what is the difference between the number of electrons in an atom of selenium and the number of electrons in an atom of aluminum

The atomic number of Selenium is 34. This means that Selenium possesses 34 electrons.

The atomic number of Aluminium is 13. This means that Aluminium has 13 electrons.

Hence, there is a difference of 21 between the number of electrons in an atom of selenium and the number of electrons in an atom of aluminium.

Selenium has 6 electrons in it's outer most shell whereas aluminium has 3 electrons in its outer most shell. As a result, aluminium will have a greater tendency to lose one of its outer most electrons to become stable.