Question 1: Compare the properties of electrons, protons, and neutrons.
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Location | Present outside the nucleus of an atom. | Present in the nucleus of an atom. | Present in the nucleus of an atom. |
| Charge | Negatively charged. | Positively charged. | Neutral. |
| Mass | Considered negligible. | Approximately 2000 times the mass of an electron. | Nearly equal to the mass of a proton. |
Question 2: What are the limitations of J.J. Thomson's model of the atom?
Answer: The limitations of J.J. Thomson's model of the atom are:
- It could not explain the results of the scattering experiment performed by Rutherford.
- It did not have any experimental support.
Question 3: What are the limitations of Rutherford's model of the atom?
Answer: The limitations of Rutherford's model of the atom are:
- It failed to explain the stability of an atom.
- It doesn't explain the spectrum of hydrogen and other atoms.
Question 4: Describe Bohr's model of the atom.
Answer:
- The atom consists of a small positively charged nucleus at its center.
- The whole mass of the atom is concentrated at the nucleus, and the volume of the nucleus is much smaller than the volume of the atom.
- All the protons and neutrons of the atom are contained in the nucleus.
- Only certain orbits known as discrete orbits of electrons are allowed inside the atom.
- While revolving in these discrete orbits, electrons do not radiate energy. These orbits or cells are represented by the letters K, L, M, N, etc. or the numbers, n = 1, 2, 3, 4, etc.
Question 5: Compare all the proposed models of an atom given in this chapter.
| Thomson’s model | Rutherford’s model | Bohr’s model |
|---|---|---|
| An atom consists of a positively charged sphere and the electrons are embedded in it. | The atom consists of a positively charged center in the atom called the nucleus. | Bohr agreed with almost all points as said by Rutherford except regarding the revolution of electrons for which he added that there are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. |
| The negative and positive charges are equal in magnitude. As a result, the atom is electrically neutral. | The size of the nucleus is very small as compared to the size of the atom. | While revolving in its discrete orbits, the electrons do not radiate energy. |
Question 6: Summarize the rules for writing the distribution of electrons in various shells for the first eighteen elements.
The rules for writing the distribution of electrons in various shells for the first eighteen elements are as follows:
- If n represents the number of orbit or energy level, then 2n² gives the maximum number of electrons possible in a given orbit or energy level. Thus, the:
- First orbit or K-shell will have 2 electrons.
- Second orbit or L-shell will have 8 electrons.
- Third orbit or M-shell will have 18 electrons.
- If it is the outermost orbit, then it should have not more than 8 electrons.
- There should be step-wise filling of electrons in different orbits, i.e., electrons are not accompanied in a given orbit if the earlier orbits or shells are incompletely filled.
Question 7: Define valency by taking examples of silicon and oxygen.
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.
Valency of Silicon: It has electronic configuration: 2,8,4. Thus, the valency of silicon is 4 as these electrons can be shared with others to complete octet.
Valency of Oxygen: It has electronic configuration: 2,6. Thus, the valency of oxygen is 2 as it will gain 2 electrons to complete its octet.
Question 8: Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
(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: These are atoms of the same element having the same atomic number, but different mass numbers. For example, chlorine has two isotopes with atomic number 17 but mass numbers 35 and 37.
(iv) Isobars: These are atoms having the same mass number, but different atomic numbers, i.e., isobars are atoms of different elements having the same mass number. For example, Ne has atomic number 10 and sodium has atomic number 11, but both of them have mass numbers as 22.
Two uses of isotopes:
1. One isotope of uranium is used as a fuel in nuclear reactors.
2. One isotope of cobalt is used in the treatment of cancer.
Question 9: Na+ has completely filled K and L shells. Explain.
The atomic number of sodium is 11. So, a neutral sodium atom has 11 electrons, and its electronic configuration is 2, 8, 1. But Na+ has 10 electrons. Out of 10, K-shell contains 2 and L-shell 8 electrons respectively. Thus, Na+ has completely filled K and L shells.
Question 10: If a bromine atom is available in the form of, say, two isotopes 79 / 35Br (49.7%) and 81 / 35Br (50.3%), calculate the average atomic mass of a bromine atom.
It is given that two isotopes of bromine are 79 / 35Br (49.7%) and 81 / 35Br (50.3%). Then, the average atomic mass of the bromine atom is calculated as follows:
Question 11: The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 16 / 8 X and 18 / 8 X in the sample?
It is given that the average atomic mass of the sample of element X is 16.2 u. Let the percentage of isotope 18 / 8 X be y%. Thus, the percentage of isotope 16 / 8 X will be (100 - y)%.
Therefore, 18y + 1600 - 16y = 1620
2y + 1600 = 1620
2y = 1620 - 1600
y = 10
Therefore, the percentage of isotope 18 / 8 X is 10%, and the percentage of isotope 16 / 8 X is (100 - 10)% = 90%.
Question 12: If Z = 3, what would be the valency of the element? Also, name the element.
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.
Question 13: Composition of the nuclei of two atomic species X and Y are given as under:
X:
- Protons = 6
- Neutrons = 6
Y:
- Protons = 6
- Neutrons = 8
Give the mass numbers of X and Y. What is the relation between the two species?
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.
Question 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.
False
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
False
(c) The mass of an electron is about 1 / 2000 times that of a proton.
True
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
False
Question 15: Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
(a) Atomic nucleus
Question 16: Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
(c) different number of neutrons
Question 17: Number of valence electrons in Cl - ion are:
(a) 16
(b) 8
(c) 17
(d) 18
(b) 8
Question 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
(d) 2, 8, 1
Question 19: Complete the following table.
