Line/Dot Formulas

The better name is Lewis Electron Dot formulas - then the lines come in later. We shall tackle all it.

As shown in the last section, we find it extremely helpful to actually show the valence shell electrons (the s and p ones) as dots surrounding the element symbol. Put your new found knowledge of electron configurations to work for you. Count those valence electrons - yes, the outermost s and p ones. I'll give you a big hint, the number of valence electrons EQUALS the ones-digit of the group number of the periodic table (skipping the d-transition metals). So group 1 and group 2 have 1 and 2 valence electrons. Now jump over to the p-block and you have groups 13-18. That will be 3 valence electrons all the way up to 8 valence electrons. You just put the dots symmetrically around the symbol and you've got it. Here is the electron dot "formula" for the 2nd row elements.

Figure showing dots representing electorns surrounding each element symbol from the second row.

Easy breezy. There is really no wrong way to surround the element - however, there ARE some assumed rules that most everyone follows - so let's just pretend its a real rule (although it's not). Think up, down, left, right for positioning of the electron dots. On a clock face that is 12, 6, 9, and 3. Pretend those positions are orbitals and then fill them in order... right, left, up, down for the first 4 dots, then again with pairing in the positions. You don't have to go in any order, but it looks nice when you try to be somewhat symmetric in your choices.

Covalent Bonds with Dots

So let's ignore Li, Be, and B - they're a bit odd in their rules and we don't need them at all (sorry). Much much more useful is hydrogen and it's one dot, like this H· Ok? see one dot, one electron. Now lets remember how all those non-metals would just love to have a full valence shell which is s2p6 electron configuration around it. Neon (see above) is already there which is why neon does NOT ever react with anyone else. Neon is happy and content being neon - aloof and distant, one could even say noble. So how do the other elements come up with 8 e ? By sharing! So carbon, needs to gain another 4 electrons via sharing and it will share 4 electrons in return. Nitrogren needs another 3 electrons and will share 3 in return. Oxygen needs 2 more electrons and will share 2 in return. Hopefully you get the idea here. Below are several dot structures for some common compounds.


Lines!

Now lets convert those same structures above into line/dot formulas. We swap out any bonding pairs of electrons with lines. A single shared pair will be a single line or single bond. Two shared pairs will be a double line or double bond. And, three shared pairs will be a triple line or triple bond. All this is shown below.


Recognizing Lone Pair and Bonding Pair Electrons

Below is a line/dot structure of the amino acid glycine (H2NCH2COOH). Note the structure and how to identify the lone pair electrons and the bonding pair electrons. Be able to both create the structure (draw it) and analyze the structure (count features within the structure).



S = N – A   Rule

The S = N – A rule is a simple "formula" that helps you at least identify how many electrons will be in chemical bonds (bonding pairs of electrons) - which also means you know how many are not in chemical bonds (lone pair electrons). Let's tell you what each of those letters stands for:

  • S : the number of SHARED electrons in the structure. These are bonding electrons and if you divide S by 2 you'll have the number of bonds (or lines) in the structure when completed.
  • N : the number of electrons NEEDED based on the octet rule. Assume that every atom in the structure will get 8 needed electrons to satisfy the octet rule. The only exception is for hydrogens (H) which only need 2 electrons for it's "octet" rule.
  • A : the number of valence electrons that are AVAILABLE in the structure. This is just the total number of valence electrons for all the atoms in the structure. You will also adjust this up or down if the species is an ion. Anions will need more electrons and cations will need less.

Skeletal Line Structures

When organic structures start to get larger and larger - showing every atom and bond can get very cluttered. A skeletal line structure is basically a "shorthand" for doing structures. Many of the carbons are implied by simply drawing lines and bends. If there is not an explicit symbol there, it is intended to be a carbon. In addition, all implied carbons are assumed to be complete with a full octet and any missing bonds are just assumed/implied to be hydrogens. So the hydrogens aren't shown at all but are known to be there. And, again, to streamline the skeletal structure, lone pair electrons are often not show as well. Below is a skeletal structure for aspirin (acetylsalicylic acid, C9H8O4). It's an animated gif that first shows the plain skeletal structure (its from Wikipedia) then the carbons show up in red, then the hydrogens show up in green, and finally, the lone pair electrons on all the oxygens.

You will want to develop the skill to know and "see" all those implied atoms and lone pairs (this is the kind of thing we would ask on an exam). Like - check this out, here is the skeletal structure for a very popular and consumed substance. Can you come up with the correct empirical formula?

Q1: How many C's? H's? lone pairs? What's the empirical formula? What is the substance?   answers▴

You DO need at least 3 or more atoms in a chain in order to show a skeletal structure. Too few carbons will just look silly. Like — is ethane. Yes, that's a short little single line, technically ethane, but don't do it. At least with propane you can put a bend in the line to imply 3 carbons. Skeletal structures make much more sense when things get much much bigger. Like this.


Q2: How many C's? H's? lone pairs? What's the empirical formula? What is this substance?   answers▴



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