Chem 117 Week 3 Lecture 3B Number of Covalent Bonds

This video walks students though a strategy to predict the number of bonds an element will form.

So our first step to deciding how atoms are going to be connected together when we’re looking at a chemical formula is to determine how many bonds each of the elements that are in the molecule is likely to have and there’s exceptions to this especially if we’re thinking about ions or covalent molecules that have an overall charge but the rule that works really

Well in general again is that the number of bonds for a molecule will equal or for an element will equal 8 minus the number of outer electrons let’s do this for the kind of common elements in that that are non-metals in that second row of the periodic table we’ll use these ones a lot uh so carbon so let’s uh set this up as the equation uh so we’ll have 8 minus the

Number of outer electrons and remember the easiest way to determine this number of outer electrons is to still look at which column or group the element is in and so for our main group elements we have and we have we have two columns and we have all these transition metals and lanthanide and actinides and we’re ignoring those because they’re not main group and

Then we have um another six and so we can kind of count one over so uh we’ll have one outer electron for everything in the first column two outer electrons for our second column skip your transition metals then we’ll have three for the the column that has boron in it and borons and metalloids i’m kind of skipping that one for thinking through this exercise and

Focusing on non-metals and then our first non-metal that comes up next is in the fourth column of the main group elements and that’s going to be carbon and then we’ve got five and six and seven and then eight and that that’s the easy way to kind of get to that number of outer electrons and they’re all the same for every element in that that group so for carbon it

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Is located in that fourth column and so it will have four outer electrons and i’m going to use this symbol e with a negative sign this equals electron it’s my little shorthand way of writing electron uh because chemists love to abbreviate things that’s our favorite thing to do besides blow things up and make crystals and look at pretty color changes so carbon will

Be eight minus four we expect carbon will form four bonds now let’s look at nitrogen so nitrogen is one over on the periodic table from carbon it is in the fifth group of the main group elements and so it’ll have five outer electrons so we’ll subtract five from eight and that’ll equal three bonds that we expect it to form um we could be also writing the electron

Dot symbols that we we learned about previously so r carbon has those four valence electrons right there right or outer electrons and it would need to form four bonds our nitrogen has five so it’ll look like this for our electron dot symbols all right let’s do oxygen next eight minus and now it’s one over from nitrogen so oxygen is in the sixth main group column

It’ll have six outer electrons sorry about that so it’s likely to form two bonds let’s do fluorine it’s next to oxygen it has seven we actually saw an example of it just above it has seven outer electrons so it typically forms one bond all right let’s write our electron dot symbols for these two remember our electron dot symbols are just the element symbol and

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Then you take the number of outer electrons and you add them around the four sides of the elect of the chemical symbol and then you double them up uh and i’d like to show a little correlation right here check this out the number of unpaired electrons that we have that is just one electron in our element symbol or electron dot symbol is equal to the number of bonds

That is likely to form see we’ve got two for oxygen and two unpaired electrons similarly if we look at fluorine we have one unpaired electron right and we typically form one bond so i actually tend to just draw my element symbol or my electron dot symbol and look for the number of unpaired electrons i’m like oh that’s that’s where i need to form some bonds i need i

Need one for fluorine and two for oxygen and it holds for the other ones too if you look at uh here’s nitrogen has these three and it’ll typically form three bonds uh let’s give me the color our carbon has two three four and it forms typically four bonds um and so i use that a lot to kind of guide to quickly help me figure out how many bonds to form to something

But this kind of gives the the reason for it here’s a summary table of it as well um and this one is correlating something else so this one is looking at the number of bonds and then the number of unpaired electrons that it would have and remember we have any bonds that aren’t going to participate in the sharing of the electrons are going to form as lone pairs and

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We’ll represent them around our element symbol in our electron dot symbol as just paired up dots um and so those lone pairs are right here so we’ve got one on nitrogen we have two on oxygen and if this x was fluorine we’d have three uh which we can see right here right so for fluorine we’ve got one two three sets of lone pairs or six total electrons that are not

Bonding there’s another way to look at it and so again hydrogen is kind of the exception to the rule because it doesn’t follow the octet but carbon nitrogen oxygen and fluorine these all follow the octet rule and you can see that if something has four bonds it’ll have zero lone pairs but as you decrease the number of lone pair or the number of bonds you increase

The number of lone pairs and so you could actually take the sum of these and look they all equal four so you could say all together we have four pairs of lone pairs or bonds so that kind of bonds plus lone pairs equals four and really um this is just kind of another one of these correlations and these correlations are helpful for kind of double checking yourself

When you’re building a structure if you’re looking at someone else’s it can help you um noticing these things can help you kind of interpret it a little bit faster and build it a little bit faster

Transcribed from video
Chem 117 Week 3 Lecture 3B Number of Covalent Bonds By Dr. Pikul \u0026 Chemistry