Chemistry Atomic Structure NUMS 2022
PMDC Verified Question 65 of 95
Proton number of \( _{11}\text{Na}^{23} \) is:
A
\( 23 \)
B
\( 12 \)
C
\( 11 \)
D
\( 34 \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( 11 \)
Concept: The standard atomic symbol notation \( ^A_Z\text{X} \) clearly defines both the mass number and the atomic number of an element.

Formula: $$ \text{Proton Number} = Z $$

Solution:
  • In the notation \( _{11}\text{Na}^{23} \), the lower left subscript is the atomic number (\( Z \)).
  • The atomic number is exactly equal to the number of protons in the nucleus.
  • Therefore, the proton number is 11.


Why other options are incorrect: 23 is the mass number (protons + neutrons). 12 is the number of neutrons (\( 23 - 11 \)). 34 is the sum of the mass and atomic numbers, which has no physical meaning.

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