In the previous section, we saw the details about electron. In this section we will see proton and neutron
• In the previous section, we saw the cathode ray discharge tube. In that tube, a hole was made in the anode
• The coating of fluorescent material was given at the portion behind the anode. Now we do the reverse:
1. A hole is made in the cathode
• Coating of fluorescent material is given at the portion behind the cathode
2. In this new devise also, when voltage is applied, we see bright spots on the fluorescent material
• This indicates that, some particles started from the anode and moved towards the cathode
• This ray of particles is called anode rays or canal rays
3. These particles are attracted towards the negatively charged cathode
• So they must be positively charged
• So our next aim is to find the charge and mass of these positively charged particles
• The results can be summarized as follows:
(i) In the case of cathode rays, we saw that, the characteristics of the particles do not depend upon the type of gas
♦ This is because, the characteristics of electrons of all the elements are the same
• But in the case of anode rays, the characteristics of the particles do depend upon the type of gas
• The reason can be written as follows:
♦ The positively charged particles that make up the anode rays are in fact positively charged ions
♦ These ions are formed from the atoms of the gas filled in the tube
♦ The electrons, while moving towards the anode, bombard the gas atoms
♦ As a result, electrons are knocked out from the atoms
♦ Thus the gas atoms become positive ions
• These positive ions move towards the cathode, and thus we get the anode rays
• Are all ions thus formed of the same type ?
The answer can be obtained as follows:
♦ Atoms of some gases are so strong that, only one electron can be knocked out
♦ Atoms of some other gases are not so strong. Two electrons can be knocked out
• Thus we see that, 'the type of ions' will depend upon 'the type of gas'
• That means, 'the type of particles in the anode rays' will depend upon the 'type of gas'
(ii) In the case of cathode rays, we calculated the charge to mass ratio
• In that case, we get the same ratio for all gases.
♦ This is because, the charge e is same
♦ Mass me is also same for all particles
• But in our present case, if one electron was knocked out, the charge is one unit
♦ If two electrons were knocked out, the charge is two units
♦ so on . . .
• Also the corresponding masses will also vary
• So the charge to mass ratio will be different for different gases
(iii) In the case of some gases, the particles carry a single positive charge
♦ For some other gases, the particles carry two positive charges
♦ so on . .
• The reason has been explained in (ii)
(iv) The behavior of these positively charged particles is opposite to those observed for electrons or cathode rays
• This can be expected because, the charges are opposite
• If an electric field is applied,
♦ the cathode rays will deflect towards the positive electrode
♦ the anode rays will deflect towards the negative electrode
1. We saw that, each gas has it’s own anode ray particles
• The experiment was done with hydrogen
2. The particles in it’s anode rays were the smallest in size
• Also, those particles were the smallest in mass
3. Take out one particle from the anode ray of hydrogen
• It is the smallest positively charged particle in terms of mass
• So it can be considered as the basic unit
4. It was given the name proton
• This basic positive particle is formed by knocking off one electron from the hydrogen atom
• So the magnitude of the positive charge will be same as that of the negative charge
• The signs will be opposite
• We can write:
♦ Charge of one electron = -1.602176 ×10-19 C
♦ Charge of one proton = +1.602176 ×10-19 C
♦ The atom has negatively charged electrons
♦ The negative charges are neutralized by the positively charged protons
♦ So the atom is electrically neutral
1. In 1932 British Physicist James Chadwick bombarded a thin sheet of beryllium by 𝜶 particles
(We will see details about '𝜶 particles' later in this section)
2. Some unknown particles were emitted from the beryllium sheet
• These new particles were electrically neutral
• They had masses slightly greater than those of protons
