Structure of the Atom Class 9 Worksheet Science Chapter 4

Multiple Choice Questions

Q1: The nucleus of an atom consists of ______.
(a) Protons
(b) Electrons and neutrons
(c) Protons and neutrons
(d) Neutrons 
Ans: (c)
Sol: 

• The nucleus of an atom consists of protons and neutrons.
• These particles are collectively referred to as nucleons.
• The mass of an atom is primarily due to the presence of these nucleons in the nucleus.

Q2: What is the maximum number of electrons which can be present in K and L shells in an atom?
(a) 2 and 8
(b) 2 and 18
(c) 2 and 32
(d) 8 and 18
Ans: (a)
Sol: 

• The K shell can hold a maximum of 2 electrons.
• The L shell can accommodate up to 8 electrons.

Q3: Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Ans: (a)
Rutherford's alpha-particle scattering experiment led to the discovery of the atomic nucleus. Key findings from the experiment include:

• Most alpha particles passed through the gold foil, indicating that atoms are mostly empty space.
• Only a few particles were deflected, suggesting that the positive charge of the atom is concentrated in a small area.
• A small fraction of particles rebounded, showing that the positive charge and mass are located in the nucleus.

This experiment fundamentally changed our understanding of atomic structure.

Q4: Isotopes of element have:
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers
Ans: (c)
Sol: 

• Isotopes are atoms of the same element.
• They have the same atomic number but different mass numbers.
• This difference is due to varying numbers of neutrons.

Q5: Number of valence electrons in Cl^– ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Ans:  (b)
Sol: Cl^– Ion has 18 electrons (17+1). Hence the number of valence electrons in Cl^– ion is 8. Electronic distribution:
Cl – 2,8,7
Cl^– – 2,8,8

Q6: An element has electronic configuration 2,8,4 it will be classified as
(a) Metal
(b) Non metal
(c) Metalloid
(d) None of these
Ans: (c)

• An element with the electronic configuration 2, 8, 4 typically shows characteristics of both metals and non-metals.
• This dual nature classifies it as a metalloid.

Q7: Amongst element X (2,8,6) and (2,8,8) which is more reactive and why ?
(a) X because it is a metal
(b) Y because it is non metal
(c) X because it has 6 valence electrons
(d) Y because it is gas
Ans: (c)
Element X is more reactive due to its 6 valence electrons. This means it is more likely to gain 2 electrons to achieve a stable octet configuration.

• X has 6 valence electrons.
• It tends to gain 2 electrons.
• This helps it achieve a stable octet.

Q8: The nucleus of the hydrogen atom is called as
(a) Neutron
(b) Electron
(c) Proton
(d) Nucleons
Ans: (c)
The nucleus of a hydrogen atom contains a single proton.

Q9: Cathode rays get deflected in a electric field towards
(a) Positive plate
(b) Negative Plate
(c) No deflection takes place
(d) First towards negative plate and then towards positive plate
Ans: (a)

• Cathode rays are negatively charged particles.
• They are attracted to the positive plate in an electric field.
• This attraction occurs because opposite charges attract each other.

Q10: The atomic number of an element ‘y’ is 20. The electronic configuration of the ion having inert gas configuration is
(a) 2,8,10
(b) 2,18
(c) 2,10,8
(d) 2,8,8
Ans: (d)

• The ion with an inert gas configuration has lost 2 electrons.
• This results in an electronic configuration of 2, 8, 8.

Fill in the Blank

Q1: According to Maharishi Kanad, the tiniest to tiny particle of a pure substance is called _________.
Ans: According to Maharishi Kanad, the tiniest to tiny particle of a pure substance is called Anu.
Maharishi Kanad proposed that the smallest indivisible particle of matter is called 'Anu', which translates to 'atom' in modern terminology.

Q2: An atom is the smallest unit of an element which takes part in a _________.
Ans: An atom is the smallest unit of an element which takes part in a Chemical reaction.
An atom is the fundamental building block of matter and is involved in chemical reactions, forming compounds and molecules.

Q3: Mass of an electron is 1/1837 times less than the mass of one atom of__________.
Ans: Mass of an electron is 1/1837 times less than the mass of one atom of Hydrogen.
This statement highlights the relative mass of an electron compared to hydrogen, which is the lightest element.

Q4: The K-shell of any atom cannot have more than _________ electrons.
Ans: The K-shell of any atom cannot have more than Two electrons.
The K-shell, being the closest shell to the nucleus, can only accommodate two electrons due to the principles of quantum mechanics.

Q5: Isotopes are the atoms of ___________ element, having same atomic number but different mass number.
Ans: Isotopes are the atoms of Same element, having same atomic number but different mass number.
Isotopes differ in mass number because they contain different numbers of neutrons, even though they have the same number of protons.

True or False

Q1: Radioactive isotope of iodine is used for making the medicine called tincture iodne.
Ans: False
Tincture iodine is a solution made from iodine and alcohol. It does not contain a radioactive isotope of iodine.
Q2: J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
Ans: False
J.J. Thomson is known for proposing the existence of electrons, but he did not claim that the nucleus of an atom contains only nucleons. His model suggested:

• An atom consists of a positively charged sphere.
• Electrons are embedded within this sphere.

