Chapter 4 Structure of Atom Notes | Study Science - Short Notes Class IX - Class 9

 Page 1


Chapter 4 Structure of Atom 
Atoms – The smallest constituent particles of matter that cannot be broken down further. 
Sub atomic particles – Particles that make up an atom 
There are three major sub atomic particles –  
1. Protons – discovered by E. Goldstein in 1886. They are positively charged (+1). The mass of proton is 
taken as 1 unit.  
2. Electrons – discovered by J.J. Thomson in 1897. They are negatively charged (-1) but have negligible 
mass. 
3. Neutrons – Discovered by James Chadwick in 1932. They do not have any charge. The mass of neutrons 
is approximately equal to that of protons and hence is taken as 1 unit. 
Neutrons and protons are present in the center pf an atom inside the nucleus and the protons revolve around 
them. 
Thomson’s Model of Atom 
He thought of an atom similar to watermelon where the red part was positively charged and the seeds as the 
electrons. 
 Key features – 
a. An atom is a positively charged sphere. 
b. Electrons are embedded in that positively charged sphere. 
c. The negative and the positive charges are equal in magnitude and that is why an atom is neutral as a 
whole. 
 
                                                            Fig. Thomson’s model of an atom 
Drawbacks –  
? Thomson’s model of atom could not explain the Rutherford’s a-particles scattering experiment. 
? It could not explain the experiment of other scientists as well such as Neil Bohr. 
 
Rutherford’s Model of an Atom 
Rutherford’s Experiment 
? He selected a very thin gold foil (1000atoms thick)  
Electrons 
Positive Sphere 
Page 2


Chapter 4 Structure of Atom 
Atoms – The smallest constituent particles of matter that cannot be broken down further. 
Sub atomic particles – Particles that make up an atom 
There are three major sub atomic particles –  
1. Protons – discovered by E. Goldstein in 1886. They are positively charged (+1). The mass of proton is 
taken as 1 unit.  
2. Electrons – discovered by J.J. Thomson in 1897. They are negatively charged (-1) but have negligible 
mass. 
3. Neutrons – Discovered by James Chadwick in 1932. They do not have any charge. The mass of neutrons 
is approximately equal to that of protons and hence is taken as 1 unit. 
Neutrons and protons are present in the center pf an atom inside the nucleus and the protons revolve around 
them. 
Thomson’s Model of Atom 
He thought of an atom similar to watermelon where the red part was positively charged and the seeds as the 
electrons. 
 Key features – 
a. An atom is a positively charged sphere. 
b. Electrons are embedded in that positively charged sphere. 
c. The negative and the positive charges are equal in magnitude and that is why an atom is neutral as a 
whole. 
 
                                                            Fig. Thomson’s model of an atom 
Drawbacks –  
? Thomson’s model of atom could not explain the Rutherford’s a-particles scattering experiment. 
? It could not explain the experiment of other scientists as well such as Neil Bohr. 
 
Rutherford’s Model of an Atom 
Rutherford’s Experiment 
? He selected a very thin gold foil (1000atoms thick)  
Electrons 
Positive Sphere 
? He passed fast moving a-particles through it. 
? a-particles are doubly charged Helium atoms (He
2+
). They have a positive charge of two units and a 
mass of 4u. 
Observations 
? Most of the fast moving a-particles passed straight without any deflection. 
? Some of the a-particles were deflected by small angles. 
? One out of 12000 particles bounced back after hitting the gold foil. 
      Inference or Conclusion –  
? Most of the space in an atom is empty because most of the a-particles passed without deflection. 
? Very few a-particles deflected with small angle indicating that the positive charge in an atom occupied 
very less space. 
? A very a-particles bounced back with an angle of 180
0
 indicating that most of the positive charge and 
mass of an atom is concentrated in a very small volume within the atom. 
                         Fig. Rutherford’s a-particles scattering experiment. 
 
Structure of Atom according to Rutherford –  
? Nearly all the mass is concentrated in a positively charged nucleus which is present in the center of the 
atom. 
? The electrons revolve around the nucleus in well-defined orbits. 
? The size of the nucleus is very small as compared to the size of an atom. Most of the space in an atom is 
empty. 
 
