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Structure of the Atom — Science STD 9 — Question

Gujarat BoardEnglish MediumSTD 9ScienceStructure of the Atom1 Mark
MCQ
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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