ionic compounds. does what it says on the box: an ionic compound is a compound (so, more than one...
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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 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