What do carbon 12 carbon 13 and carbon 14 have in common?

They have all the same number of protons and electrons (6)

Explanation:

Carbon 12, Carbon 13 and Carbon 14 are all isotopes of Carbon.

The definition of an isotope is: They have the same number of protons (and electrons), but a different number of neutrons. Different isotopes of the same element have different masses.

Carbon 12 means an isotope with mass 12 u

it has 6 protons, (and 6 electrons) and 6 neutrons : 6 neutrons + 6 protons = 12

Carbon 13 is an isotope with mass 13 u

It has the same amount of protons (and electrons) as Carbon 12, so 6 protons and 6 electrons

13 - 6 = 7 ⇒ Carbon 13 has 7 neutrons

Carbon 14 is an isotope with mass 14

it has the same of protons (and electrons) as Carbon 12 and Carbon 13, so 6 protons and 6 electrons

14 - 6 = 8 ⇒ Carbon 14 has 8 neutrons

We can conclude that Carbon 12, Carbon 13 and Carbon 14 are 3 isotopes of Carbon. They all have the same number of protons (and electrons) = 6.

This means the isotopes will also have the same atomic number, because they are all isotopes of the same element.