Introduction: -i ) In 1895, Jean Perin proved that electrons ( known as cathode rays that time ) are negatively charged.
ii ) Shortly thereafter, J. J. Thomson determined charge ( e ) to mass ( m ) ratio of electrons to be of the order of 1011. This meant that m / e of electron is of the order of 10- 11 and that the mass of electron is very small.
iii ) In 1909, Millikan measured the magnitude of the charge of an electron.
iv ) Studies on X-rays, discovered in 1895, resulted in the discovery of radioactivity.
v ) Rutherford’s experiments on radioactivity proved emission of a-particles besides electrons in radioactive radiations. Thus one more particle was discovered.
vi ) In the 19th century, scientists were trying to measure wavelengths of radiations emitted by different gases filled in discharge tubes using the diffraction grating discovered by Henry Rowland. These wavelengths were found to be discrete and dependent on the type of gas filled in the discharge tube.
vii ) Same time Max Planck presented the photon theory and showed that the black body radiation is discrete. Einstein explained photoelectric effect using photon theory of light for which he received the Nobel prize.
viii ) In 1902, J. J. Thomson presented an atomic model according to which positive charge is distributed uniformly in a small spherical space of atom and electrons are embedded inside it like the seeds of watermelon embedded in its pulp. Hence, the model was called watermelon model or plum pudding model. The magnitude of positive charge was taken equal to the total negative charge of electrons to explain electrical neutrality of atom. But, the electrons embedded in the uniform distribution of positive charges should experience a force towards the centre of the atom directly proportional to the distance from the centre. Hence, they should perform either SHM or uniform circular motion. As both these motions are accelerated, electrons should emit continuous radiation according to the electromagnetic theory of Maxwell. This made it difficult to understand the emission of discrete wavelengths from atoms. Also such a model of atom cannot form a stable structure. To overcome these problems, Thomson assumed the charges to remain stationary unless disturbed from outside and thought about different arrangement of electrons in different atoms. He also estimated the size of atoms to be of the order of 10- 8 cm from the wavelengths of radiations emitted. Despite all these efforts, he could not explain why the radiation consisted of discrete wavelengths.
ix ) In 1906, Rutherford observed that a-particles passing through a slit provided in the chamber and incident on a photographic plate do not give sharp image of the slit. But on evacuating the chamber, the image became sharp. From this, he concluded that the a-particles must be scattered by the air particles in the chamber.
Rutherford’s Atomic Model:Rutherford proposed that the entire positive charge of an atom resides in a very small region at its centre, where almost all of its mass is concentrated. The negatively charged electrons move around this small central region, called the nucleus, in circular orbits. According to classical mechanics, An electron can revolve around the nucleus in any radius depending on its energy. But such a circular motion being accelerated, radiates energy in the form of electromagnetic radiation. As it loses energy, its orbit radius keeps on deceasing resulting in its motion being spiral and terminating in the nucleus. In this case, the atom cannot remain stable. Thus model failed to explain the stability of the atom.
NUCLEUS: - It discovered by a great scientist named as RUTHERFORD in 1911. A nucleus consists of electrically neutral neutrons and positively charged protons. The nucleus of hydrogen has only one proton and no neutrons. Protons and neutrons are commonly known as nucleons. The charge of proton = the charge of electron. Also the number of proton and electron must be equal in any atom so therefore Atom is generally neutrall. If proton or electron is not equal then its mean there is a charge produced on that atom which is either positive or negative. If number of proton is greater than electron then the charge of an atom is positive and if the number of electron is greater then charge of an atom is negative. e.g. Sodium. In this atom proton is 11 electron is also 11 but the neutron is 12. so that in first orbit 2 electron, second orbit 8 electron and in 3rd orbit only 1 electron. So when he gave out one electron from its outer most shell then electron is 10 and proton is already 11 so one proton is increase than electron so the charge of an atom is POSTIVE. Which is called ION.
But remember that the total sum of electron or proton is called atomic number of an atom and the total number of proton and neutron is called atomic mass or atomic weight. its mean that total mass of an atom lies in the centre of atom which is called NUCLEUS.
Nuclear Forces: - Despite Coulombian force of repulsion between the protons in the nucleus, nucleus does not
break up. This is because of strong nuclear force of attraction between ( i ) protons and protons, ( ii ) neutrons and neutrons and ( iii ) protons and neutrons which is more than the Coulombian force of repulsion. As far as this force is concerned, there is no difference between protons and neutrons. Hence they are commonly known as nucleons. This strong force is a short range force which exists between the neighbouring nucleons but is negligible between the nucleons far away from each other as in the large nuclei.
