professor Dave here, let's learn how to draw Lewis dot structures
we've learned about how ions come together to form ionic compounds but much more
interesting is the way atoms form networks of covalent bonds. in order to
talk about this we're going to have to learn how to draw Lewis dot structures
when we draw these we will represent an atom by its chemical symbol. here's
carbon. we then draw that atom's valence electrons as dots around the atom. carbon is
one 1s2 2s2 2p2 so that's four electrons in the outermost shell, which
you can also tell by counting over from the left
for carbon and other small atoms we assume that there are four coordination
sites that can accommodate electrons or bonds because this is how carbon can
fill its octet, or its
outermost shell. when we draw the valence electrons we put one in each
coordination site first before pairing them up so carbon will look like this
not like this
elements in the same group will have similar looking lewis dot symbols
because they have the same number of valence electrons. now to draw a lewis dot
structure for a molecule just draw each atom with all of its valence electrons
unpaired electrons from two different atoms can come together to make covalent
bonds. when that happens instead of two dots we make a line. this is a covalent
bond which always contains two electrons. for our purposes here we can assume that
electrons need to be paired so a good lewis dot structure will have all the
electrons either inside covalent bonds or in lone pairs. the number of valence
electrons an atom has dictates how many bonds it tends to form. carbon likes to
make four bonds. nitrogen, with five valence electrons likes to have three
bonds and one lone pair. oxygen likes to have two bonds and two lone pairs. and
fluorine likes to have one bond and three lone pairs. when two atoms have one
covalent bond between them we call that a sigma bond, but atoms can have double
or even triple bonds. look at carbon dioxide. after we make a sigma bond
between the atoms there are still unpaired electrons. if adjacent atoms
each have unpaired electrons they can become another covalent bond. the second
covalent bond in a double bond is called a pi bond. the second and third bonds in
a triple bond are also pi bonds. single covalent bonds are the longest, double
bonds are a little shorter, and triple bonds are shorter still. more on these
later. sometimes an atom in a lewis dot structure will have a formal charge. this
happens if the number of electrons an atom is contributing to a lewis dot
structure is different from its typical valence. nitrogen has five valence
electrons so in ammonia
where it contributes those 5, 1 per covalent bond plus the lone pair, it will
be a neutral nitrogen atom. for the ammonium ion, nitrogen is now
contributing four electrons to the lewis dot structure, one per bond. that's one
less than its typical valence and one fewer negatively charged particles means
the nitrogen will have a formal positive charge. that makes this a polyatomic ion
these elements want to fill their octet by having eight electrons around them
filling n=2 shell. hydrogen is very small so that only needs two to
fill the n=1 shell, and larger atoms like phosphorus and sulfur can
make five or six covalent bonds. just draw valence electrons and start
combining unpaired ones. let's check comprehension
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