An isotope of Uranium, Z = 92 and A = 235, decays by emitting an alpha particle. Calculate the number of neutrons in the nucleus left behind after the radioactive decay.

141

Explanation:

The atomic number (Z) corresponds to the number of protons:

Z = p

while the mass number (A) corresponds to the number of protons+neutrons:

A = p + n

So the number of neutrons in a nucleus is equal to the difference between mass number and atomic number:

n = A - Z

For the initial nucleus of Uranium, Z = 92 and A = 235, so the initial number of neutrons is

n = 235 - 92 = 143

An alpha particle carries 2 protons and 2 neutrons: so, when the isotope of Uranium emits an alpha particle, it loses 2 neutrons. Therefore, the number of neutrons after the decay will be

n = 143 - 2 = 141