Drawback 
? A particle in circular motion always undergoes acceleration. During acceleration a charged particle 
radiates energy. Hence, an electron revolving around the nucleus will keep on radiating energy, keep 
coming closer to the nucleus and finally fall into it 
? It will make an atom highly stable. 
? However, as we know that atom is quite stable but cannot be explained by Rutherford’s model. 
 
Bohr’s Model of an atom 
 Postulates of Neil Bohr 
? Electrons revolve only on discrete or special fixed orbits around the nucleus, in an atom. 
? Electrons do not radiate energy while revolving. 
Page 3


Chapter 4 Structure of Atom 
Atoms – The smallest constituent particles of matter that cannot be broken down further. 
Sub atomic particles – Particles that make up an atom 
There are three major sub atomic particles –  
1. Protons – discovered by E. Goldstein in 1886. They are positively charged (+1). The mass of proton is 
taken as 1 unit.  
2. Electrons – discovered by J.J. Thomson in 1897. They are negatively charged (-1) but have negligible 
mass. 
3. Neutrons – Discovered by James Chadwick in 1932. They do not have any charge. The mass of neutrons 
is approximately equal to that of protons and hence is taken as 1 unit. 
Neutrons and protons are present in the center pf an atom inside the nucleus and the protons revolve around 
them. 
Thomson’s Model of Atom 
He thought of an atom similar to watermelon where the red part was positively charged and the seeds as the 
electrons. 
 Key features – 
a. An atom is a positively charged sphere. 
b. Electrons are embedded in that positively charged sphere. 
c. The negative and the positive charges are equal in magnitude and that is why an atom is neutral as a 
whole. 
 
                                                            Fig. Thomson’s model of an atom 
Drawbacks –  
? Thomson’s model of atom could not explain the Rutherford’s a-particles scattering experiment. 
? It could not explain the experiment of other scientists as well such as Neil Bohr. 
 
Rutherford’s Model of an Atom 
Rutherford’s Experiment 
? He selected a very thin gold foil (1000atoms thick)  
Electrons 
Positive Sphere 
? He passed fast moving a-particles through it. 
? a-particles are doubly charged Helium atoms (He
2+
). They have a positive charge of two units and a 
mass of 4u. 
Observations 
? Most of the fast moving a-particles passed straight without any deflection. 
? Some of the a-particles were deflected by small angles. 
? One out of 12000 particles bounced back after hitting the gold foil. 
      Inference or Conclusion –  
? Most of the space in an atom is empty because most of the a-particles passed without deflection. 
? Very few a-particles deflected with small angle indicating that the positive charge in an atom occupied 
very less space. 
? A very a-particles bounced back with an angle of 180
0
 indicating that most of the positive charge and 
mass of an atom is concentrated in a very small volume within the atom. 
                         Fig. Rutherford’s a-particles scattering experiment. 
 
Structure of Atom according to Rutherford –  
? Nearly all the mass is concentrated in a positively charged nucleus which is present in the center of the 
atom. 
? The electrons revolve around the nucleus in well-defined orbits. 
? The size of the nucleus is very small as compared to the size of an atom. Most of the space in an atom is 
empty. 
 
Drawback 
? A particle in circular motion always undergoes acceleration. During acceleration a charged particle 
radiates energy. Hence, an electron revolving around the nucleus will keep on radiating energy, keep 
coming closer to the nucleus and finally fall into it 
? It will make an atom highly stable. 
? However, as we know that atom is quite stable but cannot be explained by Rutherford’s model. 
 
Bohr’s Model of an atom 
 Postulates of Neil Bohr 
? Electrons revolve only on discrete or special fixed orbits around the nucleus, in an atom. 
? Electrons do not radiate energy while revolving. 
? These orbits are called energy levels and are denoted as K, L, M, N ………. or n = 1,2, 3, 4 . . . . .. 
? An electron can lose energy to move into a lower energy level while it can gain energy and move into 
higher energy shell.  
                             Fig. Neil Bohr’s model of Atom 
 
Number of Electrons in different shells: 
? The maximum number a shell can have is given by a formula 2n
2
. 
? The maximum number of electrons that can be accommodated in the last shell is 8. 
? Only if the inner shells are filled, electrons can be accommodated in an outer shell. 
Example –  
Atomic number or carbon is 6 
It means carbon has 6 protons and 6 electrons. 
No. of electrons in K shell = 2n
2
 
For K shell n = 1 and hence no. of electrons in K shell = 2*1
2
 
    = 2 
Maximum No. of electrons left = 6-2 = 4 
Maximum No. of electrons that can be accommodated in L shell = 2*2
2
 = 8 
Hence, the remaining 4 electrons will come in L shell. 
 
