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NCERT Class 9 Science Important Chapter 8 Journey Inside the Atom
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Journey Inside the Atom
Chapter – 8
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. Who first suggested that matter is made of indivisible particles called ‘parmanus’?
Ans: Acharya Kanada, an ancient Indian thinker, suggested that the smallest indivisible particles of matter are called ‘parmanus’.
2. What does the Greek word ‘atomos’ mean?
Ans: The Greek word ‘atomos’, proposed by philosophers Leucippus and Democritus, means indivisible.
3. Who proposed the first scientific description of how matter is made?
Ans: John Dalton proposed his atomic theory in 1808, describing atoms as the fundamental, indivisible building blocks of matter.
4. Which subatomic particle did J.J. Thomson discover?
Ans: J.J. Thomson discovered the electron, a tiny, negatively charged subatomic particle.
5. What was Thomson’s model of an atom compared to?
Ans: It was compared to a ‘plum pudding’ or a watermelon, where the positive charge is like the red pulp and the electrons are spread throughout like seeds.
6. What type of particles were used in Rutherford’s gold foil experiment?
Ans: Rutherford used alpha (α) particles, which are tiny, positively charged particles emitted from certain radioactive elements.
7. What did Rutherford conclude about the atom’s structure from his experiment?
Ans: He concluded that most of the atom is empty space and that its positive charge and mass are concentrated in an extremely small central region called the nucleus.
8. Why did Rutherford’s model fail to explain atomic stability?
Ans: It failed because an accelerating electron orbiting the nucleus should theoretically lose energy, spiral inward, and fall into the nucleus, causing the atom to collapse.
9. How did Niels Bohr explain the stability of atoms?
Ans: Bohr proposed that electrons revolve around the nucleus in fixed circular paths called stationary states or energy levels, and they do not lose energy while moving in these fixed shells.
10. What formula is used to calculate the maximum number of electrons a shell can hold?
Ans: The maximum number of electrons in a shell is calculated using the formula 2n², where ‘n’ is the shell number.
11. Which subatomic particle was discovered by James Chadwick in 1932?
Ans: James Chadwick discovered the neutron, a neutral subatomic particle with a mass nearly equal to that of a proton.
12. What are nucleons?
Ans: The protons and neutrons present together tightly packed inside the nucleus of an atom are called nucleons.
13. Which international organization approves the names and symbols of elements?
Ans: The International Union of Pure and Applied Chemistry (IUPAC) approves the names and symbols of elements.
14. Why is the chemical symbol for Iron ‘Fe’?
Ans: The symbol ‘Fe’ is derived from Iron’s Latin name, ferrum.
15. What does the atomic number (Z) of an element represent?
Ans: The atomic number represents the exact number of protons present in the nucleus of an atom of that element.
16. How is the mass number (A) of an atom calculated?
Ans: The mass number is calculated by adding the total number of protons and the total number of neutrons present in the nucleus.
17. Define the valency of an element.
Ans: Valency is the combining capacity of an atom, determined by the number of electrons it needs to gain, lose, or share to complete its outermost shell (octet).
18. What are isotopes?
Ans: Isotopes are atoms of the same element that have the same atomic number (number of protons) but different mass numbers (due to different numbers of neutrons).
19. Mention one medical application of an isotope.
Ans: Cobalt-60 is a radioactive isotope used in radiation treatment for cancer, and Iodine-131 is used to treat goitre and thyroid cancer.
20. What are isobars?
Ans: Isobars are atoms of different elements that have the same mass number but different atomic numbers, such as calcium and argon, which both have a mass number of 40.
Fill in the Blanks:
1. J.J. Thomson discovered the __________.
Ans: Electron.
2. The central dense region of an atom is called the __________.
Ans: Nucleus.
3. The maximum number of electrons in a shell is given by the formula __________.
Ans: 2n².
4. The atomic number of an element is equal to the number of __________ in its nucleus.
Ans: Protons.
5. Atoms of the same element having different mass numbers are called __________.
Ans: Isotopes.
