Chapter U1.15, Problem 4E

Interpretation Introduction

Interpretation:

The reason for change in the mass of an atom when an alpha particle is emittedis to be explained.

Concept introduction:

Elements are found in one or more isotopic forms. Some isotopes have stable nuclei while some have unstable nuclei. Stability of nucleus determines the stability of an isotope. Unstable nucleus will undergo radioactive decay.

Answer to Problem 4E

When an alpha particle is emitted from an atom, mass number of the atom decreases by 4 and atomic number decreases by 2. Since alpha particle has 2 protons and 2 neutrons, the mass of the atom decreases.

Explanation of Solution

Radioactive isotopes are unstable. To become stable they eject a particle from the nucleus. The process of ejecting particles from the nucleus of an atom is called radioactive decay. Various kinds of subatomic particles are emitted such as alpha particles and beta particles.

When an alpha particle is ejected, the process is called alpha decay. An alpha particle consists of 2 protons and 2 neutrons. It is the same as the nucleus of a helium atom. Hence it can be represented by ${}_2^4\text{He}$.

When an atom such as uranium undergoes alpha decay, an atom of new element thorium is produced and an alpha particle is produced. New element thorium has 2 protons and 2 neutrons less than uranium. Hence the mass number of uranium decreases by 4 (2 protons and 2 neutrons).

The reaction can be written as:

${}_{92}^{238}\text{U}{\to }_{90}^{234}\text{Th}{+}_2^4\text{He}$

When an alpha particle is emitted, mass of the atom also changes.

Conclusion

When an alpha particle is emitted from an atom, mass number of the atom decreases by 4 and atomic number decreases by 2. Mass of an atom is concentrated mainly in nucleus. When mass number of an atom changes, its atomic mass also changes.