When an element ejects an alpha particle, the mass number of that element A. reduce by 4.

B. remains unchanged.

C. increase by 1.

D. reduce by 1.

A.reduce by 4.

Explanation:

In Chemistry, an alpha particle which is typically a helium nucleus has an atomic mass (number of nucleons) of four (4). Therefore, the ejection of an alpha particle by any chemical element simply means that, the mass number of that particular chemical element would be reduced by four (4).

Basically, this process is referred to as an alpha decay and can be defined as a radioactive disintegration of a chemical element that causes it to emit an alpha particle and as a result reducing its mass number by four (4).

Hence, when an element ejects an alpha particle, the mass number of that element reduces by 4.