When an alpha particle is emitted from an unstable nucleus, the atomic mass numberof the nucleus
9)

a) increases by 4.
b) decreases by 2.
c) increases by 2.
d) decreases by 4.

The correct option is D.

Explanation:

Radioactive substances usually emit different types of particles when they are decaying. Such particles include alpha particles, beta particles and gamma ray. When an alpha particle is emitted from an unstable radioactive nucleus such nucleus usually lost an atomic mass that correspond to that of helium atom. Note that an alpha particle is made up of two protons and two neutrons, which result in mass number of 4. Thus, a nucleus that emit an alpha particle will have its mass number (atomic mass) reduce by 4 and atomic number that is reduced by 2.  

metric time was an attempt to create a standard unit of time based off of 10, but it never took off an is not included in the metric system (si)