Notes On Atomic Theory of Matter - ICSE Class 10 Physics
The structure of matter has long continued to fascinate man. One of the earliest attempts to understand the structure of matter was made by Maharishi Kanad, an Indian sage who lived around 6th century BC. According to him, if matter is broken into smaller and smaller pieces, a stage will come when the matter cannot be broken further. Kanad called these tiniest of tiny particles ‘parmanu’. Around the same time that Maharishi Kanad proposed the existence of parmanu, Greek philosophers such as Democritus and Epicurus suggested that matter is made up of tiny indivisible particles called atoms. Incidentally, the word ‘atom’ is derived from the Greek word ‘atomos’, which means indivisible or uncut. However, the views expressed by Democritus and Epicurus as well as Kanad were philosophical in nature and not backed by any scientific experiment. Then several centuries later in 1803, an English scientist named John Dalton published a theory, explaining the nature of matter. Known as the Dalton’s Atomic Theory, this particular theory revolutionised man’s perspective about matter and its constituents. Some Important Postulates of the Dalton’s Atomic Theory All matter is composed of elements, which in turn are made up of small indivisible particles called atoms. Atoms of a same element have same mass and properties; whereas atoms of different elements have different masses. Every single atom of a particular element maintains its distinct identity throughout various chemical and physical changes. Atoms of different elements combine with one another to form compounds. Example: oxygen and carbon atoms combine to form carbon monoxide. On the other hand, atoms of the same elements combine in different ratio to form different compounds. Example: Two oxygen atoms and one carbon atom combine to form carbon dioxide. Though the Dalton’s Atomic Theory enabled scientists to gain more knowledge about matter, it also drew criticisms as this theory had several drawbacks. Drawbacks of Dalton’s Atomic Theory Dalton had suggested that an atom is indivisible. However, scientists later proved that an atom could be further broken down into protons, neutrons and electrons. scientists also proved that atoms of the same element can have slightly different masses. For instance, the isotopes of oxygen have different atomic masses. Example: Isotopes of Oxygen. This contradicted the Dalton’s Atomic Theory, which stated that atoms of the same element have same atomic mass. Another major drawback of the Dalton’s Atomic Theory was that it could not explain the differences in the properties of charcoal, graphite and diamond though all the three are made up of carbon atoms. Later Thomson discovered the presence of a negatively charged particle having a mass equal to 1/1837 times the mass of hydrogen. Since hydrogen is the lightest atom, Thomson concluded that this particular particle was a subatomic particle. He named this negatively charged subatomic particle as ‘electron’. The discovery of electron made it clear to Thomson that since an atom is electrically neutral; it must possess positively charged particles to compensate for the negatively charged electrons. Later, these positively charged particles came to be known as protons. Based on his studies, J. J. Thomson proposed a new model for atom in 1903; almost 100 years after Dalton had proposed his atomic theory. J. J. Thomson for atom According to this model, an atom is made up of positively charged particles, which are spread thinly in the form of a sphere. Embedded inside this sphere, are the electrons just as raisins are embedded in a plum pudding. Hence, this model also came to be known as Thomson’s Plum Pudding Model. Draw backs of Thomson’s Plum Pudding Model This theory could not explain how positively charged particles remain shielded from the negatively charged electrons without getting neutralised. Rutherford’s Experiment Thomson’s student, Ernest Rutherford conducted an experiment using gold foil which disproved Thomson’s model. In this particular experiment, Rutherford bombarded a thin foil of pure gold with positively charged particles called alpha particles. Observations made from alpha ray scattering experiment  He observed that while most of the alpha particles passed through the gold foil without deviating from their original part, a few others deviated slightly while others got completely deflected. These observations enabled Rutherford to put forth a new model on the structure of atom. Postulates of Rutherford nuclear model Rutherford nuclear model stated that the mass of an atom remains concentrated in small space called the nucleus. This nucleus is positively charged, that is, it contains the protons, which explains why certain alpha particles deviated and rebounded. Revolving around this nucleus are the negatively charged electrons. Incidentally, the number of electrons is always equal to the number of protons, which explains