##[Cr(NH_3)_6][(NO_3)_3]## Number of elements? Number of atoms per element? Total number of atoms in compound?

Here's what I got.

You're actually dealing with a coordination compound called hexamminechromium(III) nitrate, which consists of the hexaamminechromium(III) complex ion, ##"Cr"("NH"_3)_6^(3+)##, and the nitrate anion, ##"NO"_3^(-)##.

More specifically, the coordination compound consists of one hexamminechromium(III) complex ion and three nitrate anions.

So, how many are a part of this compound? To figure that out, break up the complex ion and the nitrate anion into their respective constituent elements.

Notice that ##6## subscript that follows the ammonia ligands. This means that the complex ion contains six ammonia molecules, ##"NH"_3##, so make sure that you keep track of all these atoms.

You can thus say that ##"Cr"("NH"_3)_6^(3+)## contains

• one chromium atom, ##"Cr"##
• six nitrogen atoms, ##"N"##
• eighteen hydrogen atoms, ##"H"##

Now focus on the nitrate anion. Notice that it has a ##3## subscript, so keep that in mind when listing the number of atoms it contains.

You can say that three nitrate anions, ##3 xx"NO"_3^(-)##, will contain

• three nitrogen atoms, ##"N"##
• nine oxygen atoms, ##"O"##

So, this coordination compound contains four distinct elements

• chromium
• hydrogen
• nitrogen
• oxygen

Add up all the atoms we've listed above to get the total number of atoms per element

• chromium: ##1 ->## from the complex ion
• hydrogen: ##18 ->## all from the complex ion
• nitrogen: ##9 ->## ##6## from the complex ion and ##3## from the three nitrate anions
• oxygen: ##9 ->## all from the nitrate anions

The total number of atoms present in the compound will be

##"no. of atoms" = 1 + 18 + 9 + 9 = 37##