I understanding how to read and write chemical formulas

[TWalker]

I am in a Chem 105 course. I don't have a homework assignment but we do have a test coming up and I have some worksheets to help me practice writing formulas. My question is how do you know if the formula has a charge. For example: Sodium Nitrate - I know Sodium is Na and I know Nitrate is a form of Nitrogen and its symbol is Na - so, would the formula be SNa?

 

[gendou2]

It should go without saying that you can look this information up on the internet:

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

I should add that a nitrate molecule has a Nitrogen (N) atom with 3 Oxygen (O) atoms bound to it. This ion has a negative charge, so it binds to positively charged ions, like Sodium (Na).

 

[TWalker]

I have actually been on the internet as well as in 3 different chemistry books and I am still struggling with understanding the material. Thanks. Does the distinguishing electrons have anything to do with the writing of formulas?

 

[gendou2]

In chemistry, one need not sweat the subatomic details.
This is because most of the rules of chemistry were written before the atom's structure was fully understood.
The chemical formula notation is not very complex.
Different atoms have different abbreviations, which can be seen on the periodic table.
These abbreviations have either one or two letters, where the first is capitalized.
A chemical formula is a string of these abbreviations with not spaces.
When more than one of a given atom is present in a chemical, the number of times this atom is present is placed as a _subscript just after the atom's abbreviation.
When the formula for a chemical ion is written, the positive or negative charge is written as a number in ^superscript at the end of the formula.

In chemistry, there are names for certain groupings which commonly appear in notable chemicals. The aforementioned nitrate (NO₃^-) is one such example. Notice how a the nitrate ion has a negative (-) charge of magnitude one, two different atoms, one nitrogen (N) and three oxygen (O).

 

[TWalker]

I have actually been on the internet as well as in 3 different chemistry books and I am still struggling with understanding the material. Thanks. Does the distinguishing electrons have anything to do with the writing of formulas?

Maybe I am not asking my question right. O.K. the two columns are s with a positive charge of 1+ or 2+. The 6 columns are p with a negative charge of 1-,2-,3- so, do these charges along with the number help to form the formula? I really apreciate any help with this!

 

[TWalker]

O.K. Here's a example off a worksheet: silver chloride = AgCl
silver oxide= AgO- Is this correct?

 

[gendou2]

Both have neutral charge.

Please search for yourself next time:

Silver Chloride: AgCl -- http://en.wikipedia.org/wiki/Silver_chloride
Silver Oxide: Ag₂O -- http://en.wikipedia.org/wiki/Silver_oxide

 

[Redbelly98]

O.K. Here's a example off a worksheet: silver chloride = AgCl
silver oxide= AgO- Is this correct?

You need to know the charge states of the elements involved. There should be a table in your textbook showing this. Also, we want the final charge state of the compound to be zero.


From my textbook:

Ag has a charge +1
Cl has a charge -1

So the charge on AgCl is +1 -1 = 0, and that is a correct formula

Oxygen has a charge of -2.
So AgO has a charge of +1 -2 = -1, as you have figured out.

However, that net charge should be zero for a correct chemical formula.

To get zero net charge, we combine two Ag's with one O:

Ag₂O
Charge = 2⋅(+1) + 1⋅(-2) = 2-2 = 0

So Ag₂O is the correct formula.

 

[Borek]

It is not necesarilly charge that is responsible for the final formula. Ionic compounds are made of charged ions, like sodium chloride - NaCl made of two ions, one charged positive (Na^{+[/sup) and one charged negative (Cl-). Other compounds - like carbon dioxide - are not ionic, and to find out their formulas you should use valence of elements (see for example http://en.wikipedia.org/wiki/Valence_(chemistry ) for explanation).}

 

[Redbelly98]

That's true, with many compounds one does not use charge balance to determine the formula. But I had the impression that TWalker's class is working with ionic compounds right now. If so, then charge would be the way to do it.

 

[Borek]

That's true, with many compounds one does not use charge balance to determine the formula. But I had the impression that TWalker's class is working with ionic compounds right now. If so, then charge would be the way to do it.

That's right, it just have to be charge of cations and anions on the whole - so in the case of composite anion like nitrate you just have to remember that sulfate is SO₄^2- - one sulfur atom, four oxygen atoms, total charge of -2. In the case of chlorides situation is simpler, as it is just Cl^-, so one atom and single negative charge.

Cations are much easier, with the exception of NH₄⁺ almost all cations will be just a single atom (although you have to know the charge). There are more complicated cations (like uranyl, UO₂⁺, for example), but they are rarely present in basic syllabi.