Nuclear Stability: - In the nuclei of lighter elements, the number of protons ( Z ) and neutrons ( N ) are almost equal but in case of heavy elements, the number of neutrons is comparatively more. Stable nucleus lies on or very close to the stability line. Initially stability line is on Z = N line and then lies below it which is needed for the stability of the nucleus. As the size of nucleus increases, the number of protons inside it also increase resulting in increase in the Coulombian repulsive force. To balance it, the nuclear force should also increase. Every additional proton exerts Coulombian repulsive force on all other protons inside the nucleus as this force is long ranged. But an additional neutron cannot exert the strong nuclear force on all the nucleons in a large nucleus as it is a short ranged force. Hence to balance the
repulsive Coulombian force in a large nucleus, the number of neutrons have to be more than the number of protons.
Natural Radioactivity: - Heavy elements like uranium are unstable and emit invisible radiations spontaneously to gain stability. This phenomenon is called “natural radioactivity”. This was accidentally discovered by Becquerel in 1896 while studying the relation between X-rays discovered by Rontgen in
1895 and the phenomenon of fluorescence. He called them Becquerel rays. Madame Curie and her husband Pierre Curie isolated radium and polonium from an ore of uranium called pitch-blend which showed much larger radioactivity than uranium. Later thorium and actinium possessing radioactivity were also discovered. Radiations from radioactive elements are called radioactive radiations. Radioactive radiations are spontaneous and instantaneous and are not affected by pressure, temperature, electric and magnetic fields etc. Their emission rates also cannot be changed by any means not even by combining radioactive elements chemically with other elements to form different compounds.
Radioactive Radiations: - During radioactive radiations, a-particles, b-particles and g-rays are emitted. a-particles are material particles which are nuclei of helium having two protons and two neutrons and carry a charge of +2e. b-particles are electrons. The velocity of emission of a- and b-particles depends upon the radioactive element from which the emission occurs. g-rays are not material particles but are electromagnetic rays.All these radiations affect a photographic plate, produce fluorescence and ionize the medium through which they pass. The extent unto which they can penetrate a medium depends upon their energy and their interactions with the constituent particles of the medium.
ii ) Shortly thereafter, J. J. Thomson determined charge ( e ) to mass ( m ) ratio of electrons to be of the order of 1011. This meant that m / e of electron is of the order of 10- 11 and that the mass of electron is very small.
iii ) In 1909, Millikan measured the magnitude of the charge of an electron.
iv ) Studies on X-rays, discovered in 1895, resulted in the discovery of radioactivity.
v ) Rutherford’s experiments on radioactivity proved emission of a-particles besides electrons in radioactive radiations. Thus one more particle was discovered.
vi ) In the 19th century, scientists were trying to measure wavelengths of radiations emitted by different gases filled in discharge tubes using the diffraction grating discovered by Henry Rowland. These wavelengths were found to be discrete and dependent on the type of gas filled in the discharge tube.
vii ) Same time Max Planck presented the photon theory and showed that the black body radiation is discrete. Einstein explained photoelectric effect using photon theory of light for which he received the Nobel prize.
viii ) In 1902, J. J. Thomson presented an atomic model according to which positive charge is distributed uniformly in a small spherical space of atom and electrons are embedded inside it like the seeds of watermelon embedded in its pulp. Hence, the model was called watermelon model or plum pudding model. The magnitude of positive charge was taken equal to the total negative charge of electrons to explain electrical neutrality of atom. But, the electrons embedded in the uniform distribution of positive charges should experience a force towards the centre of the atom directly proportional to the distance from the centre. Hence, they should perform either SHM or uniform circular motion. As both these motions are accelerated, electrons should emit continuous radiation according to the electromagnetic theory of Maxwell. This made it difficult to understand the emission of discrete wavelengths from atoms. Also such a model of atom cannot form a stable structure. To overcome these problems, Thomson assumed the charges to remain stationary unless disturbed from outside and thought about different arrangement of electrons in different atoms. He also estimated the size of atoms to be of the order of 10- 8 cm from the wavelengths of radiations emitted. Despite all these efforts, he could not explain why the radiation consisted of discrete wavelengths.
ix ) In 1906, Rutherford observed that a-particles passing through a slit provided in the chamber and incident on a photographic plate do not give sharp image of the slit. But on evacuating the chamber, the image became sharp. From this, he concluded that the a-particles must be scattered by the air particles in the chamber.
