For the nuclide ²¹₉X, find the number of protons and neutrons in its nucleus.

Solution & explanation

1. Given: Mass number A = 21; atomic number Z = 9.

2. Atomic number is the number of protons.

   $$p=Z=9$$

3. Mass number counts protons and neutrons together.

   $$A=n+p$$

4. Subtract the proton number from mass number.

   $$n=A-Z=21-9$$

5. Carry out the subtraction.

   $$n=12$$

6. State the result with units: 9 protons and 12 neutrons.

9 protons; 12 neutrons