The mass number of an atom is 31. If the atom has 5 electrons in M-shell, calculate the number of neutrons.

Explanation

To solve the problem, we need to follow these steps:

### Step 1: Understand the Mass Number
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. In this case, the mass number is given as 31.

### Step 2: Determine the Number of Electrons
The problem states that the atom has 5 electrons in the M-shell. The M-shell can hold a maximum of 18 electrons, but we are only concerned with the number of electrons in this case, which is 5.

...