[narrator] How to form molecular compounds and draw Lewis Structures is the subject of
another video, whatever suffice it to say that molecular compounds differ from ionic
compounds in that we share electrons to form a bond, a chemical bond, rather than transferring
electrons to form an ionic bond. We’re not transferring, we’re now sharing, so that
each of these chlorine atoms can be said to have a full valence shell of electrons. Sharing
to have eight for each when they don’t have eight individually. If we’re looking at
the periodic table we’re going to name the most electropositive element first. Just like
with ionic compounds metals are the most electropositive element and so if we divide the periodic table
between the metals and the non-metals the metals are going to appear on the left-hand
side and the non-metals are going to appear at the top and to the right. Metals came first,
they’re the most electropositive. In ionic compounds anions came last, they’re the
most electronegative. We’re going to follow the same trend, except with molecular compounds
we’re going to have a non-metal and another non-metal, rather than a metal and a non-metal.
If we have no prefix and, again, most electronegative would be fluorine. Fluorine’s always going
to go last and then things that are up and to the right are more electronegative, so
if you’re further left or down on the periodic table you’re going to be named first in
the molecular compounds. Fluorine would always be last. If we name the most electropositive
element first, we’re not going to put a prefix if it’s a single atom. However, we
must use a prefix if there’s more than one. The prefixes that we’re going to use, 1-10.
Doubt we’ll ever use 7, 8, 9 and 10 but we might. Mono-, di-, tri-, tetra-, penta-,
hexa-, hepta-, octa-, nona-, and deca-. For one, two, three, four, five, six, seven, eight,
nine, ten atoms. The second thing we must do is name the most electronegative element
last and we have to use a prefix no matter how many there are, and then a special case
when it’s an oxide is the second and we use a mono- prefix instead of “monooxide”
we would drop one of the o’s and just use “monoxide” instead. We’d much rather
have it sound monoxide than monooxide. Let’s do a few examples, that’s the best way to
learn this. Two nitrogens; we have to use a prefix, di-. Name it just like we would
a metal, dinitrogen. Then the most electronegative element last must use a prefix and we have
to still use the -ide ending, just to make sure that we are still paying attention to
that. Dinitrogen monoxide is the first. Then we have carbon. Notice we don’t use the
mono- prefix. Carbon monoxide, that’s the deadly gas. Always open your tents. Don’t
burn candles in a closed space, or lanterns, and open the garage when running your car
engine. Then, finally here, we have sulfur hexafluoride. Not fluoride, its fluoride.
Hexa- being the prefix for six. Not too bad, just got to learn those prefixes. Then a lot
of textbooks and classes throw in linear alkanes and I’ll throw those in here. With alkanes,
they’re a hydrocarbon containing only single bonds. A hydrocarbon that contains only single
bonds is going to have a molecular formula that’s some number of carbons and some number
of hydrogens, which is double that number of carbons plus two. For example, if
we have methane that would be one carbon and then two times one two plus two, CH4. Then
ethane would be C2H6, propane would then be C3--three times two is six, plus two is eight,
and you get the idea. Just keep going with that. Again, prefix plus -ane. For example,
with four carbons we would have butane. Not tetraane, different set of prefixes here.
Let’s look at the whole list of them. With one through ten they’re not the same. Methane,
ethane, propane, butane, pentane--okay, pentane picks up the same, so penta but pent- is the
prefix--hexane, heptane, octane, nonane is familiar, decane and then we get to 11 and
12, undecane, duodecane, tridecane, tetradecane. Repeats the prefixes we learned before all
the way to twenty which is eicosane. That’s a lot but let’s only learn, for my class,
let’s learn up to ten, shall we? For now. When organic chemistry rolls around we’ll
get as many as you like.