why atoms are electrically neutral. Rutherford’s atomic model thus gave a clearer picture about the structure of an atom. However, this theory had several drawbacks. Drawbacks of Rutherford’s atomic model This model, did not explain the position of electrons with respect to each other and the nucleus. Likewise, the model was unsuccessful in explaining the reason as to why the atomic mass of an element was far too greater than the mass of the protons, in spite of the fact that the mass of an atom is concentrated within its protons. However, Rutherford’s atomic model acted as a springboard for other scientists such as Niels Bohr and James Chadwick who further refined our knowledge about the atomic structure. Discovery of neutron Chadwick,discovered neutron. The discovery of neutron enabled scientists to understand as to why atomic mass of an element was far too greater than the mass of the protons. Today, the Modern Atomic Model which is based on the models proposed by Bohr, Rutherford, J. J. Thomson to name a few, states that an atom consists of three subatomic particles, namely, the electrons, neutrons and protons. Each of these three subatomic particles has unique properties. Properties of electron, proton and neutron    Parameters            Electron        Proton                   Neutron Position Present outside the nucleus and revolves in the orbits Present inside the nucleus Present inside the nucleus Mass 9.108X10-28 g 1.67X10-24g 1.67X10-24 g Charge -1.602 X10-19coulombs 1.602 X               10-19coulombs         Zero Representation   e-   p+   n Incidentally, protons and neutrons, collectively called nucleons, form the nucleus – the central core of an atom. Moreover, the nucleons are held together inside the nucleus by nuclear forces. While the protons and neutrons form the nucleus, electrons revolve around this nucleus in fixed orbits known as energy levels or shells. Interestingly, electrons present in the inner shells or orbits are held by a strong electric pull exerted by the protons. Such electrons are called bound electrons. On the other hand, electrons present in the outermost shell experience an extremely weak nuclear force due to a greater distance between their orbits and the protons. Such electrons are called valence electrons. Thus, our knowledge about electrons as well as other subatomic particles such as protons and neutrons is basically due to the pioneering work done by the likes of Bohr, Rutherford, J. J. Thomson and Dalton.

#### Summary

The structure of matter has long continued to fascinate man. One of the earliest attempts to understand the structure of matter was made by Maharishi Kanad, an Indian sage who lived around 6th century BC. According to him, if matter is broken into smaller and smaller pieces, a stage will come when the matter cannot be broken further. Kanad called these tiniest of tiny particles ‘parmanu’. Around the same time that Maharishi Kanad proposed the existence of parmanu, Greek philosophers such as Democritus and Epicurus suggested that matter is made up of tiny indivisible particles called atoms. Incidentally, the word ‘atom’ is derived from the Greek word ‘atomos’, which means indivisible or uncut. However, the views expressed by Democritus and Epicurus as well as Kanad were philosophical in nature and not backed by any scientific experiment. Then several centuries later in 1803, an English scientist named John Dalton published a theory, explaining the nature of matter. Known as the Dalton’s Atomic Theory, this particular theory revolutionised man’s perspective about matter and its constituents. Some Important Postulates of the Dalton’s Atomic Theory All matter is composed of elements, which in turn are made up of small indivisible particles called atoms. Atoms of a same element have same mass and properties; whereas atoms of different elements have different masses. Every single atom of a particular element maintains its distinct identity throughout various chemical and physical changes. Atoms of different elements combine with one another to form compounds. Example: oxygen and carbon atoms combine to form carbon monoxide. On the other hand, atoms of the same elements combine in different ratio to form different compounds. Example: Two oxygen atoms and one carbon atom combine to form carbon dioxide. Though the Dalton’s Atomic Theory enabled scientists to gain more knowledge about matter, it also drew criticisms as this theory had several drawbacks. Drawbacks of Dalton’s Atomic Theory Dalton had suggested that an atom is indivisible. However, scientists later proved that an atom could be further broken down into protons, neutrons and electrons. scientists also proved that atoms of the same element can have slightly different masses. For instance, the isotopes of oxygen have different atomic masses. Example: Isotopes of Oxygen. This contradicted the Dalton’s Atomic Theory, which stated that atoms of the same element have same atomic mass. Another major