# Structure of the Atom : NCERT Exercise Questions

Q.1     Compare the properties of electrons, protons and neutrons.
Sol.

 Electron Proton Neutron (i) Electron are present outside the nucleus of an atom. (i) Proton are present in  the nucleus of an atom. (i) Neutron are present in the  nucleus of an atom. (ii) Electron are negatively charged (ii) Proton are positively charged. (ii) Neutron are neutral. (iii) The mass of an electron is considered to negligible. (iii) The mass of a proton is  approximately 2000 times as the mass of an electron (iii) The mass of neutron is nearly equal to the mass  of a proton.

Q.2     What are the limitations of J.J. Thomson’s model of the atom?
Sol.      According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the center of the atom called the nucleus, and the electrons revolve around the nucleus.

Q.3    What are the limitations of Rutherford’s model of the atom?
Sol.     According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience
acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the
nucleus. In such a case, the atom would be highly unstable and collapse.

Q.4     Describe Bohr’s model of the atom.
Sol.     Bohr’s model of the atom Niels Bohr proposed the following postulates regarding the model of the atom.
(i) Only certain orbits known as discrete orbits of electrons are allowed inside the atom.

(ii) While revolving in these discrete orbits, the electrons do not radiate energy.These discrete orbits or
shells are shown in the following diagram
. The first orbit (i.e., for n = 1) is represented by letter K. Similarly, for n = 2, it is L − shell, for n = 3, it is M − shell and for n = 4, it is N − shell. These orbits or shells are also called energy levels.

Q.5     Compare all the proposed models of an atom given in this chapter
Sol.

 Thomson's model Rutherford's model Bohr's model An atom consists of a positively charged sphere with electrons embedded in it. An atom consists of a positively charged particles concentrated at the center known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electron revolve around the nucleus in well - defined orbits. There are only certain orbits know as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

Q.6     summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Sol.     The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.
(i) The maximum number of electrons that a shell can accommodate is given by the formula  '$2{n^2}$', where 'n' is the orbit number or energy level index (n = 1, 2, 3...).
The maximum number of electrons present in an orbit  of n = 1 given by
$2{n^2} = 2 \times {1^2} = 2$
Similarly, for second orbit, it is  $2{n^2} = 2 \times {2^2} = 8$
For third orbit, it is $2{n^2} = 2 \times {3^2} = 18$
And so on .....
(ii) The outermost orbit can be accommodated by a maximum number of  8 electrons.
(iii) Shells are filled with electrons in a step wise manner i.e., the outer shell is not  occupied with electrons unless the inner shells are completely filled with electrons.

Q.7     Define valency by taking examples of silicon and oxygen.
Sol.      The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element. If the number of
valence electrons of the atom of an element is less than or equal to four, then the valency of that element

is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons.
Thus, the valency of silicon is four. On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.

Q.8     Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Sol.
(i) Atomic number
The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
(ii) Mass number
The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.
(iii) Isotopes And, the percentage of isotope is (100 − 10) % = 90%.

Q.12     If Z = 3, what would be the valency of the element? Also, name the element.
Sol.        By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).
Therefore, the element with Z = 3 is lithium.

Q.13     Composition of the nuclei of two atomic species X and Y are given as under X Y
Protons = 6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two species?
Sol.        Mass number of X = Number of protons + Number of neutrons
= 6 + 6
= 12
Mass number of Y = Number of protons + Number of neutrons
= 6 + 8
= 14
These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.

Q.14     For the following statements, write T for ‘True’ and F for ‘False’.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Sol.       (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton combining together. Therefore, itis neutral. (F)
(c) The mass of an electron is about 1/2000 times that of proton. (T)
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

Q.15     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the following question: Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
Sol.       Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus ( – )
(b) Electron ( x )
(c) Proton ( x )
(d) Neutron ( x )

Q.16     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the following
question: Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
Sol.       Isotopes of an element have
(a) the same physical properties ( x )
(b) different chemical properties ( x )
(c) different number of neutrons ( – )
(d) different atomic numbers ( x )

Q.17     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the
following question:
Number of valence electrons in Cl− ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Sol.       Number of valence electrons in Cl− ion are:
(a) 16 ( x )
(b) 8 ( – )
(c) 17 ( x )
(d) 18 ( x )

Q.18     Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
Sol.        (d) The correct electronic configuration of sodium is 2, 8, 1.
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