3. Chadwick named those particles as neutrons
Electrons, protons and neutrons are the fundamental particles in an atom
Their properties are written below:
1. Electron:
Symbol: e
Absolute charge: -1.602176 ×10-19 C
Relative charge: -1
Mass in kg: 9.10939 ×10-31 kg
Mass in u: 0.00054 u
Approximate mass in u: 0
2. Proton:
Symbol: p
Absolute charge: 1.602176 ×10-19 C
Relative charge: 1
Mass in kg: 1.67262 ×10-27 kg
Mass in u: 1.00727 u
Approximate mass in u: 1
2. Neutron:
Symbol: n
Absolute charge: 0
Relative charge: 0
Mass in kg: 1.67493 ×10-27 kg
Mass in u: 1.00867 u
Approximate mass in u: 1
• We have seen details about 'u' in the previous classes
♦ It is an unit for measuring very small masses
♦ 'u' is the symbol for 'atomic mass unit'
♦ Details can be seen here
1. French scientist Henri Becquerel noticed high energy rays coming out from pitchblende
• Pitchblende is an ore of uranium
2. Large quantities of pitchblende will contain only minute quantities of uranium
• In other words, to obtain even minute quantities of uranium, very large quantities of pitchblende will have to be analyzed
3. Polish scientist Marie Curie and her husband French scientist Pierre, under took the task of isolating pure uranium from pitchblende
• They found out that, uranium spontaneously emits high energy radiation
■ This spontaneous emission of radiation is called radioactivity
('Spontaneous emission' means emission without an application of any external energies)
4. This radiation was further studied by the British scientist Ernest Rutherford
• He discovered that, the radiation coming out from uranium consists of three different radiations
♦ 𝜶 radiation
♦ 𝜷 radiation
♦ 𝜸 radiation
5. Consider the radiation coming out from uranium
• Place an electric field in it’s path (fig.2.4 below)
(In the fig., uranium is kept inside a lead block)
♦ The 𝜶 radiation will get separated and will be deflected towards the negative electrode
♦ The 𝜷 radiation will get separated and will be deflected towards the positive electrode
♦ The 𝜸 radiation will not be affected by the electric field. It will continue in it’s original path
6. Studies made by Rutherford gave the following information:
(a) 𝜶 particles
(i) 𝜶 rays consist of fast moving particles. These particles are named as 𝜶 particles
• 𝜶 particles have a positive charge of +2
(ii) We know that the helium atom has 2 electrons, 2 protons and 2 neutrons
• If a helium atom loses 2 electrons, then that atom will become an ion with a positive charge of +2
• Even when it is in the ionic form, it's mass will be 4 u because there are still 2 protons and 2 neutrons in the nucleus
• If those ions receive 2 electrons each, they will become helium atoms
(iii) Rutherford showed that, the 𝜶 particles indeed combines with two electrons to give helium atoms
• Also each 𝜶 particle has a mass of 4 u
(iv) So we can write:
■ An 𝜶 particle is in fact the positively charged core of a helium atom
(b) 𝜷 particles
(i) 𝜷 rays also consist of fast moving particles. These particles are named as 𝜷 particles
• 𝜷 particles have a negative charge of -1
(ii) 𝜷 particles are in fact, fast moving electrons
• We know that, each electron has a negative charge of -1
(c) 𝜸 radiation
• 𝜸 radiation is a high energy radiation. It does not contain particles
• Also it has no charge
7. 𝜶 particles have the least penetrating power. It can be stopped by ordinary materials like a few sheets of paper
• 𝜷 particles have a greater penetrating power. Thick aluminium sheets will be required to stop them
• 𝜸 rays have the greatest penetrating power. Heavy blocks of lead or concrete are required to stop them
8. X-rays
(i) We saw that cathode rays are produced in side a cathode ray tube
• We also saw that cathode rays consist of electrons
(ii) When the electrons strike a dense metal, a new type of rays are produced
• This was discovered by the German scientist Wilhelm Rontgen
• Since he did not know which rays they were, he named it as X-rays
X-rays like 𝜸 rays, have great penetrating power
(iii) Both are electromagnetic radiations. We will see details about electromagnetic radiations in later sections of this chapter
• In the previous section, we saw the cathode ray discharge tube. In that tube, a hole was made in the anode
