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 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 + 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 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 NaClMgSO 4 RhOH 2 O covalentionicionicioniccovalent Sc(OH) 3 BrClNa ioniccovalentnot 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: NaClMgO Na is +1Mg is +2 Cl is -1O 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 formula units into water, I will have water with 200 Ca 2+ 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