The helium isotope $^3_2He$

Analyze the question: We're asked about the make-up of the nucleus of the helium isotope written as ³₂He.

Concept: In nuclear notation ᴬ_ZX:

• Z (the lower number) is the atomic number, which equals the number of protons.
• A (the upper number) is the mass number, which equals protons + neutrons.
• Number of neutrons = A − Z.

Solution:

• Protons = Z = 2 (this is always true for helium)
• Mass number A = 3
• Neutrons = A − Z = 3 − 2 = 1

So ³₂He has 2 protons and 1 neutron.

Checking the options:

• A. 3 protons and 2 neutrons: wrong, since the proton count would make it lithium.
• B. 2 protons and 1 neutron: correct.
• C. 2 protons and 3 neutrons: this would give A = 5, not 3.
• D. 1 proton and 2 neutrons: this describes tritium (³₁H), not helium.
• E. Wrong. Tritium has the same mass number (3), but a different atomic number (1). Isotopes and isobars like this can coexist, and ³₂He is a real, stable isotope (helium-3).

Tip: Remember "Z = protons, A − Z = neutrons." The element's name fixes the number of protons, so for helium, protons are always 2. Just subtract to find the neutrons.