An aluminum isotope consists of 13 protons, 13 electrons, and 14 neutrons. its mass number is

In an atom, protons and neutrons have a relative mass of 1 whilst electrons have a relative mass of 0. So, you must add the number of protons and neutrons in the isotope together to find the mass number:

13+14=27

The mass number of the aluminium isotope is 27.