Ionic Compounds

Ionic Compounds

Does what it says on the box: an ionic compound is a compound element) made up of ions (so, more than one (charged atoms).

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

NOT Bonds

You 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 there are no bonds.

+ ions and Cl ions all crammed together in a repeating pattern, but 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

Step 1: melt it. (Ionic compounds don’t tend to melt at temperatures below a couple thousand degrees Celsius) Step 2: Shove a battery and light bulb in it.

Step 3: Profit Step 3: If it lights up, the compound conducts electricity, which only happens if there’s charges moving…so it must be ionic

Method 2

Look 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) CO 2 covalent Sc (OH) 3 ionic Na Cl ionic BrCl covalent Mg SO 4 ionic Rh O ionic H 2 O covalent Na 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 Cl is -1 Mg is +2 O is -2 (these are charges you should know from where they are on the table)

What if I Want Mg

2+

with Cl

-

?

This is perfectly ok; I just need to use a different ratio: The formula is MgCl 2 I now have two chlorines (-1 each) balancing out the magnesium (+2) Al 3+ and Cl ? Just use a 1:3 ratio: AlCl 3 Ok, but what about Al 3+ and O 2 ?

Make them Both 6!

Al 2 O 3 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!

Al 2 O 3 Ratios should always be in their lowest possible terms.

Ca 2 O 2 does balance (Ca is 2+, O is 2-) but 2:2 is the same as 1:1 CaO is the actual chemical formula.

Important Point

CaCl 2 This is NOT made of Ca and Cl 2 ; it is one Ca 2+ and two Cl .

When an ionic compound dissolves in water, it splits apart into the individual ions. If I put 200 CaCl 2 water, I will have water with 200 Ca 2+ formula units into ions and 400 Cl ions all floating around freely.

Summary

Ionic compounds are made of charged atoms Held together by charges, not bonds Recognize them by looking for a metal