Lewis Dot Structure Polyatomic Ions

Now that you lot've mastered the art of cartoon the Lewis structures of neutral covalent compounds, it'southward fourth dimension to draw the Lewis structures of polyatomic ions. If yous haven't mastered the art of drawing Lewis structures yet, yous can check out the tutorial by clicking HERE. Seriously, unless you check out that tutorial, this probably won't make a lot of sense.

Allow'due south get started!

What do polyatomic ions accept to practice with covalent compounds?

Well, it depends. In a very strict sense, polyatomic ions take zippo to exercise with covalent compounds. Subsequently all, covalent compounds don't contain any ions at all, which is why we need dissimilar sections for ionic and covalent compounds.

Withal, it turns out that the polyatomic ions that are present in ionic compounds take atoms which are covalently-bonded to each other. In other words, ionic compounds sometimes have covalent ions in them.

You'd retrieve this wouldn't make whatever difference, and if you're looking at the basic properties of ionic compounds you'd exist correct¹. Nevertheless, when you dissolve an ionic compound in water, those polyatomic ions are now free to wander around and react with whatsoever covalent compounds are present. Every bit a result, the structures of these ions turn out to be of import after all.²

Fig1tnt

The formation of TNT involves the nitrate ion. I've included a foreign version of this diagram for our Russian friends. России-замечательная страна!³

How to draw the Lewis structures of a polyatomic ion

Before yous can practise this, you actually demand to empathise the method for drawing Lewis structures that I talked well-nigh in the previous tutorial. Go ahead… I'll wait for you lot to come back.

To sympathize how to practise this, let's use the example of the hydroxide ion: OH-

Step 1: Detect the number of valence electrons.

This is done in the same way every bit for neutral ions, except that when you're done, you need to modify the number of electrons to compensate for the charge on the ion. Here'due south how:

If it'south an anion, add the negative accuse to the number of valence electrons.
If it's a cation, decrease the negative charge from the number of valence electrons.

Let's find the valence electrons for OH-:

0xygen: ane atom x six valence electrons = vi valence electrons

hydrogen: 1 atom 10 1 valence electron = 1 valence electron

6 + one = 7 valence electrons

The additional step: OH- has a -1 charge, and so we need to add one to the number we plant, giving us a total of 8 valence electrons.

Pace 2: Notice the number of octet electrons.

The rules for doing this are pretty much the same as we saw earlier, with a few minor modifications:

hydrogen always wants two electrons
beryllium always wants four electrons
boron wants six electrons in neutral compounds, but 8 electrons in polyatomic ions
all other elements ever want 8 electrons

In the case of the OH- ion:

oxygen: 1 cantlet x viii octet electrons = 8 octet electrons

hydrogen: 1 cantlet x 2 octet electrons = 2 octet electrons

Grand total: 8 + 2 = 10 octet electrons

Stride three: Find the number of bonding electrons.

Again, this is done the same manner as it is for neutral compounds, where yous need to subtract the valence electrons from the octet electrons. For us:

10 octet electrons – 2 valence electrons = ii bonding electrons

Step four: Find the number of bonds.

Divide the number of bonding electrons past 2 to get the number of bonds:

2 bonding electrons /ii = ane bail

Step 5: Draw information technology!

When drawing polyatomic ions, we do pretty much the same matter every bit we did with neutral covalent compounds. However, the rules for doing this are a little different. I dear making charts, so here's a chart that shows what I mean: Fig2bonding

This chart is basically the same thing you lot saw in footstep five in the first Lewis structure tutorial, except that it the right column gives a somewhat different set of rules when drawing the Lewis structures of polyatomic ions.⁴

For the case of OH-, we've institute that in that location's one bond. Since there are two atoms, it's pretty simple to see that this bond lies betwixt them:

Fig3OHnoelectrons

Step 6: Add lone pairs

The basic procedure is the same as for neutral compounds, except that the number of electrons everything wants reflects the rules shown in #ii above. (In example you don't feel like scrolling up, hydrogen and the halogens ever want two electrons, beryllium wants four electrons, boron wants 6 electrons in neutral compounds and eight in polyatomic ions, and everybody else wants 8 electrons).

Looking at OH-, we see that oxygen has simply two electrons around it (the pair of electrons it shares with H), and hydrogen too has those same two electrons effectually it. While hydrogen only wants two electrons, oxygen wants 8. Every bit a outcome, nosotros need to add six electrons in the course of lone pairs:

Fig4OHelectrons

Crawly!

Step 7: Figure out which atom is charged

Recall that the whole reason we're doing a tutorial here is that some covalent things have either positive or negative charges. Every bit a result, we've got to figure out which cantlet is positively or negatively charged.

