The nucleon and proton numbers of a neutral atom of an element are 238 and 92 respectively. Determine the number of neutrons in the atom.

Analyze the question You're given the nucleon number (mass number) A = 238 and the proton number (atomic number) Z = 92. You need the number of neutrons, N.

Concept The nucleon number is the total count of particles in the nucleus, which means protons plus neutrons:

A = Z + N

So the number of neutrons is:

N = A − Z

Solution

• A = 238
• Z = 92

N = 238 − 92 = 146

(The atom is neutral, so it has 92 electrons too, but electrons don't affect the neutron count. This is uranium-238.)

Checking the options

• A. 119: not equal to 146.
• B. 330: this is 238 + 92, so it comes from adding instead of subtracting.
• C. 165: not equal to 146.
• D. 146: matches our calculation.

Correct answer: D (146)

Tip: Remember "Neutrons = Nucleon number − Number of protons". Both start with N-words that connect: Nucleon minus Proton number gives Neutrons. Also, the word "neutral" in the question is a distractor, since it only tells you electrons equal protons.