In the previous section, we saw the details about electrons, protons and neutrons which are present inside an atom. The next task is to find out how these particles are distributed inside the atom. Many models were proposed by scientists at the beginning of the twentieth century. In this section we will see some of those models
1. Consider the edible portion of a water melon. It is a red sphere
• Note that, the edible red portion is not a ‘shell’
♦ A 'shell' is a hollow sphere. It has only an outer layer. The inside is hollow
• But in a water melon, the edible portion occupies the entire volume of the sphere
2. Thomson’s model resembles this edible portion of the water melon
• The red edible portion represents the entire volume of the atom
• All the ‘matter content’ (mass of protons and neutrons) of the atom is distributed uniformly in this volume
3. This 'uniformly distributed matter' is positively charged
4. The seeds of the water melon represents the electrons
• In a water melon, the seeds are distributed in the volume
♦ The seeds do not occupy any particular spot or particular layer
• In the same way, the electrons are distributed in the volume of the atom
5. This 'distribution of electrons' is done in such a way that, the most stable electrostatic arrangement is obtained
6. Note that the red edible portion has very large mass when compared to the mass of the seeds
• This is true for atom also. The ‘protons and neutrons’ have a very large mass when compared to the mass of the electrons
7. This model was called plum-pudding model or water melon model because, it resembles a plum pudding. It also resembles a water melon
• This model was able to explain the overall neutrality of the atom
• But it could not explain the results obtained from many later experiments. So this model did not gain acceptance
1. Consider the gold foil in fig.2.5 below
• It is a very thin foil. It’s thickness is approximately 100 nm
♦ 1 nanometer is 10-9 m
♦ So 100 nm is 10-7 m
• It is indeed very thin
2. But in spite of being so thin, the foil has hundreds of 'layers of atoms'. Let us see the reason:
(i) The diameter of a gold atom is approximately 166 pm
♦ 1 picometer = 0.001 nm = 10-12 m
♦ So 166 pm = 166 × 10-12 m
(ii) So number of layers in 10-7 m
= $\mathbf\small{\frac{10^{-7}}{166\times 10^{-12}}}$ = 0.006 × 105 = 600
• So there will be approximately 600 layers of gold atoms in that foil
3. The foil is bombarded with 𝜶 particles from a radioactive source
• This source is kept inside a lead block
4. A circular screen is placed around the foil
• This screen is coated with fluorescent material zinc sulphide
• So whenever an 𝜶 particle strikes the screen, a bright spot will be seen
• Thus the 𝜶 particles scattered from the foil can be detected
• The screen is made into a circular shape so that, 𝜶 particles scattered in ‘any direction’ can be detected
5. This experiment was done by the scientist Ernst Rutherford in 1910
• The three results obtained were as follows:
(i) Most of the 𝜶 particles passed through the gold foil undeflected
(ii) Some 𝜶 particles were deflected by small angles
(iii) A very few 𝜶 particles (approximately 1 in 20000) bounced back. That is., deflected by nearly 180o angle
6. Let us analyse the first of the three results:
■ Most of the 𝜶 particles passed through the gold foil undeflected
• If the atom was like a water melon, would we get this result?
Answer can be written in 4 steps:
(i) In the water melon model, the whole volume of the atom is filled with ‘materials’
(ii) So all the 𝜶 particles would surely encounter some resistance. They would not pass freely
(iii) As a result, there would be some deflection for almost all the 𝜶 particles
(iv) So if the atom was like a water melon, we would not get this result
• Thus the first result enables us to question the water melon model
7. Let us analyse the second result:
■ Some 𝜶 particles were deflected by small angles
• If the atom was like a water melon, would we get this result?
Answer can be written in 3 steps:
(i) This result is consistent with the water melon model
(ii) But remember that, we would expect all the 𝜶 particles to suffer small deflection
(iii) According to this result, only some 𝜶 particles suffer deflection
8. Let us analyse the third result
■ A very few 𝜶 particles (approximately 1 in 20000) bounced back. That is., deflected by nearly 180o angle
• If the atom was like a water melon, would we get this result?
Answer can be written in 5 steps:
(i) This result is totally unexpected
(ii) According to the water melon model, there is nothing inside the atom to cause such a huge deflection
(iii) To suffer such a huge deflection, the 𝜶 particle should hit on to some thing which is ‘massive’
(‘massive’ when compared to the 𝜶 particle)
(iv) Also that 'massive obstruction’ must have a large positive charge. Only then a ‘bouncing back’ can occur
(v) So like the first, this third result also enables us to question the water melon model
9. After detailed studies, Rutherford was able to give an explanation in 1911
• He proposed a new model. This model can be demonstrated with the help of fig.2.6 below:
(i) We saw that, the gold foil has a large number of layers
• Two of those layers are shown in fig.2.6
(ii) The incoming 𝜶 particles are represented by yellow arrows
• When they enter the foil, what do they see?
