1) Compare the properties of electrons, protons and neutrons.
ANSWER:-Electrons- Electrons are Negatively charged particles Protons-Protons are Positively charged particles. Neutron-Neutrons do not carry any charge and are neutral
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 result of scattering experiment performed by Rutherford.
→ It did not have any experiment support.
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.
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.
6) Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.
ANSWER:-→ If n gives the number of orbit or energy level, then 2n 2 gives the maximum number of electrons possible in a given orbit or energy level. Thus, 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.
7) Define valency by taking examples of silicon and oxygen.
ANSWER:-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.
8) Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
ANSWER :-( 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.
(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.
Two uses of isotopes:
→ One isotope of uranium is used as a fuel in nuclear reactors.
→ One isotope of cobalt is used in the treatment of cancer.
9) Na + has completely filled K and L shells. Explain.
ANSWER:-The atomic number of sodium is 11. So, 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.
10). If 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 bromine atom.
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?
ANSWER:-Let the percentage of isotope 18 / 8 X be y%. Thus, the percentage of isotope 16 / 8 X will be (100 - y) %.
18y + 1600 - 16y = 1620
2y + 1600 = 1620
2y = 1620 - 1600
Therefore, the percentage ofisotope 18 / 8 X is 10%.
And, the percentage of isotope
16 / 8 X is (100 - 10) % = 90%.
12) If Z = 3, what would be the valency of the element? Also, name the element.
ANSWER:-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.
13) Composition of the nuclei of two atomic species X and Y are given as under
Protons = 6 6
Neutrons = 6. 8
Give the mass numbers of X and Y. What is the relation between the two species?
ANSWER:-Mass number of X = Number of protons + Number of neutrons
= 6 + 6
Mass number of Y = Number of protons + Number of neutrons
= 6 + 8
These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes
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 / 2000times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
15) Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
ANSWER:-a) Atomic nucleus
16) Isotopes of an element have
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers
ANSWER:-different number of neutrons
17) Number of valence electrons in Cl - ion are:
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
ANSWER:-d) 2, 8, 1
19) Complete the following table. Atomic num. Mass num. No of Neutrons
9. 19. 10
16. 32. 16
12. 24. 12
1. 2. 1
1. 1. 0