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QUESTION

# [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