Acertain 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 .

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:

The mass number of daughter atom is 228.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)