Question

A nucleus emits a one-alpha particle and forms a nucleus. The mass and charge of the new nucleus are changed in which of the following manners?
A. Mass is reduced by 2 and charge reduced by 2
B. Mass is reduced by 4 and charge reduced by 2
C. Mass is reduced by 4 and charge reduced by 2
D. Mass is reduced by 2 and charge reduced by 4

Answer 1

Hint: Before we answer this question, the definitions of mass number and atomic number are important.
The atomic number is equal to the number of protons in the nucleus of an atom. It is represented by Z.
The mass number is equal to the number of protons and neutrons of an atom. It is represented by A.
In order to understand the change in the nucleus of an atom during a radioactive emission, it is important to understand the kind of particles or emission that emerges out of the nucleus when it is a radioactive element.

Complete step by step answer:
The phenomenon of radioactivity was discovered in the year 1896 by a French scientist by name Henri Becquerel when he was working with phosphorescent materials. These materials glowed in the dark when exposed to light.
He thought that the glow produced in cathode ray tubes by X-rays could be associated with phosphorescence. So, he wrapped a photographic plate in black paper and placed different phosphorescent salts on it. All the results turned out to be negative until he used the uranium salts. which caused the blackening of the plate in spite of the plate being wrapped in black paper. These radiations were given the name Becquerel Rays.
However, in 1913, the two scientists by name, Frederick Soddy and Kazimierz Fajans arrived at a law known as the Radioactive displacement law of Fajans and Soddy, which dealt with the change in the structure, which they called as a transmutation of huge radioactive elements into different smaller elements in the field of radioactivity.
The law introduces two important kinds of decay, known as the alpha decay and the beta decay.
i) Alpha decay: In alpha decay, the heavier nuclear splits into 2 lighter nuclei with atomic mass reduced by 2, and the mass number reduced by 4.
ii) Beta decay: In beta decay, the mass number does not change but the atomic number increases by one.
If the radioactive element X consists of atomic mass A and mass number Z, if it undergoes
i) Alpha decay:
$X_Z^A \to Y_{Z - 2}^{A - 4} + \alpha _2^4$
ii) Beta decay:
$X_Z^A \to M_{Z + 1}^A + \beta _1^0$
As you can see, in alpha decay, there is an alpha particle emitter and the atomic number reduces by 2 and the mass number reduces by 4. The reduction in atomic number by 2 results in a reduction of charge by 2 and the reduction of the mass number represents a reduction in mass.

Hence, the correct option is Option C.

Note:
There is another kind of radioactive decay known as Gamma decay. In this kind of decay, there is no change in the structure of the nucleus like reduction or addition of nuclear particles. However, there is a change in the energy state of the nucleus and the nucleus emits this energy in the gamma radiation of the electromagnetic spectrum.
${X^ * } \to X + \gamma$