Hydrogen isotopes: how many neutrons are present in the nucleus of tritium (written as 1H3)?

Introduction / Context:
Isotope notation quickly conveys the composition of a nucleus. Hydrogen’s three common isotopes—protium, deuterium, and tritium—are classic examples for counting protons and neutrons and practicing the relation between mass number and atomic number.

Given Data / Assumptions:

• Tritium symbol: 1H3 (sometimes written as H-3).
• Atomic number Z for hydrogen = 1 (protons).
• Mass number A = 3 (protons + neutrons).

Concept / Approach:
For any nuclide, neutrons N = A − Z. For tritium, A = 3 and Z = 1, so N = 3 − 1 = 2. Protium (1H1) has 0 neutrons; deuterium (1H2) has 1 neutron; tritium (1H3) has 2 neutrons. This counting is fundamental to nuclear reaction bookkeeping and decay chain analysis.

Step-by-Step Solution:
Write N = A − Z.Substitute A = 3 and Z = 1.Compute N = 3 − 1 = 2.Select “2.”

Verification / Alternative check:
Textbook tables for hydrogen isotopes confirm the neutron counts: 0, 1, and 2 for protium, deuterium, tritium respectively.

Why Other Options Are Wrong:
1 / 3 / 5: Incorrect arithmetic from N = A − Z for 1H3.0: This would correspond to protium, not tritium.

Common Pitfalls:
Mixing up mass number with atomic number or forgetting that the hydrogen symbol “H” always has Z = 1 regardless of isotope.

Final Answer:
2