 
   Fig. Structure of Carbon Atom 
Page 4


Chapter 4 Structure of Atom 
Atoms – The smallest constituent particles of matter that cannot be broken down further. 
Sub atomic particles – Particles that make up an atom 
There are three major sub atomic particles –  
1. Protons – discovered by E. Goldstein in 1886. They are positively charged (+1). The mass of proton is 
taken as 1 unit.  
2. Electrons – discovered by J.J. Thomson in 1897. They are negatively charged (-1) but have negligible 
mass. 
3. Neutrons – Discovered by James Chadwick in 1932. They do not have any charge. The mass of neutrons 
is approximately equal to that of protons and hence is taken as 1 unit. 
Neutrons and protons are present in the center pf an atom inside the nucleus and the protons revolve around 
them. 
Thomson’s Model of Atom 
He thought of an atom similar to watermelon where the red part was positively charged and the seeds as the 
electrons. 
 Key features – 
a. An atom is a positively charged sphere. 
b. Electrons are embedded in that positively charged sphere. 
c. The negative and the positive charges are equal in magnitude and that is why an atom is neutral as a 
whole. 
 
                                                            Fig. Thomson’s model of an atom 
Drawbacks –  
? Thomson’s model of atom could not explain the Rutherford’s a-particles scattering experiment. 
? It could not explain the experiment of other scientists as well such as Neil Bohr. 
 
Rutherford’s Model of an Atom 
Rutherford’s Experiment 
? He selected a very thin gold foil (1000atoms thick)  
Electrons 
Positive Sphere 
? He passed fast moving a-particles through it. 
? a-particles are doubly charged Helium atoms (He
2+
). They have a positive charge of two units and a 
mass of 4u. 
Observations 
? Most of the fast moving a-particles passed straight without any deflection. 
? Some of the a-particles were deflected by small angles. 
? One out of 12000 particles bounced back after hitting the gold foil. 
      Inference or Conclusion –  
? Most of the space in an atom is empty because most of the a-particles passed without deflection. 
? Very few a-particles deflected with small angle indicating that the positive charge in an atom occupied 
very less space. 
? A very a-particles bounced back with an angle of 180
0
 indicating that most of the positive charge and 
mass of an atom is concentrated in a very small volume within the atom. 
                         Fig. Rutherford’s a-particles scattering experiment. 
 
Structure of Atom according to Rutherford –  
? Nearly all the mass is concentrated in a positively charged nucleus which is present in the center of the 
atom. 
? The electrons revolve around the nucleus in well-defined orbits. 
? The size of the nucleus is very small as compared to the size of an atom. Most of the space in an atom is 
empty. 
 
Drawback 
? A particle in circular motion always undergoes acceleration. During acceleration a charged particle 
radiates energy. Hence, an electron revolving around the nucleus will keep on radiating energy, keep 
coming closer to the nucleus and finally fall into it 
? It will make an atom highly stable. 
? However, as we know that atom is quite stable but cannot be explained by Rutherford’s model. 
 
Bohr’s Model of an atom 
 Postulates of Neil Bohr 
? Electrons revolve only on discrete or special fixed orbits around the nucleus, in an atom. 
? Electrons do not radiate energy while revolving. 
? These orbits are called energy levels and are denoted as K, L, M, N ………. or n = 1,2, 3, 4 . . . . .. 
? An electron can lose energy to move into a lower energy level while it can gain energy and move into 
higher energy shell.  
                             Fig. Neil Bohr’s model of Atom 
 
Number of Electrons in different shells: 
? The maximum number a shell can have is given by a formula 2n
2
. 
? The maximum number of electrons that can be accommodated in the last shell is 8. 
? Only if the inner shells are filled, electrons can be accommodated in an outer shell. 
Example –  
Atomic number or carbon is 6 
It means carbon has 6 protons and 6 electrons. 
No. of electrons in K shell = 2n
2
 
For K shell n = 1 and hence no. of electrons in K shell = 2*1
2
 
    = 2 
Maximum No. of electrons left = 6-2 = 4 
Maximum No. of electrons that can be accommodated in L shell = 2*2
2
 = 8 
Hence, the remaining 4 electrons will come in L shell. 
 
