Ionic Compounds

Ionic Compounds

Does what it says on the box:

an ionic compound is a compound (so, more than one element) made up of ions (charged atoms).

These are called “salts” in general. Table salt (NaCl) is just a specific example of a salt

NOT BondsYou will frequently see the term “ionic bond”

I do not like this phrase; it is misleading. In an ionic compound, there are no bonds. Instead, you have formula units.

Example: NaCl

If you look at a crystal of NaCl, you find that there are a bunch of Na+ ions and Cl- ions all crammed together in a repeating pattern, but there are no bonds.

So….what holds them together?

CHARGES!Opposite charges attract.

The Na+ is attracted to all the Cl- around it, and vice-versa.

(This is called Coulombic attraction, by the way)

There is no such thing as in individual “NaCl”.

Instead, “NaCl” is the formula unit—the proportion of sodium to chlorine… 1 to 1

Is it ionic?So how do you know if it’s ionic in the first place?

Method 1: experimental

Method 2: on paper

(Different kind of ionic entirely)

Method 1

Method 2Look at the formula. Is there a metal?

If yes: it’s ionic

If no: it’s covalent (the other kind—we’ll get to it)

CO2 NaCl MgSO4 RhO H2Ocovalent ionic ionic ionic covalent

Sc(OH)3 BrCl Na ionic covalent not even a compound in the first place

One of the most important things all year

Overall, everything is neutral; charges must balance.

If you have a +2, there are two negatives somewhere.

This is true of ionic compounds.

Examples:

NaCl MgO

Na is +1 Mg is +2Cl is -1 O is -2

(these are charges you should know from where they are on the table)

What if I Want Mg2+ with Cl-?This is perfectly ok; I just need to use a different ratio:

The formula is MgCl2

I now have two chlorines (-1 each) balancing out the magnesium (+2)

Al3+ and Cl-? Just use a 1:3 ratio: AlCl3

Ok, but what about Al3+ and O2-?

Make them Both 6!Al2O3

I now have two aluminums (3+) for every three oxygens (2-)

So the total charge of the aluminums is 6+, and the total charge of the oxygens is 6-. Balance!

In an ionic compound, the subscripts (2 and 3, in this case), are just the proportion of each ion in the formula unit.

If I have 2000 formula units, that’s 4000 aluminum ions and 6000 oxygen ions.

It’s a ratio!Al2O3

Ratios should always be in their lowest possible terms.

Ca2O2 does balance (Ca is 2+, O is 2-)

but 2:2 is the same as 1:1

CaO is the actual chemical formula.

Important PointCaCl2

This is NOT made of Ca and Cl2; it is one Ca2+ and two Cl-.

When an ionic compound dissolves in water, it splits apart into the individual ions. If I put 200 CaCl2 formula units into water, I will have water with 200 Ca2+ ions and 400 Cl- ions all floating around freely.

SummaryIonic compounds are made of charged atoms

Held together by charges, not bonds

Recognize them by looking for a metal