What is the name of the isotope produced when Californium-251 emits an alpha particle?

Curium-247 i. e. ²⁴⁷₉₆Cm

Explanation:

Alpha decay is given by following general equation,

ᵃₓA → ⁴₂He + ᵃ⁻⁴ₓ₋₂B

Where;

A = Parent Isotope

B = Daughter Isotope

ᵃ = Mass Number

ₓ = Atomic Number

Californium-251 is the parent isotope in our case and it has 98 protons (atomic number) and is given as,

²⁵¹₉₈Cf

The alpha decay reaction of Californium-251 will be as,

²⁵¹₉₈Cf → ⁴₂He + ²⁴⁷₉₆B

The symbol for B with atomic number 96 was found to be the atom of Curium (Cm) by inspecting periodic table. Hence, the final equation is as follow,

²⁵¹₉₈Cf → ⁴₂He + ²⁴⁷₉₆Cm