notes for class 9cbse structure of the atoms
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The Structure of an Atom
The discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton's atomic theory. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.
Thomson's Model of an Atom
Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere.
Thomson proposed that:I. An atom consists of a positively charged sphere and the electrons are embedded in it.
II. The negative and positive charge are equal in magnitude. So, the atom as a whole is electrically neutral.
Drawbacks of Rutherford's model of the atom
The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.
Bohr's Model of Atom
Neils Bohr put forward the following postulates about the model of an atom:
Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.While revolving in discrete orbits the electrons do not radiate energy.These orbits or shell are called energy levels.
Neutrons
In 1932, J Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as 'n'. The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
How are Electrons distributed in Different Orbits (Shells)?
The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.
The number of electrons in different energy levels or shells:
The maximum number of electrons present in a shell is given by the formula 2n2 , where 'n' is the orbit number or energy level index, 1,2,3,..... Hence the maximum number of electrons in different shells.The maximum number of electrons that can be accommodated in the outermost orbit is 8.Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.Valency
The electrons in an atom are arranged in different shell/orbits. The electrons present in the outer most shell of an atom are known as the valence electrons.
The outermost shell of an atom can accommodate a maximum of 8 electrons. It was observed that the atoms of elements, having a completely filled outer most shell show little chemical activity. In other words, their combining capacity or valency is zero, of these inert elements, the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outer most shell.
The combining capacity of the atoms of other elements, that is, their tendency to react and form molecules with atoms of the same or different elements was thus explained as an attempt to attain a fully-filled outer most shell. An outermost-shell, which had eight electrons, was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outer most shell. This was done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency discussed in the previous chapter. For example, hydrogen/lithium/sodium atoms contain one electron each in their outermost shell; therefore each one of them can lose one electron. So they are said to have valency of one. The valency of magnesium and Aluminium is two and three, respectively, because magnesium has two electrons in its outer most shell.
The discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton's atomic theory. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.
Thomson's Model of an Atom
Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere.
Thomson proposed that:I. An atom consists of a positively charged sphere and the electrons are embedded in it.
II. The negative and positive charge are equal in magnitude. So, the atom as a whole is electrically neutral.
Drawbacks of Rutherford's model of the atom
The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.
Bohr's Model of Atom
Neils Bohr put forward the following postulates about the model of an atom:
Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.While revolving in discrete orbits the electrons do not radiate energy.These orbits or shell are called energy levels.
Neutrons
In 1932, J Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as 'n'. The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
How are Electrons distributed in Different Orbits (Shells)?
The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.
The number of electrons in different energy levels or shells:
The maximum number of electrons present in a shell is given by the formula 2n2 , where 'n' is the orbit number or energy level index, 1,2,3,..... Hence the maximum number of electrons in different shells.The maximum number of electrons that can be accommodated in the outermost orbit is 8.Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.Valency
The electrons in an atom are arranged in different shell/orbits. The electrons present in the outer most shell of an atom are known as the valence electrons.
The outermost shell of an atom can accommodate a maximum of 8 electrons. It was observed that the atoms of elements, having a completely filled outer most shell show little chemical activity. In other words, their combining capacity or valency is zero, of these inert elements, the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outer most shell.
The combining capacity of the atoms of other elements, that is, their tendency to react and form molecules with atoms of the same or different elements was thus explained as an attempt to attain a fully-filled outer most shell. An outermost-shell, which had eight electrons, was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outer most shell. This was done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency discussed in the previous chapter. For example, hydrogen/lithium/sodium atoms contain one electron each in their outermost shell; therefore each one of them can lose one electron. So they are said to have valency of one. The valency of magnesium and Aluminium is two and three, respectively, because magnesium has two electrons in its outer most shell.
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