Writing Binary Ionic Formulae From Transition Metal Ions

I’ve stayed away from certain areas.
One of these areas is the transition metal area. We’ve been doing
main group ions.
We do have a couple of ions you can make from iron. One of them is
called the ferrous ion and the other is called the ferric ion. This is called
the classic system. So while the “ferrous” and the “ferric” tell you
it’s iron (it’s latin). It doesn’t tell you what the charge is.
As it turns out, the ferrous ion has a 2+ and the
ferric ion has a 3+ charge. The classic system was developed
to handle what we call multivalent ions. Iron is a multivalent ion.
You can have 2+ ions. You can have 3+ ions.
It’s not like sodium. When sodium forms an ions, it’s 1+.
So the classic system helps dispel that doubt. OK, well you can see
both the ferrous and the ferric ions have different charges.
But, how do you know what the charge is? Perhaps the “ous” is ferrous
means 2+?
Let’s look at the stannous and the stannic ions. Stannous, tin,
is 2+. So maybe that holds?
Stannic... oops... stannic is 4+ not 3+.
The “ous” means a smaller charge than the “ic”.

We’d like to use something better so we actually know what the
charge is instead of memorizing a bunch of charges. So, we are going
to use the Stock system. It was introduced a few decades ago and
uses Roman numerals to explicitly state the charge on the ion.
So, ferrous... how would we name that?
It’s iron, so we name it the iron(II) ion.
That’s because the “II” is the charge on it, 2+.
Ferric... that becomes iron(III).
Stannous becomes tin(II) and stannic becomes tin(IV).
Now tin is not a transition metal. I realize that.
Remember that I stayed away
also from column 4a. We did 1, 2, and 3a cations.
5, 6, and 7a, anions. We ignored 8a because those are the
noble gases. But we didn’t do anything about 4a. This is what we are
going to do. This is how you are going to tell when you use
the Stock number. You are going to look at the cation. OK, first, it’s a
cation... alright... it’s a metal. You are going to ask yourself if it’s a
transition metal, tin, or lead? Tin and lead are in column 4a.
So the nonmetals in column 4a tend to form molecular substances.

And then tin and lead form ionic substances.
So, if you have an element and it’s one of those three, then
you’re going to use a Stock number on it. With these
three exceptions. There are many other exceptions in the
transition metal area. I’m only going to hold you responsible for
these three. Silver ion has a 1+ charge. Cadmium and zinc both
have 2+ charges. They are not multivalent.
Now, once you commit to memory that silver, cadmium, and zinc
have charges that you have to remember, then you can use the
periodic table. Silver is in 1b: 1+. Cadmium and zinc are both in 2b: 2+.
Let’s put this into practice.
Stock number- we are only going to use it with cations.
And only cations in cases where they are multivalent.
You can use the Stock number for much more than what we are
doing here, but it’s not commonly done that way anymore. It’s more
common to do it this way. In other words, just use it for the
multivalent cations. So, lead(IV) ion. You know the charge- it’s a 4.
And it’s a metal, it’s lead. You know it’s 4+. Pb4+.
Mercury(II)... you know the charge on that.
It is 2+. So it’s Hg2+.
What if you go to the name. I see Mn3+.
You ask yourself if it’s a transition metal, is it tin, is it lead?
It’s a transition metal.
Is it one of the three- silver, cadmium, or zinc? No, it it’s not.
Therefore, you must use a Stock number. So we’ll call it
the manganese(III) ion.
Similar deal- copper. Is it a transition metal, tin, or lead?
Yes, it is a transition metal.
So use the Stock number: copper(I) ion.

So let’s write some names out. Mercury(II) iodide.
You like it when you look at the name because
you know the ions. Mercury(II), Hg2+.
Remember, mercury is “Hg” not “Mg”. “Mg” is magnesium.
Don’t mix them up!
Iodide is 1- because it is in column 7a.
Now yo do your charge balancing.
The least common multiple of 2 and 1 is 2 so you take
One mercury(II) ion and 2 iodide ions(I).
Silver nitride!
Silver is one you have to memorize.
Silver is 1+. Nitride 3-.
Because it’s in column 5a.
Now do your charge balancing. What is the least common multiple
of one and three. It’s three. So Ag3N.
Three silver ions... one nitride.
Chromium(III) sulfide... chromium(III)..
You know the charge, it’s Cr3+.
Sulfide- it’s in column 6a so it’s 2-.
The last common multiple of “3” and “2” is six so you need
two chromiums and three sulfides.
The Stock number is only put into the name. The Stock number
is not put into the chemical formula.
Mercury(I) chloride.
Mercury(I)... Hg1+.

Chloride... 7a... Cl1-. Then if you were to look at
a chemical reagent bottle for it, you’d see that the chemical formula
is Hg2Cl2. Oh oh!
Something is wrong! Either the name is wrong
or the formula for the ion is wrong. Normally you would take the
Hg2Cl2 and reduce it down to HgCl.
Actually, mercury(I) is one you have to commit to memory.
The mercury(I) ion is Hg22+

It’s believed that the mercury(II) ion, at least the little bit that I’ve read,
when it dissociates, it dissociates into a mercury atom and a
mercury(II) ion. So it’s two mercury atoms stuck together
and the net charge on the whole thing is 2+.
That’s the only metal-metal thing we are going to be working with.
So mercury(I) is one you have to memorize.