 
   Fig. Structure of Carbon Atom 
Valency 
The number of electrons required or need to be reduced (whichever is less) to completely fill the outer most shell is called 
the valency of an element. 
For e.g.  
1. Carbon 
Atomic number of carbon is 6 
            There are 2 electrons in K shell and 4 electrons in L shell but L shall can have 8 electrons. 
 4 electrons can either be added or removed to completely fill the outermost shell. 
 Hence, the valency of carbon is 4. 
2. Sodium 
 
Atomic number of carbon is 11 
            There are 2 electrons in K shell and 8 electrons in L shell and 1 in M shell. 
 M is the outer most shell and can have a maximum of 8 electrons. 
             So, if 1 electron is removed sodium will be left with 8 electrons in the outermost shell that is L and it will be 
completely filled. 
Hence, its valency is 1 
 
Electronic configuration 
It is the method of representation of the number of electrons in each shell of an atom 
E.g. Atomic number of Sodium is 11 
 Electronic configuration = 2, 8, 1 (Because there are 2, 8 and 1 electrons in K, L and M shell respectively) 
Atomic number of Oxygen is 8 
 Electronic configuration = 2, 6 
 
Atomic Number  
Number of protons present in the nucleus of an atom is known as its atomic number. 
It is denoted by ‘Z’. 
 
Atomic Mass 
Total number protons and neutrons in an atom is known as atomic mass. 
It is denoted by ‘A’ 
Notation or representing atomic mass and number of an element 
                           
Page 5


Chapter 4 Structure of Atom 
Atoms – The smallest constituent particles of matter that cannot be broken down further. 
Sub atomic particles – Particles that make up an atom 
There are three major sub atomic particles –  
1. Protons – discovered by E. Goldstein in 1886. They are positively charged (+1). The mass of proton is 
taken as 1 unit.  
2. Electrons – discovered by J.J. Thomson in 1897. They are negatively charged (-1) but have negligible 
mass. 
3. Neutrons – Discovered by James Chadwick in 1932. They do not have any charge. The mass of neutrons 
is approximately equal to that of protons and hence is taken as 1 unit. 
Neutrons and protons are present in the center pf an atom inside the nucleus and the protons revolve around 
them. 
Thomson’s Model of Atom 
He thought of an atom similar to watermelon where the red part was positively charged and the seeds as the 
electrons. 
 Key features – 
a. An atom is a positively charged sphere. 
b. Electrons are embedded in that positively charged sphere. 
c. The negative and the positive charges are equal in magnitude and that is why an atom is neutral as a 
whole. 
 
                                                            Fig. Thomson’s model of an atom 
Drawbacks –  
? Thomson’s model of atom could not explain the Rutherford’s a-particles scattering experiment. 
? It could not explain the experiment of other scientists as well such as Neil Bohr. 
 
Rutherford’s Model of an Atom 
Rutherford’s Experiment 
? He selected a very thin gold foil (1000atoms thick)  
Electrons 
Positive Sphere 
? He passed fast moving a-particles through it. 
? a-particles are doubly charged Helium atoms (He
2+
). They have a positive charge of two units and a 
mass of 4u. 
Observations 
? Most of the fast moving a-particles passed straight without any deflection. 
? Some of the a-particles were deflected by small angles. 
? One out of 12000 particles bounced back after hitting the gold foil. 
      Inference or Conclusion –  
? Most of the space in an atom is empty because most of the a-particles passed without deflection. 
? Very few a-particles deflected with small angle indicating that the positive charge in an atom occupied 
very less space. 
? A very a-particles bounced back with an angle of 180
0
 indicating that most of the positive charge and 
mass of an atom is concentrated in a very small volume within the atom. 
                         Fig. Rutherford’s a-particles scattering experiment. 
 
Structure of Atom according to Rutherford –  
? Nearly all the mass is concentrated in a positively charged nucleus which is present in the center of the 
atom. 
? The electrons revolve around the nucleus in well-defined orbits. 
? The size of the nucleus is very small as compared to the size of an atom. Most of the space in an atom is 
empty. 
 
Drawback 
? A particle in circular motion always undergoes acceleration. During acceleration a charged particle 
radiates energy. Hence, an electron revolving around the nucleus will keep on radiating energy, keep 
coming closer to the nucleus and finally fall into it 
? It will make an atom highly stable. 
? However, as we know that atom is quite stable but cannot be explained by Rutherford’s model. 
 
