Chapter 4 Structure Of The Atom Solutions
Question - 11 : - How will you find the valency of chlorine, sulphur and magnesium?
Answer - 11 : -
If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.
The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively.
Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively.
Thus, the valency of chlorine = 8 −7 = 1
The valency of sulphur = 8 − 6 = 2
The valency of magnesium = 2
Question - 12 : - If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom and (ii) what is the charge on the atom?
Answer - 12 : -
(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.
(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.
Question - 13 : - With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Answer - 13 : -
Mass number of oxygen = Number of protons + Number of neutrons
= 8 + 8
= 16
Mass number of sulphur = Number of protons + Number of neutrons
= 16 +16
= 32
Question - 14 : - For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Answer - 14 : -
| Symbol | Proton | Neutron | Electron |
| H | 1 | 0 | 1 |
| D | 1 | 1 | 1 |
| T | 1 | 2 | 1 |
Question - 15 : - Write the electronic configuration of any one pair of isotopes and isobars.
Answer - 15 : - Two isotopes of carbonare 
and 
The electronicconfiguration of is 2, 4. The electronic configuration of is 2, 4.[Isotopes have the same electronic configuration]
and 
are a pair of isobarsCompare the properties of electrons, protons and neutrons.
Answer - 16 : -
| Electron | Proton | Neutron |
| (i) | Electrons are present outside the nucleus of an atom. | (i) | Protons are present in the nucleus of an atom. | (i) | Neutrons are present in the nucleus of an atom. |
| (ii) | Electrons are negatively charged. | (ii) | Protons are positively charged. | (ii) | Neutrons 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. |
Question - 17 : - What are the limitations of J.J. Thomson’s model of the atom?
Answer - 17 : -
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 centre of the atom called the nucleus, and the electrons revolve around the nucleus.
Question - 18 : - What are the limitations of Rutherford’s model of the atom?
Answer - 18 : -
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.
Question - 19 : - Describe Bohr’s model of the atom.
Answer - 19 : -
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.
Compare all the proposed models of an atom given in this chapter.
Answer - 20 : -
| 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 centre known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electrons revolve around the nucleus in well-defined orbits. | There are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving. |