In-Text Questions and Answers
- What are canal rays?
Answer: Canal rays are streams of positively charged particles discovered by E. Goldstein. They are also known as anode rays. These rays are produced in a gas discharge tube and move in a direction opposite to that of electrons.
- If an atom contains one electron and one proton, will it carry any charge or not?
Answer: No, the atom will not carry any charge. The negative charge of the electron balances the positive charge of the proton, making the atom electrically neutral.
- On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson’s model, an atom consists of a positively charged sphere with negatively charged electrons embedded in it, like seeds in a watermelon. The positive and negative charges are equal in magnitude, so they cancel each other out, making the atom neutral overall.
- On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: According to Rutherford’s model, the nucleus of an atom contains positively charged particles called protons.
- Draw a sketch of Bohr’s model of an atom with three shells.
Answer: Bohr’s model shows a nucleus at the center with electrons orbiting in three shells around it. The shells are labeled K (first shell), L (second shell), and M (third shell).
- What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: If a metal other than gold is used, similar observations would be made but with fewer deflections. This is because other metals may have smaller nuclei and less dense atomic structures, resulting in fewer α-particles being deflected at large angles.
- Name the three sub-atomic particles of an atom.
Answer: The three sub-atomic particles are:
- Protons (p⁺): positively charged particles in the nucleus.
- Neutrons (n): neutral particles in the nucleus.
- Electrons (e⁻): negatively charged particles orbiting the nucleus.
- Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer: Number of neutrons = Atomic mass – Number of protons
Number of neutrons = 4 u – 2 = 2
So, helium has two neutrons.
- Write the distribution of electrons in carbon and sodium atoms.
Answer:
- Carbon (Atomic number 6):
Electron distribution: K shell = 2, L shell = 4
Electronic configuration: 2, 4 - Sodium (Atomic number 11):
Electron distribution: K shell = 2, L shell = 8, M shell = 1
Electronic configuration: 2, 8, 1
- Carbon (Atomic number 6):
- If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: K shell can hold 2 electrons, L shell can hold 8 electrons.
Total electrons = 2 + 8 = 10
So, the atom has 10 electrons.
- How will you find the valency of chlorine, sulphur, and magnesium?
Answer:
- Chlorine (Atomic number 17):
Electronic configuration: 2, 8, 7
Valence electrons = 7
Valency = 8 – 7 = 1 - Sulphur (Atomic number 16):
Electronic configuration: 2, 8, 6
Valence electrons = 6
Valency = 8 – 6 = 2 - Magnesium (Atomic number 12):
Electronic configuration: 2, 8, 2
Valence electrons = 2
Valency = 2
- Chlorine (Atomic number 17):
- 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?
Answer: The atomic number is equal to the number of protons, so it is 8.(ii) What is the charge on the atom?
Answer: The atom is neutral because the number of protons (positive charges) equals the number of electrons (negative charges), so the net charge is zero. - With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Answer:
- Oxygen:
Number of protons = 8
Number of neutrons = 8
Mass number = Protons + Neutrons = 8 + 8 = 16 - Sulphur:
Number of protons = 16
Number of neutrons = 16
Mass number = 16 + 16 = 32
- Oxygen:
- For the symbols H, D, and T, tabulate three sub-atomic particles found in each of them.
Answer:
Symbol Protons Neutrons Electrons H 1 0 1 D 1 1 1 T 1 2 1 - Write the electronic configuration of any one pair of isotopes and isobars.
Answer:
- Isotopes (Carbon-12 and Carbon-14):
Both have atomic number 6.
Electronic configuration for both: 2, 4 - Isobars (Calcium-40 and Argon-40):
Calcium (Atomic number 20): 2, 8, 8, 2
Argon (Atomic number 18): 2, 8, 8
- Isotopes (Carbon-12 and Carbon-14):
Exercise Questions and Answers
- Compare the properties of electrons, protons, and neutrons.
Answer:
- Electron (e⁻):
Charge: -1
Mass: Approximately 1/2000 of a proton
Location: Outside the nucleus in shells - Proton (p⁺):
Charge: +1
Mass: 1 atomic mass unit (u)
Location: Inside the nucleus - Neutron (n):
Charge: 0 (neutral)
Mass: 1 atomic mass unit (u)
Location: Inside the nucleus
- Electron (e⁻):
- What are the limitations of J.J. Thomson’s model of the atom?
Answer:
- It could not explain the results of the gold foil experiment.
- It did not account for the existence of a concentrated nucleus.
- It failed to explain the arrangement of electrons around the nucleus.
- It could not explain the stability of an atom.
- What are the limitations of Rutherford’s model of the atom?
Answer:
- It could not explain why atoms are stable despite electrons revolving around the nucleus.
