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 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 different from its typical valence. nitrogen has five valence electrons so in ammonia where it contributes those five, one 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 the n equals 2 shell. hydrogen is very small so that only needs two to fill the n equals 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 thanks for watching guys, subscribe to my channel for more tutorials, and as always feel free to email me