Question

the octet rule involves elements trying to get a full outer shell of electrons (outermost seats)
a true
b false

oxidation numbers are the numbers used when doing the \criss-cross\ method of forming compounds.
a true
b false

using the \criss-cross\ method and your oxidation number chart, choose the correct compound below
chart with oxidation numbers

aluminium (al) bonding with oxygen (o)
a alo
b al₃o₂
c al₂o₃
d al₁o₁

Explanation:

• First question: The octet rule states that atoms tend to gain, lose, or share electrons to have 8 electrons in their outer shell (except for some smaller atoms like hydrogen which aim for 2). So the statement about the octet rule is correct.
• Second question: Oxidation numbers are indeed used in the criss - cross method (also known as the valence - cross method) to determine the formula of ionic compounds. For example, if element A has an oxidation number of \(+x\) and element B has an oxidation number of \(-y\), the formula of the compound formed between A and B is \(A_yB_x\) (after simplifying if possible).
• Third question: Aluminium (\(Al\)) has an oxidation number of \(+ 3\) (\(Al^{3+}\)) and oxygen (\(O\)) has an oxidation number of \(-2\) (\(O^{2 -}\)). Using the criss - cross method, we swap the absolute values of the oxidation numbers. So we get \(Al_2O_3\) (since \(\frac{3}{1}\) for \(Al\) and \(\frac{2}{1}\) for \(O\), and we simplify the ratio \(3:2\) to get the formula where the sub - scripts are the smallest whole numbers that balance the charges: \(2\times(+3)+3\times(- 2)=6 - 6 = 0\)).

Answer:

1. a. TRUE
2. a. TRUE
3. c. \(Al_2O_3\)