How many valence electrons are there is aluminum-25?

Answer 1

Number of valence electrons depends on number of electrons in uncharged atom(= atomic number), not on mass number. Al has atomic number = 13 and it belongs to group 13( III A) of Periodic Table so it's uncharged atom has 3 valence electrons. But valence electrons in ions is different.

No. of valence electrons in an ion = No. of valence electrons in uncharged atom

+ negative charge or - positive charge1.

1. for charges only their magnitude is taken i.e. if charge is +2 or -2 then 2 is taken and put in equation.