sodium ion on losing one electron. 
Types of ions 
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses 
electron. Metals usually form cations. 
2. Anions – A negatively charged ion is called anion. Anions are formed when an atom/molecule gains 
electrons. 
 
Chemical Formula 
The symbolic representation of the composition of elements in a compound is called its chemical formula. 
*Valency – The combining capacity of an element of molecule is called its valency. 
*Details are given in the next chapter. 
Valency of some Ions 
 
 
 
 
 
 
 
 
 
 
 
Rules to write a chemical formula: 
? Valencies of the ion should be balanced (the compound should not have any charge as a whole). 
? If a compound consists of a metal and a non-metal, name of symbol of the metal should be written 
first followed by the name or symbol of the non-metal. 
? If the compound is formed from polyatomic ion, the ions should be enclosed in a bracket before 
writing the ratio. 
 
Examples 
1. Formula of sodium chloride 
Name             Sodium  Chloride 
Symbol Na   Cl 
Valency 1     1 
Formula           NaCl  
 
Valency Metallic 
ions 
Symbol Non-metallic 
ions 
Symbol Polyatomic 
ions 
Symbol 
 
 
       1 
Sodium Na
+ 
Hydrogen H
+ 
Ammonium NH4
+ 
Potassium K
+ 
Hydride H
-
 Hydroxide OH
- 
Silver Ag
+ 
Chloride Cl
- 
Nitrate NO3
- 
Copper Cu
+ 
Bromide Br
-
 bicarbonate HCO3
- 
  Iodide I
-
   
       
 
 
        2 
Magnesium Mg
2+ 
Oxide O
2-
 Carbonate CO3
2- 
Calcium Ca
2+ 
Sulphide S
2-
 Sulphite SO3
2- 
Zinc Zn
2+
   Sulphate SO4
2- 
Iron (II) Fe
2+
     
Copper (II) Cu
2+
     
       
        
       3 
Aluminium Al
3+
 Nitride N
3- 
Phosphate PO4
3- 
Iron (III) Fe
3+
     
2. Formula for Calcium chloride 
Name             Calcium  Chloride 
Symbol           Ca   Cl 
Valency            2     1 
Formula             CaCl 2 
 
3. Formula for Calcium oxide 
 
Name             Calcium  Oxide 
Symbol           Ca   O 
Valency            2    2 
Formula             Ca 2O 2 but the formula is always written in the simplest ratio and 2/2 = 1:1 so Formula for calcium 
chloride is CaO. 
 
 
 
4. Formula for Ammonium carbonate 
Name             Ammonium Carbonate 
Symbol           NH4  CO3 
Valency           1     2 
Formula          (NH) 2CO 3 
 
Molecular Mass and Mole concept 
 
Molecular mass – Sum of atomic masses of all the atoms present in a molecule. 
   For e.g. Atomic mass of Na2O = 2 x atomic mass of Na + atomic mass of oxygen 
            2 x 23 + 16 
        = 62u. 
Formula unit mass – Sum of atomic masses of all the atoms present in a formula unit or empirical formula of 
that compound. 
Empirical formula of a compound represents the elements of a compound in their simplest ratio regardless of 
their chemical composition. 
E.g.  Disulphur dioxide (S2O2) contains two atoms of both sulphur and oxygen.  
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