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Science Assertion (A) & Reason (B) MCQ

Structure of the Atom · CBSE STD 9 (CBSE - English Medium). 5 questions.

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Assertion (A) & Reason (B) MCQ

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5 questions · timed · auto-graded

MCQ 11 Mark
In the following questions, a statement of Assertion $(A)$ is followed by a statement of Reason $(R)$.
Mark the correct choice as:
Assertion: The valency of an atom of flourine is $7$.
Reason: The number of electrons present in the outermost shell of an atom are known as the valence electrons.
  • A
    If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  • B
    If both Assertion and Reason are true but Reason is not the correct explanationi of Assertion.
  • C
    If Assertion is true but Reason is false.
  • ✓
    If Assertion is false but Reason is true.
Answer
Correct option: D.
If Assertion is false but Reason is true.
Boron has a valency of $3$ and carbon has a valency of four; since the outermost shell has $3$ and $4$ electrons respectively. Since fluorine has $7$ electrons in the outermost orbit it loses one electron to attain the octet configuration. So the valency of fluorine is $1$.
The number of electrons present in the outermost shell of an atom are known as the valence electrons.
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MCQ 21 Mark
In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
Assertion: $Na, K, Ca, Mg$ are metals because they can lose electrons to form positive ions (cations).
Reason: $F, CI, N, P$ are non-metals because they can gain electrons to form anions $(-)$.
  • ✓
    Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
  • B
    The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
  • C
    Assertion is true but the Reason is false.
  • D
    The statement of the Assertion is false but the Reason is true.
Answer
Correct option: A.
Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
Atom is electrically neutral as it has same number of electrons and protons. When electrons are removed, a positively charged ion called cation is formed and when electrons are added, negatively charged ion called anion is formed.
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MCQ 31 Mark
In the following questions, a statement of Assertion $(A)$ is followed by a statement of Reason $(R)$. Mark the correct choice as:
Assertion: Bohr's orbits are called stationary orbits.
Reason: Electrons remain stationary in these orbits for some time.
  • A
    If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  • B
    If both Assertion and Reason are true but Reason is not the correct explanationi of Assertion.
  • ✓
    If Assertion is true but Reason is false.
  • D
    If Assertion is false but Reason is true.
Answer
Correct option: C.
If Assertion is true but Reason is false.

Bohr's orbits are called as stationary states because as we know that if an object moves in circular path it will lose energy but, according to bohr electrons which is moving in the circular path does not lose energy. If electrons lose energy they will fall into nucleus and get burnt.

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MCQ 41 Mark
In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
Assertion: Atomic number is equal to number of protons only in case of ions.
Reason: Atomic number is equal to number of protons or electrons even in an ion.
  • A
    Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
  • ✓
    The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
  • C
    Assertion is true but the Reason is false.
  • D
    The statement of the Assertion is false but the Reason is true.
Answer
Correct option: B.
The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
A neutral atom has the same number of protons and electrons.
The proton number is the atomic number of the element, while the electron number is the atomic number minus the charge.
A negatively-charged ion or anion has more electrons than protons.
Its nucleus also contains two neutrons. Since $2 + 2 = 4$, we know that the mass number of the helium atom is $4$.
Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons.
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MCQ 51 Mark
In the following questions, a statement of Assertion $(A)$ is followed by a statement of Reason $(R)$. Mark the correct choice as:
Assertion: According to Bohr's model, the orbits of an atom are also called energy levels.
Reason: Orbits are the spheres containing definite value of energy.
  • ✓
    If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  • B
    If both Assertion and Reason are true but Reason is not the correct explanationi of Assertion.
  • C
    If Assertion is true but Reason is false.
  • D
    If Assertion is false but Reason is true.
Answer
Correct option: A.
If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Electrons in atoms orbit the nucleus. The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the “stationary orbits”) at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels.
These orbits are stable and called "stationary" orbits. Electrons in these orbits do not radiate their energy away. Each orbit is associated with a definite value of the energy and the angular momentum.
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Assertion (A) & Reason (B) MCQ - Science STD 9 Questions - Vidyadip