• The coating of fluorescent material was given at the portion behind the anode. Now we do the reverse:
1. A hole is made in the cathode
• Coating of fluorescent material is given at the portion behind the cathode
2. In this new devise also, when voltage is applied, we see bright spots on the fluorescent material
• This indicates that, some particles started from the anode and moved towards the cathode
• This ray of particles is called anode rays or canal rays
3. These particles are attracted towards the negatively charged cathode
• So they must be positively charged
• So our next aim is to find the charge and mass of these positively charged particles
• The experiment with anode rays was repeated several times
(i) In the case of cathode rays, we saw that, the characteristics of the particles do not depend upon the type of gas
♦ This is because, the characteristics of electrons of all the elements are the same
• But in the case of anode rays, the characteristics of the particles do depend upon the type of gas
• The reason can be written as follows:
♦ The positively charged particles that make up the anode rays are in fact positively charged ions
♦ These ions are formed from the atoms of the gas filled in the tube
♦ The electrons, while moving towards the anode, bombard the gas atoms
♦ As a result, electrons are knocked out from the atoms
♦ Thus the gas atoms become positive ions
• These positive ions move towards the cathode, and thus we get the anode rays
• Are all ions thus formed of the same type ?
The answer can be obtained as follows:
♦ Atoms of some gases are so strong that, only one electron can be knocked out
♦ Atoms of some other gases are not so strong. Two electrons can be knocked out
• Thus we see that, 'the type of ions' will depend upon 'the type of gas'
• That means, 'the type of particles in the anode rays' will depend upon the 'type of gas'
(ii) In the case of cathode rays, we calculated the charge to mass ratio
• In that case, we get the same ratio for all gases.
♦ This is because, the charge e is same
♦ Mass me is also same for all particles
• But in our present case, if one electron was knocked out, the charge is one unit
♦ If two electrons were knocked out, the charge is two units
♦ so on . . .
• Also the corresponding masses will also vary
• So the charge to mass ratio will be different for different gases
(iii) In the case of some gases, the particles carry a single positive charge
♦ For some other gases, the particles carry two positive charges
♦ so on . .
• The reason has been explained in (ii)
(iv) The behavior of these positively charged particles is opposite to those observed for electrons or cathode rays
• This can be expected because, the charges are opposite
• If an electric field is applied,
♦ the cathode rays will deflect towards the positive electrode
♦ the anode rays will deflect towards the negative electrode
• Our next aim is to find the actual charge and mass of these particles in the anode rays
• The experiment was done with hydrogen
2. The particles in it’s anode rays were the smallest in size
• Also, those particles were the smallest in mass
3. Take out one particle from the anode ray of hydrogen
• It is the smallest positively charged particle in terms of mass
• So it can be considered as the basic unit
4. It was given the name proton
• This basic positive particle is formed by knocking off one electron from the hydrogen atom
• So the magnitude of the positive charge will be same as that of the negative charge
• The signs will be opposite
• We can write:
♦ Charge of one electron = -1.602176 ×10-19 C
♦ Charge of one proton = +1.602176 ×10-19 C
• So there are protons and electrons in an atom. Thus scientists had a more detailed picture of the structure of atom
♦ The negative charges are neutralized by the positively charged protons
♦ So the atom is electrically neutral
• But the continued researches gave hints about 'the presence of one more particle inside the atom'
(We will see details about '𝜶 particles' later in this section)
2. Some unknown particles were emitted from the beryllium sheet
• These new particles were electrically neutral
• They had masses slightly greater than those of protons
3. Chadwick named those particles as neutrons
So we can write:
Their properties are written below:
1. Electron:
Symbol: e
