What ion is Aluminum likely to form?

[forgetful]

Homework Statement

What ion is aluminum likely to form?

Relevant Equations

N/A

A self test question from Chemical Principles(Atkins/Jones).
Other than rote memorization I don’t see how to do this. The review section references the Periodic Table and, for elements on the right side of the table, one can use 18 minus the group number which would give Al(superscript 5+) since Al is a group 13; Which is of course wrong. The answer is shown as 3+. What am I missing? Thanks for any help.

 

[Borek]

What is the electron configuration of Al? How many valence electrons?

Group numbers work only for a "short" periodic table and fail for most d-block and f-block elements.

 

[forgetful]

Electron configuration? I think it’s $1s^22s^22p^63s^23p^1$
Valence electrons? At first I would have said 1 since there is an incomplete p orbital but reading ahead it seems that it would be all electrons in excess of the nearest noble gas as one moves to the left and that would be 3(and 7 for Cl but that seems high). If what I just wrote is correct would it be true that the number of valence electrons is also the number of ions that a element can have? Thanks for your replies.

 

[Borek]

Al³⁺ it is.

For Cl things go in the other direction - the most stable configuration is again the noble gas configuration, but the nearest one is not the one with 7 electrons removed, but the one with an electron added, so Cl^-.

Technically ions like Al⁺, Al²⁺ (and even Al⁴⁺ and so on) are all possible till you strip all electrons and leave just a bare nucleus, but in typical situation (typical for chemistry, which means presence of other substances and temperatures/pressures not much different from these we live in) Al³⁺ is the most stable one.

 

[forgetful]

Thank you

 

[dshoup]

The Al ion has a +3 charge on it. This is because the Al ion has a neon electron configuration 1s22s22p6. The noble gas neon has a stable electron configuration. All the n=2 orbitals are filled.