(iii) Consider the blue spheres
• Each of them represent an atom of gold
(iv) All the mass (protons and neutrons) of an atom is concentrated at it’s center
• This concentrated mass is represented by the small red sphere
(v) If all the mass is concentrated at the red spheres, why depict the atom as a blue sphere?
• The reason is that, though the electrons have only very low mass, we cannot ignore them. They are orbiting around the red sphere The orbits can be any where in the blue portion of the sphere.
(vi) Compared to the total volume of the blue sphere, the volume of the red sphere is very very small
• We can write a comparison as follows:
If the central red sphere is a cricket ball, then radius of the blue sphere is 5 km
(vii) So surely, most 𝜶 particles will pass through the foil without meeting with any resistance
(viii) The yellow arrows are classified into 3 categories
• The 𝜶 particles which do not meet with any resistance come under Category A
Most of the incoming 𝜶 particles fall under this category
• Some 𝜶 particles suffer a small deflection. They fall under category B
• A very few 𝜶 particles which directly hit the central red sphere will be bounced back nearly at 180o
• The large deflection is due to the enormous concentration of mass and positive charge inside the red sphere
• Since the red spheres are very small, the number of 𝜶 particles which directly hit them will be very small
• These 𝜶 particles fall under category C
(ix) In fig.b. the original path of the 𝜶 particles is shown as dashed lines
• We can see the ‘difference in deflection’ suffered by category B and category C
• The salient features of this model are written in three steps:
(i) The total positive charge possessed by the atom is densely concentrated in an extremely small region
• The total mass of the atom is also concentrated in this region
• This region was named nucleus by Rutherford
(ii) The nucleus is surrounded by electrons
• The electrons move around the nucleus at very high speeds in circular paths
• These circular paths are called orbits
• So the Rutherford’s model resembles the solar system
♦ The nucleus plays the role of the sun
♦ The electrons play the role of the planets
(iii) The nucleus and the electrons are held together by electrostatic forces of attraction
Thomson's model of atom
1. Consider the edible portion of a water melon. It is a red sphere
• Note that, the edible red portion is not a ‘shell’
♦ A 'shell' is a hollow sphere. It has only an outer layer. The inside is hollow
• But in a water melon, the edible portion occupies the entire volume of the sphere
2. Thomson’s model resembles this edible portion of the water melon
• The red edible portion represents the entire volume of the atom
• All the ‘matter content’ (mass of protons and neutrons) of the atom is distributed uniformly in this volume
3. This 'uniformly distributed matter' is positively charged
4. The seeds of the water melon represents the electrons
• In a water melon, the seeds are distributed in the volume
♦ The seeds do not occupy any particular spot or particular layer
• In the same way, the electrons are distributed in the volume of the atom
5. This 'distribution of electrons' is done in such a way that, the most stable electrostatic arrangement is obtained
6. Note that the red edible portion has very large mass when compared to the mass of the seeds
• This is true for atom also. The ‘protons and neutrons’ have a very large mass when compared to the mass of the electrons
7. This model was called plum-pudding model or water melon model because, it resembles a plum pudding. It also resembles a water melon
• This model was able to explain the overall neutrality of the atom
• But it could not explain the results obtained from many later experiments. So this model did not gain acceptance
Rutherford's model of atom
1. Consider the gold foil in fig.2.5 below
 |
| Fig.2.5 |
♦ 1 nanometer is 10-9 m
♦ So 100 nm is 10-7 m
• It is indeed very thin
2. But in spite of being so thin, the foil has hundreds of 'layers of atoms'. Let us see the reason:
(i) The diameter of a gold atom is approximately 166 pm
♦ 1 picometer = 0.001 nm = 10-12 m
♦ So 166 pm = 166 × 10-12 m
(ii) So number of layers in 10-7 m
= $\mathbf\small{\frac{10^{-7}}{166\times 10^{-12}}}$ = 0.006 × 105 = 600
• So there will be approximately 600 layers of gold atoms in that foil
3. The foil is bombarded with 𝜶 particles from a radioactive source
• This source is kept inside a lead block
4. A circular screen is placed around the foil
• This screen is coated with fluorescent material zinc sulphide
• So whenever an 𝜶 particle strikes the screen, a bright spot will be seen
• Thus the 𝜶 particles scattered from the foil can be detected
• The screen is made into a circular shape so that, 𝜶 particles scattered in ‘any direction’ can be detected
5. This experiment was done by the scientist Ernst Rutherford in 1910
• The three results obtained were as follows:
(i) Most of the 𝜶 particles passed through the gold foil undeflected
(ii) Some 𝜶 particles were deflected by small angles
(iii) A very few 𝜶 particles (approximately 1 in 20000) bounced back. That is., deflected by nearly 180o angle
6. Let us analyse the first of the three results:
■ Most of the 𝜶 particles passed through the gold foil undeflected
• If the atom was like a water melon, would we get this result?
