What is X in the equation given below? 42He + 94Be → 126C + X

Analyze the question: This is a nuclear reaction. We need to find the particle X that balances the equation. The two conserved quantities are mass number (top number) and atomic number (bottom number).

Concept: In any nuclear reaction:

• Sum of mass numbers on the left = sum of mass numbers on the right
• Sum of atomic numbers on the left = sum of atomic numbers on the right

Solution:

Equation: ⁴₂He + ⁹₄Be → ¹²₆C + ᴬ_ZX

1. Mass number balance: 4 + 9 = 12 + A 13 = 12 + A A = 1

2. Atomic number balance: 2 + 4 = 6 + Z 6 = 6 + Z Z = 0

So X has mass number 1 and atomic number 0, which is ¹₀n, a neutron.

Checking the other options:

• Electron: mass number about 0, charge -1 (⁰₋₁e), which does not fit
• Hydrogen/Proton: ¹₁H has atomic number 1, not 0
• Lithium: much heavier (mass number 6 or 7, atomic number 3)

Tip: When balancing nuclear equations, add up the top numbers and the bottom numbers separately on each side. A particle with mass number 1 and charge 0 is always a neutron. This particular reaction (alpha particle on beryllium-9) is the famous one by which Chadwick discovered the neutron in 1932, so it is worth remembering!