How many electrons are in the nucleus of an aluminum atom?

13.

Aluminium-27 is an isotope of aluminium characterized by the fact that is has a mass number equal to  27 .

Now, an atom's mass number tells you the total number of protons and of neutrons that atom has in its nucleus. Since you're dealing with an isotope of aluminium, it follows that this atom must have the exact same number of protons in its nucleus.

The number of protons an atom has in its nucleus is given by the atomic number. A quick looks in the periodic table will show that aluminium has an atomic number equal to  13 .

This means that any atom that is an isotope of aluminium will have  13

protons in its nucleus.

Since you're dealing with a neutral atom, the number of electrons that surround the nucleus must be equal to the number of protons found in the nucleus.

Therefore, the aluminium-27 isotope will have  13  electrons surrounding its nucleus.

Finally, use the known mass number to determine how many neutrons you have  mass number  =  no. of protons  +  no. of neutrons

no. of neutrons  =  27  −  13  =  14