Absolute charge: -1.602176 ×10-19 C
Relative charge: -1
Mass in kg: 9.10939 ×10-31 kg
Mass in u: 0.00054 u
Approximate mass in u: 0
2. Proton:
Symbol: p
Absolute charge: 1.602176 ×10-19 C
Relative charge: 1
Mass in kg: 1.67262 ×10-27 kg
Mass in u: 1.00727 u
Approximate mass in u: 1
2. Neutron:
Symbol: n
Absolute charge: 0
Relative charge: 0
Mass in kg: 1.67493 ×10-27 kg
Mass in u: 1.00867 u
Approximate mass in u: 1
• We have seen details about 'u' in the previous classes
♦ It is an unit for measuring very small masses
♦ 'u' is the symbol for 'atomic mass unit'
♦ Details can be seen here
Besides the discovery of the above three particles, a few more scientific discoveries were made in the nineteenth century. Let us see some basic details about those discoveries:
• Pitchblende is an ore of uranium
2. Large quantities of pitchblende will contain only minute quantities of uranium
• In other words, to obtain even minute quantities of uranium, very large quantities of pitchblende will have to be analyzed
3. Polish scientist Marie Curie and her husband French scientist Pierre, under took the task of isolating pure uranium from pitchblende
• They found out that, uranium spontaneously emits high energy radiation
■ This spontaneous emission of radiation is called radioactivity
('Spontaneous emission' means emission without an application of any external energies)
4. This radiation was further studied by the British scientist Ernest Rutherford
• He discovered that, the radiation coming out from uranium consists of three different radiations
♦ 𝜶 radiation
♦ 𝜷 radiation
♦ 𝜸 radiation
5. Consider the radiation coming out from uranium
• Place an electric field in it’s path (fig.2.4 below)
(In the fig., uranium is kept inside a lead block)
♦ The 𝜶 radiation will get separated and will be deflected towards the negative electrode
♦ The 𝜷 radiation will get separated and will be deflected towards the positive electrode
♦ The 𝜸 radiation will not be affected by the electric field. It will continue in it’s original path
 |
| Fig.2.4 |
(a) 𝜶 particles
(i) 𝜶 rays consist of fast moving particles. These particles are named as 𝜶 particles
• 𝜶 particles have a positive charge of +2
(ii) We know that the helium atom has 2 electrons, 2 protons and 2 neutrons
• If a helium atom loses 2 electrons, then that atom will become an ion with a positive charge of +2
• Even when it is in the ionic form, it's mass will be 4 u because there are still 2 protons and 2 neutrons in the nucleus
• If those ions receive 2 electrons each, they will become helium atoms
(iii) Rutherford showed that, the 𝜶 particles indeed combines with two electrons to give helium atoms
• Also each 𝜶 particle has a mass of 4 u
(iv) So we can write:
■ An 𝜶 particle is in fact the positively charged core of a helium atom
(b) 𝜷 particles
(i) 𝜷 rays also consist of fast moving particles. These particles are named as 𝜷 particles
• 𝜷 particles have a negative charge of -1
(ii) 𝜷 particles are in fact, fast moving electrons
• We know that, each electron has a negative charge of -1
(c) 𝜸 radiation
• 𝜸 radiation is a high energy radiation. It does not contain particles
• Also it has no charge
7. 𝜶 particles have the least penetrating power. It can be stopped by ordinary materials like a few sheets of paper
• 𝜷 particles have a greater penetrating power. Thick aluminium sheets will be required to stop them
• 𝜸 rays have the greatest penetrating power. Heavy blocks of lead or concrete are required to stop them
8. X-rays
(i) We saw that cathode rays are produced in side a cathode ray tube
• We also saw that cathode rays consist of electrons
(ii) When the electrons strike a dense metal, a new type of rays are produced
• This was discovered by the German scientist Wilhelm Rontgen
• Since he did not know which rays they were, he named it as X-rays
X-rays like 𝜸 rays, have great penetrating power
(iii) Both are electromagnetic radiations. We will see details about electromagnetic radiations in later sections of this chapter
So now scientists have a clear idea about the particles inside an atom. The next task is to find out how these particles are distributed inside the atom. Many models were proposed in that period. In the next section, we will see some of those models
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