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.

Answer 2

Aluminum-25 has 13 protons and 12 neutrons. To determine the number of valence electrons, we need to look at the atomic number, which is 13 for aluminum. Since aluminum is in group 13 (or group IIIA) in the periodic table, it has 3 valence electrons.