Basic compound question...

[24fan]

So I'm still VERY new to chemistry, and right now I'm just focusing on different general compounds. However, I still don't quite understand WHY exactly you put different numbers after some elements. Here are just some examples...can someone PLEASE explain to my why these numbers go where they do, and HOW I FIND THEM?

 

Examples:

Silver Carbonate - AgCO3 -- I don't understand the 3

Calcium Nitride - Ca3N2 -- I don't understand the 3 and 2

Aluminum Sulfide - Al2S3 -- I don't understand the 2 and 3

 

Thanks.

[bob000555]

short answer:

A compound is a combination of elements, the number tells how many atoms of the element before the number are in the compound.

http://en.wikipedia.org/wiki/Chemical_formula

 

long answer:

There is something called the octet rule which says atoms “try” to get a full outer shell of electrons. The compounds you gave are all ionic compounds (with the exception of the CO3 which is a polyatomic) which means they try to get the shell by donating or accepting electrons depending on weather they are a metal or nonmetal respectively.

 

Take salt ,NaCl, sodium chloride, for example Na wants to give one electron and chlorine wants to take one, so sodium gives one electron to chlorine and the two become bound by electrostatic forces.

 

But for calcium chloride we need CaCl2 because calcium wants to lose two electrons so it need two atoms of chlorine to accept them.

Edited January 22, 2009 by bob000555

[Cap'n Refsmmat]

To make that clearer:

 

I'm sure you'll agree that atoms usually have the same number of protons as electrons, and the number of protons is given as the atomic number. So we can look at an element and see how many electrons it has fairly easily.

 

As an element gets more electrons, they come in "shells" -- first a layer of two, then successive layers of eight. The element would like to have its outermost shell entirely full.

 

So look at fluorine, for example. It has two electrons in its inner shell and seven in the outer shell. It'd like to gain one electron to have eight.

 

Then look at magnesium. It has two, eight, and then two more in the outer shell. It could try to gain six, but it's easier for it to just lose the two in its outer shell, thus "exposing" the next (full) shell down.

 

If magnesium and fluorine were to bond, there's a problem: magnesium would like to give away two electrons, and fluorine would only like to gain one. So magnesium goes and finds two fluorine atoms and bonds with them. Hence [ce]MgF2[/ce].

 

This method will work for any compound where one atom wants to gain and the other wants to lose (and they have a certain difference in their electronegativity, but you'll learn about that later).

 

CO₃ is what's called a "polyatomic ion" and is merely a "gang" of atoms that would collectively like to gain two electrons.