Answer can be written in 4 steps:
(i) In the water melon model, the whole volume of the atom is filled with ‘materials’
(ii) So all the 𝜶 particles would surely encounter some resistance. They would not pass freely
(iii) As a result, there would be some deflection for almost all the 𝜶 particles
(iv) So if the atom was like a water melon, we would not get this result
• Thus the first result enables us to question the water melon model
7. Let us analyse the second result:
■ Some 𝜶 particles were deflected by small angles
• If the atom was like a water melon, would we get this result?
Answer can be written in 3 steps:
(i) This result is consistent with the water melon model
(ii) But remember that, we would expect all the 𝜶 particles to suffer small deflection
(iii) According to this result, only some 𝜶 particles suffer deflection
8. Let us analyse the third result
■ A very few 𝜶 particles (approximately 1 in 20000) bounced back. That is., deflected by nearly 180o angle
• If the atom was like a water melon, would we get this result?
Answer can be written in 5 steps:
(i) This result is totally unexpected
(ii) According to the water melon model, there is nothing inside the atom to cause such a huge deflection
(iii) To suffer such a huge deflection, the 𝜶 particle should hit on to some thing which is ‘massive’
(‘massive’ when compared to the 𝜶 particle)
(iv) Also that 'massive obstruction’ must have a large positive charge. Only then a ‘bouncing back’ can occur
(v) So like the first, this third result also enables us to question the water melon model
9. After detailed studies, Rutherford was able to give an explanation in 1911
• He proposed a new model. This model can be demonstrated with the help of fig.2.6 below:
 |
| Fig.2.6 |
• Two of those layers are shown in fig.2.6
(ii) The incoming 𝜶 particles are represented by yellow arrows
• When they enter the foil, what do they see?
(iii) Consider the blue spheres
• Each of them represent an atom of gold
(iv) All the mass (protons and neutrons) of an atom is concentrated at it’s center
• This concentrated mass is represented by the small red sphere
(v) If all the mass is concentrated at the red spheres, why depict the atom as a blue sphere?
• The reason is that, though the electrons have only very low mass, we cannot ignore them. They are orbiting around the red sphere The orbits can be any where in the blue portion of the sphere.
(vi) Compared to the total volume of the blue sphere, the volume of the red sphere is very very small
• We can write a comparison as follows:
If the central red sphere is a cricket ball, then radius of the blue sphere is 5 km
(vii) So surely, most 𝜶 particles will pass through the foil without meeting with any resistance
(viii) The yellow arrows are classified into 3 categories
• The 𝜶 particles which do not meet with any resistance come under Category A
Most of the incoming 𝜶 particles fall under this category
• Some 𝜶 particles suffer a small deflection. They fall under category B
• A very few 𝜶 particles which directly hit the central red sphere will be bounced back nearly at 180o
• The large deflection is due to the enormous concentration of mass and positive charge inside the red sphere
• Since the red spheres are very small, the number of 𝜶 particles which directly hit them will be very small
• These 𝜶 particles fall under category C
(ix) In fig.b. the original path of the 𝜶 particles is shown as dashed lines
• We can see the ‘difference in deflection’ suffered by category B and category C
• Based on his studies, Rutherford proposed a model for the atom
(i) The total positive charge possessed by the atom is densely concentrated in an extremely small region
• The total mass of the atom is also concentrated in this region
• This region was named nucleus by Rutherford
(ii) The nucleus is surrounded by electrons
• The electrons move around the nucleus at very high speeds in circular paths
• These circular paths are called orbits
• So the Rutherford’s model resembles the solar system
♦ The nucleus plays the role of the sun
♦ The electrons play the role of the planets
(iii) The nucleus and the electrons are held together by electrostatic forces of attraction
We see that Rutherford's model is an improvement from Thomson's model. But still, it had some drawbacks. We will see it in later sections
In the next section we will see how atomic number and mass number are assigned to various elements
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