I was having trouble with this too, but i found a website that helped and Ill do my best to explain it in a way that will help.
heres the link: https://www.khanacademy.org/science/che ... -compounds
When trying to find the chemical formula based on the monatomic ions, we first want to find the charge of each ion (or amount of electrons that must be removed or added in order to reach the next noble gas electron configuration. After this, we want to find a ratio of each element that will neutralize the electron charges in order to form a neutral compound.
So for 3.25, A) we are asked to find the chemical formula for magnesium arsenide.
1) find amount of valence electrons and what type of ion it is
-Magnesium --> 3s^2, meaning we have 2 valence electrons. --> Cation
-Arsenic --> 4s^2 4p^3, meaning we have 5 valence electrons --> Anion
2) Find the charge of each ion (the # of electrons to be lost/gained to reach a noble gas configuration)
-Magnesium -->Mg^2+ (because we have 2 valence electrons and Mg is a Cation, Mg must lose its 2 valence electrons to reach the configuration of
Neon, giving it a positive charge as we are losing electrons)
-Arsenic --> As^3- (because we have 5 valence electrons and As is an Anion, As must gain 3 electrons to reach the noble gas configuration of
Krypton, giving it a negative charge as we are gaining electrons)
3) find a ratio that will create a neutral compound
- we have Mg^2+ and As^3-
-in order to make this a neutral compound, we need a ratio that will make both charges equal so their charges will be equal and cancel out.
Mg^2+ (*3) = 6+
As^3- (*2) = 6-
Therefore, the chemical equation is Mg3As2
I hope this helped!!!