# Question #fb2aa

Jan 21, 2017

This is what's going on here.

#### Explanation:

The difference between atoms and ions in that context comes from the nature of the chemical substance that you're dealing with.

When you're dealing with a covalent compound, you will refer to its constituent elements as atoms because covalent compounds are formed when atoms share electrons.

On the other hand, when you're dealing with an ionic compounds, you will refer to its constituent elements as ions because ionic compounds are formed when atoms lose or gain electrons.

In other words

• $\text{covalent compounds " -> " NO ions " = " atoms}$
• $\text{ionic compounds " -> " ions}$

For example, calcium chloride is an ionic compound that contains calcium cations, ${\text{Ca}}^{2 +}$, and chloride anions, ${\text{Cl}}^{-}$

${\text{CaCl"_2 -> "Ca"^(2+) + 2"Cl}}^{-}$

Notice that each formula unit of calcium chloride contains a total of $3$ ions, $1$ calcium cation and $2$ chloride anions. This, of course, means that every mole of calcium chloride contains $3$ moles of ions.

You thus have

$\text{2 moles Cl"^(-)/"1 mole CaCl"_2" }$ and ${\text{ " "3 moles ions"/"1 mole CaCl}}_{2}$

as conversion factors to use when converting from moles of calcium chloride to moles of chloride anions or moles of ions (calcium cations and chloride anions).

Since you won't find any calcium and chlorine atoms in this substance, you will refer to them as calcium cations and chloride anions, i.e. as ions.

Nitrogen dioxide, ${\text{NO}}_{2}$, is a covalent compound that contains nitrogen atoms bonded by way of covalent bonds to oxygen atoms.

In this case, you say that each molecule of nitrogen dioxide contains $1$ atom of nitrogen and $2$ atoms of oxygen.

Your conversion factor here will be

${\text{3 moles atoms"/"1 mole NO}}_{2}$

because every mole of nitrogen dioxide contains $3$ moles of atoms.

So remember, the nature of the chemical substance tells you if you should use atoms or ions.