Answer:
21 electrons
Step-by-step explanation:
The number of electrons in an atom is equal to the atomic number of the element. Atomic number of aluminium is 13 and atomic number of selenium is 34. That means electrons of aluminium are 13 and electrons of selenium are 34. Thus, difference between number of electrons of these atoms is:
34 - 13 = 21 electrons
I hope it helps!