Assertion $(A)$: Atoms with completely filled and half-filled subshells are stable.
Reason $(R)$: Completely filled and half-filled subshells have symmetrical distribution of electrons and have maximum exchange energy.
The correct answer is:

  • A
    $(A)$ and $(R)$ are correct, $(R)$ is the correct explanation of $(A)$
  • B
    $(A)$ and $(R)$ are correct, $(R)$ is not the correct explanation of $(A)$
  • C
    $(A)$ is correct, but $(R)$ is not correct
  • D
    $(A)$ is not correct, but $(R)$ is correct

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$(a)$ How many sub-shells are associated with $n=4?$
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