- According to classical physics, electrons should lose energy and spiral into the nucleus.
- It did not explain the arrangement of electrons in energy levels.
- Describe Bohr’s model of the atom.
Answer:
- Electrons revolve around the nucleus in fixed orbits called shells.
- Each shell has a fixed energy level.
- Electrons do not radiate energy while in these shells.
- Electrons can jump between shells by absorbing or emitting energy.
- Compare all the proposed models of an atom given in this chapter.
Answer:
- Thomson’s Model:
Atom is a positive sphere with electrons embedded in it. - Rutherford’s Model:
Atom has a small, dense nucleus with electrons orbiting around it. - Bohr’s Model:
Electrons orbit the nucleus in fixed energy levels without losing energy.
- Thomson’s Model:
- Summarise the rules for writing the distribution of electrons in various shells for the first eighteen elements.
Answer:
- Maximum electrons in a shell = 2n², where n is the shell number.
- K shell (n=1) can hold up to 2 electrons.
- L shell (n=2) can hold up to 8 electrons.
- M shell (n=3) can hold up to 18 electrons, but for the first eighteen elements, it holds up to 8.
- Electrons fill lower energy shells first.
- Define valency by taking examples of silicon and oxygen.
Answer: Valency is the combining capacity of an atom to achieve a full outer shell.
- Silicon (Atomic number 14):
Electronic configuration: 2, 8, 4
Valence electrons: 4
Valency: 4 - Oxygen (Atomic number 8):
Electronic configuration: 2, 6
Valence electrons: 6
Valency: 8 – 6 = 2
- Silicon (Atomic number 14):
- Explain with examples:
(i) Atomic number:
Answer: Number of protons in the nucleus.
Example: Hydrogen has an atomic number of 1.(ii) Mass number:
Answer: Sum of protons and neutrons in the nucleus.
Example: Carbon-12 has a mass number of 12.(iii) Isotopes:
Answer: Atoms of the same element with same atomic number but different mass numbers.
Example: Hydrogen isotopes—Protium (^1H), Deuterium (^2H), Tritium (^3H).(iv) Isobars:
Answer: Atoms of different elements with same mass number but different atomic numbers.
Example: Argon-40 and Calcium-40.Uses of isotopes:
- An isotope of uranium (^235U) is used as fuel in nuclear reactors.
- An isotope of cobalt (^60Co) is used in cancer treatment.
- Na⁺ has completely filled K and L shells. Explain.
Answer: Sodium atom (Na) has electronic configuration: 2, 8, 1.
When it loses one electron to form Na⁺, the configuration becomes: 2, 8.
Now, the K and L shells are fully filled, making it more stable. - If bromine atom is available in the form of two isotopes ^79Br (49.7%) and ^81Br (50.3%), calculate the average atomic mass of bromine atom.
Answer: Average atomic mass = (79 × 49.7 + 81 × 50.3) / 100
= (3926.3 + 4084.3) / 100
= 8010.6 / 100
= 80.106 u - The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ^16X and ^18X in the sample?
Answer: Let the percentage of ^16X be x%.
Then, ^18X will be (100 – x)%
16.2 = (16 × x + 18 × (100 – x)) / 100
16.2 × 100 = 16x + 1800 – 18x
1620 = 1800 – 2x
2x = 1800 – 1620
2x = 180
x = 90%
So, ^16X is 90% and ^18X is 10%. - If Z = 3, what would be the valency of the element? Also, name the element.
Answer: Atomic number Z = 3
Element: Lithium
Electronic configuration: 2, 1
Valence electrons = 1
Valency = 1 - 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?
Answer:
- Mass number of X = 6 + 6 = 12
- Mass number of Y = 6 + 8 = 14
X and Y are isotopes of carbon (^12C and ^14C).
- 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.
Answer: F(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
Answer: F(c) The mass of an electron is about 1/2000 times that of proton.
Answer: T(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer: F - Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:
(a) Atomic Nucleus ✔
(b) Electron ✖
(c) Proton ✖
(d) Neutron ✖ - Isotopes of an element have:
(a) The same physical properties ✖
(b) Different chemical properties ✖
(c) Different number of neutrons ✔
(d) Different atomic numbers ✖ - Number of valence electrons in Cl⁻ ion are:
(a) 16 ✖
(b) 8 ✔
(c) 17 ✖
(d) 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 ✔ - Complete the following table.
Atomic Number Mass Number Number of Neutrons Number of Protons Number of Electrons Name of Atomic Species 9 19 10 9 9 Fluorine 16 32 16 16 16 Sulphur 12 24 12 12 12 Magnesium 1 2 1 1 1 Deuterium 1 1 0 1 0 Proton (H⁺ ion)