Therefore, the statement is False.

Q3: The K-shell of any atom cannot have more than _________ electrons.
Ans: True
The K-shell is the first shell of an atom and can hold a maximum of 2 electrons. This is based on the formula 2n², where n is the shell number. For the K-shell:

• n = 1
• Maximum electrons = 2 × 1² = 2

Thus, the statement is correct.

Q4: The innermost atomic shell can hold a maximum of 18 electons.
Ans: False
The innermost shell, known as the K-shell, can only hold a maximum of 2 electrons. It cannot accommodate 18 electrons.
Q5: Nuclear chemistry has nothing to do with the electrons present in an atom.
Ans: True

Nuclear chemistry focuses on the nucleus of an atom and its reactions. It does not directly involve the electrons. Key points include:

• Nuclear chemistry studies nuclear reactions, such as fission and fusion.
• Electrons are involved in chemical reactions, but not in nuclear processes.
• The nucleus contains protons and neutrons, known as nucleons.

Thus, the statement is True.

Very Short Answer Questions

Q1: Out of proton and neutron, which is heavier?
Ans: Neutron is slightly heavier (1.675 × 10^–27 kg) than proton (1.67 × 10^–27 kg).

Q2: Were neutrons known at the time Rutherford performed the scattering experiment?
Ans: No, neutrons were not known when Rutherford conducted his scattering experiment. They were discovered later by Chadwick in 1932. Rutherford's experiment took place in 1911.

Q3: Why is proton so named?
Ans: The proton (H⁺) is formed when a hydrogen atom, also known as protium, loses an electron.

Q4: What is the number of electrons in the valence shell of chlorine (Z = 17)?
Ans: The electronic configuration of chlorine (Cl) is 2, 8, 7. This means:

• The first shell contains 2 electrons.
• The second shell contains 8 electrons.
• The outermost shell (valence shell) contains 7 electrons.

Thus, chlorine has 7 valence electrons in its outer shell.

Q5. What is the basic difference between the isotopes of an element?
Ans: Isotopes of an element are atoms that have the same number of protons but differ in the number of neutrons. This leads to variations in their mass numbers. Here are the key points:

• Isotopes have the same atomic number.
• They differ in mass number due to varying neutron counts.
• For example, hydrogen has three isotopes: protium (1H), deuterium (2H), and tritium (3H).

While isotopes share similar chemical properties, their physical properties can differ.

Short Answer Questions

The valency of an element is determined by the number of valence electrons in its outermost shell. Here’s how to find the valency for chlorine, sulphur, and magnesium:

• Chlorine (Cl):
  • Atomic number: 17 (electron configuration: 2, 8, 7)
  • Valence electrons: 7
  • Needs 1 more electron to complete its octet (8).
  • Valency: 1
• Sulphur (S):
  • Atomic number: 16 (electron configuration: 2, 8, 6)
  • Valence electrons: 6
  • Needs 2 more electrons to complete its octet (8).
  • Valency: 2
• Magnesium (Mg):
  • Atomic number: 12 (electron configuration: 2, 8, 2)
  • Valence electrons: 2
  • It is easier for magnesium to lose its 2 valence electrons than to gain 6.
  • Valency: 2

Q2: Describe Bohr’s model of the atom.
Ans: To remove the drawbacks of Rutherford’s atomic model and to explain structure of atom in detail Neils Bohr in 1912 proposed a model of atom. The special features of Bohr’s model are given below:
(1) An electron revolves in the orbit of atom with well-defined energy.
(2) Energy of orbits increases from inner shell to the outer shells i.e. energy for orbit nearest the nucleus is lowest.
(3) If energy is supplied then electron moves from lower orbit to the higher orbit and if an electron jumps from higher orbit (energy level) to the lower orbit (energy level) then energy is radiated as electromagnetic waves.
(4) Each orbit or shell represents an energy level. Such orbits are represented as K,L,M,N,O……….. and named from centre to outwards.
(5) The shell or orbits are associated with certain amount of energy and energy of orbits/shells increases from inward to outwards.eg K<L<M<N<O…………

Q3: What are the limitations of Rutherford’s model of the atom?
Ans: Limitations of Rutherford's Model of the Atom

• The model does not explain the stability of the atom.
• According to the model, electrons orbiting the nucleus should emit energy due to acceleration.
• This energy loss would cause the electrons to spiral inward and eventually collide with the nucleus, leading to an unstable atom.
• However, we observe that atoms are generally quite stable.

Q4: Write the postulates of Bohr theory?
Ans: The postulates of Bohr's theory are:

• Electrons move around the nucleus in specific circular paths known as orbits.
• Each orbit is linked to a fixed amount of energy.
• The larger the radius of the orbit, the greater the energy of the electrons.
• Electrons can transition between orbits by gaining or losing a specific amount of energy.

Q5: Why metals are electropositive and non-metals are electronegative in nature?
Ans: Metals are considered electropositive because they tend to lose electrons from their outermost shell. This process allows them to achieve a more stable configuration, resulting in a positive charge. In contrast, non-metals are electronegative as they typically gain electrons to reach stability. This gain of electrons leads to a negative charge.

• Metals lose electrons to become positively charged.
• Non-metals gain electrons to become negatively charged.