Class 9 Journey Inside the Atom Mock Test (CBSE)

Topics in Journey Inside the Atom

• Roots of atomic theory
• Thomson's model of the atom
• Gold foil experiment
• Bohr's model of the atom
• Discovery of the neutron
• Symbols of elements
• Atomic number and mass number
• Distribution of electrons in energy levels
• Valency
• Isotopes and average atomic mass
• Isobars

Journey Inside the Atom practice questions with answers

1. Q1Multiple choice·1 mark
   An element has an atomic number of $12$. What is the number of valence electrons present in an atom of this element?

   1. (a)$1$
   2. (b)$2$
   3. (c)$6$
   4. (d)$8$
   Show answer and step marks

   1. Correct option: (b) $2$. The electronic configuration for atomic number $12$ is $2, 8, 2$, which means there are $2$ electrons in the outermost shell.+1

   Answer: (b)
2. Q2Assertion and reason·1 mark
   Assertion (A): The maximum number of electrons that can be accommodated in any energy level is given by the formula $2n^2$, where $n$ is the orbit number. Reason (R): The maximum number of electrons in the outermost shell is $8$, even if it can hold more according to the $2n^2$ formula.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true but Reason (R) is false.
   4. (d)Assertion (A) is false but Reason (R) is true.
   Show answer and step marks

   1. Correct option: (b) Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).+1

   Answer: (b)
3. Q3Multiple choice·1 mark
   An element has an atomic number of $17$ and a mass number of $35$. The number of neutrons in its neutral atom is:

   1. (a)17
   2. (b)18
   3. (c)35
   4. (d)52
   Show answer and step marks

   1. Correct option: (b) 18. Number of neutrons = Mass number - Atomic number = $35 - 17 = 18$.+1

   Answer: (b)
4. Q4Assertion and reason·1 mark
   Assertion (A): Isotopes of an element have identical chemical properties. Reason (R): Chemical properties of an atom are controlled by the number of electrons, which is determined by the atomic number, and isotopes have the same atomic number.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true but Reason (R) is false.
   4. (d)Assertion (A) is false but Reason (R) is true.
   Show answer and step marks

   1. Correct option: (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).+1

   Answer: (a)
5. Q5Short answer·3 marks
   Define isotopes. An element $X$ has two naturally occurring isotopes ${}^{35}_{17}X$ and ${}^{37}_{17}X$ in the percentage abundance of $75\%$ and $25\%$ respectively. Calculate the average atomic mass of element $X$.
   Show answer and step marks

   1. Define isotopes as atoms of the same element having the same atomic number but different mass numbers.+1
   2. Set up the average atomic mass formula: $\text{Average Mass} = \frac{(35 \times 75) + (37 \times 25)}{100}$.+1
   3. Calculate the final value: $\frac{2625 + 925}{100} = 35.5\text{ u}$.+1

   Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers. Average atomic mass of $X$ = $35.5\text{ u}$.
6. Q6Long answer·5 marks
   (a) Define isotopes. Give two distinct practical applications or uses of isotopes in various fields. (b) An element $X$ has an atomic number of $17$ and a mass number of $35$. Determine: (i) The number of protons, neutrons, and electrons in atom $X$. (ii) The electronic configuration of $X$ and its valency.
   Show answer and step marks

   1. Definition of isotopes: Atoms of the same element having the same atomic number but different mass numbers.+1
   2. First use of isotopes, any one of: cobalt-60 in the treatment of cancer; iodine-131 in the treatment of goitre; uranium-235 as fuel in nuclear reactors; carbon-14 in dating old objects; or any other valid use.+0.5
   3. Second use, a different one from the same list.+0.5
   4. Number of protons = $17$ (equal to the atomic number).+0.5
   5. Number of electrons = $17$ (neutral atom).+0.5
   6. Number of neutrons = $35 - 17 = 18$.+0.5
   7. Electronic configuration of $X$: $2, 8, 7$ (K, L, M shells respectively).+1
   8. Valency of $X$: it needs $1$ electron to complete its octet, so its valency is $1$.+0.5

   Answer: (a) Isotopes are atoms of the same element with the same atomic number but different mass numbers. Any two of: cobalt-60 to treat cancer; iodine-131 to treat goitre; uranium-235 as nuclear-reactor fuel; carbon-14 for dating old objects. (b)(i) Protons 17, neutrons 18, electrons 17. (ii) Configuration 2, 8, 7; valency 1.

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