Question

A certain radioactive atom has 90 protons and 142 neutrons. If this atom undergoes alpha decay, what is the mass number of the daughter atom? The mass number of the daughter atom is .

Answer 1

Answer: the mass number of the daugther atom is 232,

Explanation:

1) Alpha (α) decay is a nuclear reaction in which a nucleus (parent's nucleus) emits an alpha (α) particle and leads to a different atom (daughter atom).

2) The alpha (α) particle is a nucleus of helium atom, i,e, a nucleus with two protons and two neutrons. The symbol used for the α particles is ⁴₂He, where the superscript 4 indicates the mass number (2 protons + 2 neutrons = mass number 4), and the subscript 2 indicates the atomic number (number of protons).

3) Then, to determine the mass number of the daughter atom you just need to do a mass number balance:

mass number of the parent atom = mass number of the daugther atom + mass number of the α particle.

The mass number of the radioactive (parent) atom is 90 protons + 142 neutrons = 232.

∴ 232 = x + 4 ⇒ x = 232 - 4 = 228 ← answer.

The full equation may help you to have a wider vision of the problem:

²³²₉₀ X → ⁴₂ He + ²²⁸₈₈ Y

Note this:

• 232 = 4 + 228 (this is a mass number balance)
• 90 = 2 + 88 (this is an atomic number balance)
• X is the parent atom, and Y is the daughter atom
• You can use a periodic table to determine the identity of the unknown atoms (using the atomic numbers).

Answer 2

Answer:

The mass number of daughter atom is 228.

Explanation:

• As the mass number is the total number of protons and neutrons in an atom, So initial mass number will be 90 + 142 = 232.
• As the alpha particles are helium nuclei having mass number of 4.
• So after eliminating alpha particles, the mass number will lower by 4.
• Mass number of daughter atom = 232 - 4 = 228