Examine the following isotopes of silicon (si).
silicon-28 (28si)
silicon-29 (29si)
silicon-30 (30si)
what is true about these isotopes?
they all have the same number of protons and neutrons.
silicon-28 and silicon-29 have the same number of protons and neutrons, but silicon-30 has a different number of neutrons.
they all have the same number of protons but different numbers of neutrons.
silicon-28 and silicon-30 have the same number of protons and neutrons, but silicon-29 has a different number of neutrons.

They all have the same number of protons but different numbers of neutrons.

Explanation:

These are called isotopes. Isotopy is the existence of two or more atoms of the same element having the same atomic number but different mass numbers due to the differences in the number of neutrons in their various nuclei.

The atomic number is the number of protons in an atom. For a neutral atom, it is the same as the number of electrons.

The mass number is the number of protons and neutrons in an atom.

silicon-28 (28Si)

silicon-29 (29Si)

silicon-30 (30Si)

All of these isotopes have an atomic number of 16

Now let us chech for the neutrons:

 Number of neutrons = mass number - atomic number:

for silicon-28 (28Si)  : 28-16 = 12 neutrons

     silicon-29 (29Si) :   29-16 = 13 neutrons

     silicon-30 (30Si):    20-16 = 14 neutrons.