how to do Chemical Formulas?

I need help in trying to figure out how chemical formulas are written? I don't understand how or why.

Like these for example, Give the chemical formula for ammonium sulfide and they get answer (NH4)2S

Give the chemical formula for a salt comprised of aluminium bromide? AlBr3

I don't get how in one there's parentheses and the other no parentheses.

4 Answers

  • 9 years ago
    Favorite Answer

    It's a fairly long explanation, if you want to be complete:

  • Grover
    Lv 7
    9 years ago

    An inorganic compound is most often a +ion attracted to a -ion.

    The ions can consist of a single type of atom (monatomic) or

    a group of atoms covalently bonded together (a polyatomic ion).

    The ammonium ion (NH4^1+) is one of just a few positive polyatomic ions.

    There are many negative polyatomic ions including

    carbonate (CO3^2-)

    nitrate (NO3^1-)

    phosphate (PO4^3-)

    sulfate (SO4^2-)

    chlorate (ClO3^1-)

    You need parentheses when a formula requires more than one copy of a polyatomic ion to make the overall charge equal zero.

    In the ammonium sulfide example, you need 2 x NH4^1+ to equal the charge on the S^2-.

    (NH4)2 is notation is used to show that the subscript-two applies to the entire polyatomic ion and not just the H4.

    Sodium (Na^1+) nitrate (NO3^1-) = NaNO3.

    Calcium (Ca^2+) nitrate = Ca(NO3)2.

    By the way, if you memorize the 5 polyatomic ions listed above,

    you have the basis for knowing of figuring out

    5 IC acids and 4 OUS acids, 4 ITE polyatomic ions

    as well as other acids and ions based on family relationships in the periodic table

    (because you know chlorate, for example, you also know bromate, and iodate as well as the

    perchlorate, chlorite, hypochlorite ion and acid variations for those elements as well. That's 21 things you can deduce from knowing 1 formula)

  • 9 years ago

    in the first example of ammonium sulphide,ammonium radical has a valence of one while sulfur has a valence of two.therefore it's like they 'exchange.'(a radical is a molecule made up of more than one atom and react as a unit).this means that it will take two ammonium radicals for an atom of sulfur to react completely.we use parenthesis to show that its two radicals of ammonia that have reacted i.e.


    In the second example we follow the same principle.Br atom has a valence of one while Al has a valence of three.therefore when 'exchange' of valences occur we see that 3 Br atoms have reacted with 1 Al atom to form a compound AlBr3

  • Anonymous
    9 years ago

    (NH4)2S has parentheses because there are 2 ammonium groups for each sulphur. so there are 2 lots of NH4 to each S.

    AlBr3 is just bonded together Al to the 3 Br.

    hope this helps :)

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