Nuclear Physics

Mass number decreased by 4, atomic number decreased by 2

Step 1: An alpha particle is a helium nucleus represented as ₂He⁴. It has mass number 4 and atomic number 2. Step 2: When a parent nucleus emits an alpha particle, it loses 4 units of mass number (2 protons + 2 neutrons). Step 3: It also loses 2 units of atomic number (2 protons). Step 4: Example: ₉₂U²³⁸ → ₉₀Th²³⁴ + ₂He⁴. Here, U has mass 238 → Th has mass 234 (decreased by 4), and atomic number 92 → 90 (decreased by 2). Step 5: For beta decay: mass number stays same, atomic number increases by 1. For gamma decay: both mass and atomic number remain unchanged.

16

Step 1: In beta decay, a beta particle (electron, ₋₁e⁰) is emitted. Step 2: According to Soddy and Fajan's displacement law for beta decay: mass number remains the same, but atomic number increases by 1. Step 3: Parent nucleus: ₁₅P³² — Atomic number = 15, Mass number = 32. Step 4: After beta decay: Atomic number = 15 + 1 = 16, Mass number = 32 (unchanged). Step 5: The daughter nucleus is ₁₆S³² (Sulphur-32). The element is identified by its atomic number, not mass number. So the correct answer is 16.

3

Step 1: In nuclear fission, mass number and atomic number must be conserved (balanced) on both sides. Step 2: Left side mass numbers: 235 (U) + 1 (neutron) = 236. Step 3: Right side mass numbers: 141 (Ba) + 92 (Kr) + X(1) = 233 + X. Step 4: Balancing: 236 = 233 + X → X = 3. Step 5: This means 3 neutrons are released in each fission event. This is what makes a chain reaction possible — each fission releases neutrons that can trigger further fissions. The average energy released per fission is about 3.2 × 10⁻¹¹ J.

In controlled reaction, only one neutron is maintained to cause further fission; in uncontrolled, neutrons multiply indefinitely

Step 1: In a chain reaction, neutrons released by fission trigger further fissions, which release more neutrons. Step 2: In a controlled chain reaction, extra neutrons are absorbed by control rods (made of boron or cadmium), leaving only ONE neutron to trigger the next fission. This keeps the reaction sustained but steady. Step 3: Controlled chain reactions are used in nuclear reactors to generate electricity safely. Step 4: In an uncontrolled chain reaction, all neutrons are free to cause further fissions — the number multiplies geometrically (1 → 3 → 9 → 27...). Step 5: This results in a hug