Bohr’s Model of an atom 
 Postulates of Neil Bohr 
? Electrons revolve only on discrete or special fixed orbits around the nucleus, in an atom. 
? Electrons do not radiate energy while revolving. 
? These orbits are called energy levels and are denoted as K, L, M, N ………. or n = 1,2, 3, 4 . . . . .. 
? An electron can lose energy to move into a lower energy level while it can gain energy and move into 
higher energy shell.  
                             Fig. Neil Bohr’s model of Atom 
 
Number of Electrons in different shells: 
? The maximum number a shell can have is given by a formula 2n
2
. 
? The maximum number of electrons that can be accommodated in the last shell is 8. 
? Only if the inner shells are filled, electrons can be accommodated in an outer shell. 
Example –  
Atomic number or carbon is 6 
It means carbon has 6 protons and 6 electrons. 
No. of electrons in K shell = 2n
2
 
For K shell n = 1 and hence no. of electrons in K shell = 2*1
2
 
    = 2 
Maximum No. of electrons left = 6-2 = 4 
Maximum No. of electrons that can be accommodated in L shell = 2*2
2
 = 8 
Hence, the remaining 4 electrons will come in L shell. 
 
 
   Fig. Structure of Carbon Atom 
Valency 
The number of electrons required or need to be reduced (whichever is less) to completely fill the outer most shell is called 
the valency of an element. 
For e.g.  
1. Carbon 
Atomic number of carbon is 6 
            There are 2 electrons in K shell and 4 electrons in L shell but L shall can have 8 electrons. 
 4 electrons can either be added or removed to completely fill the outermost shell. 
 Hence, the valency of carbon is 4. 
2. Sodium 
 
Atomic number of carbon is 11 
            There are 2 electrons in K shell and 8 electrons in L shell and 1 in M shell. 
 M is the outer most shell and can have a maximum of 8 electrons. 
             So, if 1 electron is removed sodium will be left with 8 electrons in the outermost shell that is L and it will be 
completely filled. 
Hence, its valency is 1 
 
Electronic configuration 
It is the method of representation of the number of electrons in each shell of an atom 
E.g. Atomic number of Sodium is 11 
 Electronic configuration = 2, 8, 1 (Because there are 2, 8 and 1 electrons in K, L and M shell respectively) 
Atomic number of Oxygen is 8 
 Electronic configuration = 2, 6 
 
Atomic Number  
Number of protons present in the nucleus of an atom is known as its atomic number. 
It is denoted by ‘Z’. 
 
Atomic Mass 
Total number protons and neutrons in an atom is known as atomic mass. 
It is denoted by ‘A’ 
Notation or representing atomic mass and number of an element 
                           
O  
H     Atomic no. 1; mass no 1 
H    Atomic no. 1 mass no. 3 
H     Atomic no. 1 mass no. 2 
Cl    Atomic no. 17; mass no 35 
Cl    Atomic no. 17; mass no 37 
                   
Mass number 
Symbol of Element 
      
Atomic Number 
 
   
A
Element 
                                       
Z 
 
For example:   
16
 
           
 8 
 
Isotopes 
Atoms of an element having same atomic number but different mass number are known as isotopes. 
E.g.  
1. There are 3 isotopes of hydrogen  
 Protium – 
1
 
        1
 
 Deutrium 
2 
   1 
  
Tritium  
3 
  1  
 
There are two isotopes of Chlorine  
              
35                      
And  
37 
17      17 
 
 
Calculating the Mass number of elements with isotopes: 
 
It is calculated using the average percentage of those isotopes present on Earth: 
 
For example – 75% of the chlorine present is Isotope 35 and 25% is isotope 37 
 
Average atomic mass of chlorine = 35*75 + 37*25 
              100        100 
    = 105 + 37 
          4  4 
    = 35.5u 
 
Applications of Isotopes: 
? An isotope of uranium is used as nuclear fuel. 
? An isotope of cobalt is used to treat cancer. 
? An isotope of iodine is used to treat goiter. 
? Radioactive isotopes of an element are used are markers in molecular biology experiments. 
 
Isobars 
 
Elements have different atomic number but different atomic mass are known as isobars. 
E.g. Calcium and Argon both have the atomic mass of 40. The atomic number of Calcium is 20 while that of Argon is 18.