Question

How many O atoms are in the formula unit `Ba(NO_3)_2`?

Answer 1

`"6 atoms of oxygen"`

Explanation:

The idea here is that subscripts that follow a closed parenthesis are distributed to all the elements that are written inside the parentheses.

By comparison, subscripts that follow the chemical symbol of an element are distributed only to that element.

Barium nitrate, `"Ba"("NO"_3)_2`, is an ionic compound that consists of barium cations, `"Ba"^(2+)`, and nitrate anions, `"NO"_3^(-)`.

Take a look at the nitrate anion. Here you have a `3` subscript that follows the chemical symbol for oxygen, `"O"`. This means that the anion contains a total of

`"O"_color(red)(3) = color(red)(3) xx "O" = "3 atoms of oxygen"`

Now, the barium cations have a `2+` charge and the nitrate anions have a `1-` charge. This means that in order to have a neutral compound, two nitrate anions must be used to balance the overall positive charge coming from the barium cation.

Since the nitrate anion contains more than one element, you must enclose it in parentheses and add the `2` subscript

`color(blue)(2) xx "NO"_3^(-) = ("NO"_3)_color(blue)(2)`

Since you have two nitrate anions in one formula unit of barium nitrate, and since each nitrate anion contains `3` atoms of oxygen, you can say that one formula unit of barium nitrate will contain

`color(blue)(2) xx (color(red)(3) xx "O") = color(darkgreen)(ul(color(black)("6 atoms of oxygen")))`