drawback of the Dalton’s Atomic Theory was that it could not explain the differences in the properties of charcoal, graphite and diamond though all the three are made up of carbon atoms. Later Thomson discovered the presence of a negatively charged particle having a mass equal to 1/1837 times the mass of hydrogen. Since hydrogen is the lightest atom, Thomson concluded that this particular particle was a subatomic particle. He named this negatively charged subatomic particle as ‘electron’. The discovery of electron made it clear to Thomson that since an atom is electrically neutral; it must possess positively charged particles to compensate for the negatively charged electrons. Later, these positively charged particles came to be known as protons. Based on his studies, J. J. Thomson proposed a new model for atom in 1903; almost 100 years after Dalton had proposed his atomic theory. J. J. Thomson for atom According to this model, an atom is made up of positively charged particles, which are spread thinly in the form of a sphere. Embedded inside this sphere, are the electrons just as raisins are embedded in a plum pudding. Hence, this model also came to be known as Thomson’s Plum Pudding Model. Draw backs of Thomson’s Plum Pudding Model This theory could not explain how positively charged particles remain shielded from the negatively charged electrons without getting neutralised. Rutherford’s Experiment Thomson’s student, Ernest Rutherford conducted an experiment using gold foil which disproved Thomson’s model. In this particular experiment, Rutherford bombarded a thin foil of pure gold with positively charged particles called alpha particles. Observations made from alpha ray scattering experiment  He observed that while most of the alpha particles passed through the gold foil without deviating from their original part, a few others deviated slightly while others got completely deflected. These observations enabled Rutherford to put forth a new model on the structure of atom. Postulates of Rutherford nuclear model Rutherford nuclear model stated that the mass of an atom remains concentrated in small space called the nucleus. This nucleus is positively charged, that is, it contains the protons, which explains why certain alpha particles deviated and rebounded. Revolving around this nucleus are the negatively charged electrons. Incidentally, the number of electrons is always equal to the number of protons, which explains why atoms are electrically neutral. Rutherford’s atomic model thus gave a clearer picture about the structure of an atom. However, this theory had several drawbacks. Drawbacks of Rutherford’s atomic model This model, did not explain the position of electrons with respect to each other and the nucleus. Likewise, the model was unsuccessful in explaining the reason as to why the atomic mass of an element was far too greater than the mass of the protons, in spite of the fact that the mass of an atom is concentrated within its protons. However, Rutherford’s atomic model acted as a springboard for other scientists such as Niels Bohr and James Chadwick who further refined our knowledge about the atomic structure. Discovery of neutron Chadwick,discovered neutron. The discovery of neutron enabled scientists to understand as to why atomic mass of an element was far too greater than the mass of the protons. Today, the Modern Atomic Model which is based on the models proposed by Bohr, Rutherford, J. J. Thomson to name a few, states that an atom consists of three subatomic particles, namely, the electrons, neutrons and protons. Each of these three subatomic particles has unique properties. Properties of electron, proton and neutron    Parameters            Electron        Proton                   Neutron Position Present outside the nucleus and revolves in the orbits Present inside the nucleus Present inside the nucleus Mass 9.108X10-28 g 1.67X10-24g 1.67X10-24 g Charge -1.602 X10-19coulombs 1.602 X               10-19coulombs         Zero Representation   e-   p+   n Incidentally, protons and neutrons, collectively called nucleons, form the nucleus – the central core of an atom. Moreover, the nucleons are held together inside the nucleus by nuclear forces. While the protons and neutrons form the nucleus, electrons revolve around this nucleus in fixed orbits known as energy levels or shells. Interestingly, electrons present in the inner shells or orbits are held by a strong electric pull exerted by the protons. Such electrons are called bound electrons. On the other hand, electrons present in the outermost shell experience an extremely weak nuclear force due to a greater distance between their orbits and the protons. Such electrons are called valence electrons. Thus, our knowledge about electrons as well as other subatomic particles such as protons and neutrons is basically due to the pioneering work done by the likes of Bohr, Rutherford, J. J. Thomson and Dalton.

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