Ionic compounds are basically defined as
being compounds where two or more ions are held next to each other by
electrical attraction. One of the ions has a positive charge (called a
"cation") and the other has a negative charge ("anion").
Cations are usually metal atoms and anions are either nonmetals or polyatomic
ions (ions with more than one atom). The same
thing that makes the positive and negative ends of a magnet stick to each
other is what makes cations and anions stick to each other.
Usually, when we have ionic compounds,
they form large crystals that you can see with the naked eye. Table salt is
one example of this - if you look at a crystal of salt, chances are you'll be
able to see that it looks like a little cube. This is because salt likes to
stack in little cube-shaped blocks.
Sometimes when you see a salt, it looks
like a powder instead of a cube. This doesn't mean that the salt is not a
crystal - it means that the crystals are just so small that you can't see
them with the naked eye. If you were to put the powder under a microscope,
chances are that you would see little geometric blocks.
what are the main properties of salts?
- All ionic compounds form crystals. .Salts like to form crystals because when you have a whole
bunch of little electrical positive and negative charges all stuck
together, they seem to like to bunch into little stacking groups. The
arrangement that these ions like to stack into is different, and is
referred to as the "unit cell". There are ten or so different
general shapes of unit cells.
- Ionic compounds tend to have high melting and
boiling points. When I say
"high", what I mean is "very, very high." Most of the
time, when you work with ionic compounds in a chemistry class, the
melting point is hot enough that you can't melt them with a Bunsen
burner. So, why are these temperatures so high? Well, it has to do
with the way that ionic materials are held together. Remember how we
said above that ionic compounds form crystals? These crystals are
basically just great big blocks of positive and negative charges all
stuck together. To break the positive and negative charges apart, it
takes a huge amount of energy. This means that if we heat up the
compound to add energy, it takes a huge amount of energy to break it
apart.
- Ionic compounds are very hard and very
brittle. Again, this is
because of the way that they're held together. Above, we said that it
takes a lot of energy to break the positive and negative charges apart
from each other. This is the reason that ionic compounds are so hard -
they simply don't want to move around much, so they don't bend at
all. This also explains the brittleness of ionic compounds. It
takes a lot of energy to pull ionic charges apart from each other.
However, if we give a big crystal a strong enough whack with a hammer,
we usually end up using so much energy to break the crystal that the
crystal doesn't break in just one spot, but in a whole bunch of places.
Instead of a clean break, it shatters.
- Ionic compounds conduct electricity when they
dissolve in water. If we take
a salt and dissolve it in water, the water molecules pull the positive
and negative ions apart from each other. This is because of the unusual
properties of water,
Instead of the ions being right next to each other, they wander all over
the water. Now, think back to what electricity is - hopefully, you
remember that electricity is just the movement of electrons through
metals (or anything else). Now, electrons are just negatively-charged
particles, and metals have the property that they're good at letting
them wander around. Dissolved salts are the same way. When you dissolve
the salt in the water, the positive and negative ions in the water allow
electrons to flow much better than if you just had water by itself. Salt water conducts! A question you might have is
"Does electricity travel through salt crystals?" . It
doesn't. Because the ions are stuck in one place due to the structure of
the crystal, the electricity doesn't move around very well.
Another good question: "Does water without salt in it conduct
electricity?" The answer: Not very well. Water by itself is a lousy
conductor. The reason that boneheads who put the hairdryer in the
bathtub with them turn into human fritters is that when they wash
themselves, all the crud on them gets dissolved in the water. Some of
the crud is ionic, so when the dryer hits the water, they get zapped. An
interesting "thought experiment" would be to wash all the salt
off yourself and then drop a hairdryer in the bathtub with you. In
theory, you would be fine. In real life, you'd still become a crispy
critter because tap water by itself has ionic compounds dissolved in it
anyway
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