True or False:
1. Rutherford concluded that most of the atom is empty space.
Ans: True.
2. Neutrons carry a positive electric charge.
Ans: False.
3. The symbol Fe is derived from the Latin name Ferrum.
Ans: True.
4. Isobars have the same atomic number but different mass numbers.
Ans: False.
5. Valency is the combining capacity of an atom.
Ans: True.
Long Questions & Answers:
1. Describe Ernest Rutherford’s gold foil experiment, its surprising observations, and the major conclusions he drew regarding the structure of an atom.
Ans: In 1911, Ernest Rutherford and his team conducted the gold foil experiment by aiming a narrow beam of positively charged alpha particles at an extremely thin sheet of gold foil. They initially expected the particles to pass straight through or only be slightly deflected, assuming Thomson’s model where positive charge was evenly spread out. However, while most particles passed straight through undeflected, some were sharply deflected, and a few even bounced straight back. From these surprising observations, Rutherford concluded that an atom is mostly empty space, which is why the vast majority of particles passed through. He also concluded that the positive charge and almost all of the atom’s mass are concentrated in an extremely small, dense central region called the nucleus, which forcibly repelled the incoming alpha particles.
2. Explain the major limitation of Rutherford’s planetary model of the atom and how Niels Bohr’s proposed model successfully resolved this issue.
Ans: Rutherford proposed a planetary model where negatively charged electrons revolve around a dense, positively charged nucleus. However, the major limitation of this model was its complete inability to explain atomic stability, because physics dictates that a charged particle accelerating in a circular path should constantly lose energy, causing it to spiral inward and eventually crash into the nucleus. To resolve this flaw, Niels Bohr proposed in 1913 that electrons do not move randomly but rather follow fixed circular paths called stationary states, orbits, or energy levels. Bohr postulated that while moving in these specific allowed shells (labeled K, L, M, N), an electron does not radiate or lose any energy, which successfully explained why atoms remain stable without collapsing.
3. Discuss the discovery and characteristics of the neutron, and explain how protons and neutrons together determine the mass number of an atom.
Ans: Early in the 20th century, scientists were puzzled because the mass of atoms like helium was much greater than the combined mass of their protons, leading to the search for another particle. In 1932, James Chadwick discovered the neutron, a subatomic particle located in the nucleus that has no electrical charge but possesses a mass nearly equal to that of a proton. Because the mass of revolving electrons is almost negligible, the entire mass of an atom is largely determined by the tightly packed protons and neutrons in its central nucleus. The total count of these protons and neutrons together, which are collectively referred to as nucleons, is defined as the mass number of the atom.
4. Explain the concept of valency based on the electronic configuration of atoms, and describe how atoms achieve a stable state.
Ans: The exact distribution of electrons in various shells around an atom’s nucleus is called its electronic configuration, and the outermost shell is specifically known as the valence shell. Atoms are most stable and largely unreactive when they possess a complete octet, meaning they have exactly eight electrons in their valence shell, or two in the case of a single-shell element like helium. To achieve this highly stable electronic configuration, reactive atoms with incomplete outermost shells will tend to gain, lose, or share electrons with other surrounding atoms. The exact number of electrons an atom must gain, lose, or share in order to complete its octet is defined as its valency, which directly represents the combining capacity of that specific element.
5. What are isotopes? Explain why the atomic mass of elements like chlorine is often expressed as a fraction rather than a whole number.
Ans: Isotopes are atoms of the exact same element that share the identical atomic number and number of protons, but have a different number of neutrons inside their nucleus, resulting in totally different mass numbers. While they differ significantly in physical properties and mass, isotopes possess remarkably similar chemical properties because they have the exact same number of electrons and identical electronic configurations. The atomic mass of an element as found in nature is generally not just a simple whole number, but rather a weighted average that accounts for the masses of all its naturally occurring isotopes and their relative percent abundances. For example, chlorine exists as two primary isotopes with masses of 35 u and 37 u in a 3:1 natural ratio, meaning its accurate weighted average atomic mass is calculated to be exactly 35.5 u.
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