Here'south how you do it: Count the number of electrons that each atom actually owns in this Lewis structure. In our example, oxygen owns seven electrons. Why 7? Let's discuss:

The 3 lonely pairs are on oxygen and have nix to do with bonding. Thus, oxygen owns all six of these electrons outright.

The bond information technology has with hydrogen is formed when one electron from oxygen and 1 electron from hydrogen combine to grade a covalent bond. Though the bond has a total of two electrons, only one of those came from the oxygen atom. As a event, all bonds count as one electron in this stride, rather than 2.

Now that nosotros know how many electrons oxygen owns, let's compare that to how many it normally has:

Oxygen commonly has half dozen valence electrons, making information technology neutral.
In the OH- ion, oxygen really owns seven electrons. This means that information technology has one more than electron than it unremarkably does in its neutral country.
Because electrons are negatively-charged, oxygen has a -1 charge.

Hydrogen, on the other manus, doesn't have any accuse at all. In the OH- ion it owns 1 electron (i of the electrons from the bond with oxygen) and has no solitary pairs. Considering hydrogen usually has only i electron, it's neutral.

Now that nosotros know oxygen has a -ane charge, let'south finish our cartoon:

Fig5OHfinal

That's all we have to do: Put a "-1" over oxygen. If you wanted to but put a "-" over oxygen, that's fine too, equally we assume any charge without a number must be 1. In the case of something with a accuse of "-2", just write "-2".

Addressing a common student complaint:

When they first learn how to depict Lewis structures, most students mutter that Lewis structures suck. And, in all fairness, this methodis a existent pain in the barrel and totally sucks. However, it also happens to piece of work, and when you're just learning chemistry, that'southward really the most important thing to retrieve about.

Now, exerciseI describe Lewis structures this way? Nope. You've got to remember, though, that I've been doing this for 25 years, so I don'tneed to utilize this method anymore. As y'all do more examples, you'll kickoff to get the hang of things and won't have to use this method anymore, either. Until and then, only keep with it.

Another example: NH₄⁺

Step 1: Count the valence electrons

Nitrogen: 5 valence electrons x 1 atom = 5 valence electrons

Hydrogen: one valence electron ten 4 atoms = four valence electrons

Subtract ane valence electron because it's a cation with +1 charge

Total: 5 + 4 – 1 = eight valence electrons

Pace 2. Count octet electrons

Nitrogen: 8 octet electrons 10 one atom = viii octet electrons

Hydrogen: 2 octet electrons ten 4 atoms = 8 octet electrons

Total: 8 + viii = xvi octet electrons

Step three. Discover bonding electrons: 16 – 8 = viii bonding electrons

Step iv. Find number of bonds: eight / 2 = 4 bonds

Step five. Draw it:

Since nitrogen wants three or four bonds in polyatomic ions, nosotros can come across that this is A-OK:

Fig6NH4step5

Footstep 6: Add lone pairs

In this case, nitrogen already has eight electrons around it and hydrogen has its desired ii electrons. As a result, no more electrons are needed.

Step 7: Assign charge

Hydrogen never has charge on it, which would correctly imply that nitrogen must accept the +1 charge in this ion. All the same, if you lot didn't think of that, you lot can come across that nitrogen owns four electrons in this ion (i from each bond) and that it usually has 5 valence electrons to go along neutral. Equally a consequence, it has i fewer electron than information technology usually has, giving it a +1 charge:

Fig7NH4step7

I'm not going to include any examples hither, as the side by side tutorial will talk over both polyatomic ions and resonance structures (which are closely related in many cases). Bank check over there for examples.

Footnotes:

Mostly right, anyway. It's pretty hard to say that the locations of charges and/or resonance structures are totally irrelevant to the properties of ionic compounds, but it's certainly the case that they're less important than the function where the ions interact.
To put it another way, y'all can think of polyatomic ions in solution as interim a lot like any other covalent compound. Organic chemists use this a lot when building new compounds.
As measured by the burliness of Vladimir Putin, anyway.
Delight continue in mind that these are general rules and don't represent infallible secrets to the universe. If you use these rules, however, you should exist in pretty good shape for annihilation that your instructor throws at you.

Figure credits:

Russian TNT diagram: By Bauka91 91 (Ain work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons.

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For those of you who have been told to cite things incorrectly, the appointment this was published was January 26, 2015. If you'd like to do it the correct way, Google "ACS Style" or the mode rules for any of the other physical sciences. Seriously, MLA style? That'due south only used in the humanities, and we're scientists!