Q1(i). Iodine beta decay

Answer in brief

Iodine-133 is neutron rich and undergoes β⁻ decay; iodine-121 is proton rich and can lower its proton/neutron ratio through positron emission.

Key points

• For ¹³³I, N=133-53=80. In β⁻ decay, n→ p+e⁻+ν̄ₑ: ¹³³₅₃I → ¹³³₅₄Xe+e⁻+ν̄ₑ.
• For ¹²¹I, N=68. Positron emission converts p→ n+e⁺+νₑ: ¹²¹₅₃I → ¹²¹₅₂Te+e⁺+νₑ.
• Both preserve mass number and move towards a more favourable neutron/proton balance. Electron capture can compete for proton-rich nuclei; energetics determines the allowed channels.