Rutherford’s Atomic Model:Rutherford proposed that the entire positive charge of an atom resides in a very small region at its centre, where almost all of its mass is concentrated. The negatively charged electrons move around this small central region, called the nucleus, in circular orbits. According to classical mechanics, An electron can revolve around the nucleus in any radius depending on its energy. But such a circular motion being accelerated, radiates energy in the form of electromagnetic radiation. As it loses energy, its orbit radius keeps on deceasing resulting in its motion being spiral and terminating in the nucleus. In this case, the atom cannot remain stable. Thus model failed to explain the stability of the atom.
NUCLEUS: - It discovered by a great scientist named as RUTHERFORD in 1911. A nucleus consists of electrically neutral neutrons and positively charged protons. The nucleus of hydrogen has only one proton and no neutrons. Protons and neutrons are commonly known as nucleons. The charge of proton = the charge of electron. Also the number of proton and electron must be equal in any atom so therefore Atom is generally neutrall. If proton or electron is not equal then its mean there is a charge produced on that atom which is either positive or negative. If number of proton is greater than electron then the charge of an atom is positive and if the number of electron is greater then charge of an atom is negative. e.g. Sodium. In this atom proton is 11 electron is also 11 but the neutron is 12. so that in first orbit 2 electron, second orbit 8 electron and in 3rd orbit only 1 electron. So when he gave out one electron from its outer most shell then electron is 10 and proton is already 11 so one proton is increase than electron so the charge of an atom is POSTIVE. Which is called ION.
But remember that the total sum of electron or proton is called atomic number of an atom and the total number of proton and neutron is called atomic mass or atomic weight. its mean that total mass of an atom lies in the centre of atom which is called NUCLEUS.
Nuclear Forces: - Despite Coulombian force of repulsion between the protons in the nucleus, nucleus does not
break up. This is because of strong nuclear force of attraction between ( i ) protons and protons, ( ii ) neutrons and neutrons and ( iii ) protons and neutrons which is more than the Coulombian force of repulsion. As far as this force is concerned, there is no difference between protons and neutrons. Hence they are commonly known as nucleons. This strong force is a short range force which exists between the neighbouring nucleons but is negligible between the nucleons far away from each other as in the large nuclei.
Nuclear Stability: - In the nuclei of lighter elements, the number of protons ( Z ) and neutrons ( N ) are almost equal but in case of heavy elements, the number of neutrons is comparatively more. Stable nucleus lies on or very close to the stability line. Initially stability line is on Z = N line and then lies below it which is needed for the stability of the nucleus. As the size of nucleus increases, the number of protons inside it also increase resulting in increase in the Coulombian repulsive force. To balance it, the nuclear force should also increase. Every additional proton exerts Coulombian repulsive force on all other protons inside the nucleus as this force is long ranged. But an additional neutron cannot exert the strong nuclear force on all the nucleons in a large nucleus as it is a short ranged force. Hence to balance the
repulsive Coulombian force in a large nucleus, the number of neutrons have to be more than the number of protons.
Natural Radioactivity: - Heavy elements like uranium are unstable and emit invisible radiations spontaneously to gain stability. This phenomenon is called “natural radioactivity”. This was accidentally discovered by Becquerel in 1896 while studying the relation between X-rays discovered by Rontgen in
1895 and the phenomenon of fluorescence. He called them Becquerel rays. Madame Curie and her husband Pierre Curie isolated radium and polonium from an ore of uranium called pitch-blend which showed much larger radioactivity than uranium. Later thorium and actinium possessing radioactivity were also discovered. Radiations from radioactive elements are called radioactive radiations. Radioactive radiations are spontaneous and instantaneous and are not affected by pressure, temperature, electric and magnetic fields etc. Their emission rates also cannot be changed by any means not even by combining radioactive elements chemically with other elements to form different compounds.
Radioactive Radiations: - During radioactive radiations, a-particles, b-particles and g-rays are emitted. a-particles are material particles which are nuclei of helium having two protons and two neutrons and carry a charge of +2e. b-particles are electrons. The velocity of emission of a- and b-particles depends upon the radioactive element from which the emission occurs. g-rays are not material particles but are electromagnetic rays.All these radiations affect a photographic plate, produce fluorescence and ionize the medium through which they pass. The extent unto which they can penetrate a medium depends upon their energy and their interactions with the constituent particles of the medium.
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