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What two non-metals do instead of transferring electrons, and how a shared pair is counted.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to say what a covalent bond is — a pair of electrons that two nuclei both attract — and work out how many bonds each atom in a simple molecule makes, from how many electrons its outer shell is short. You will be able to build the structure of a small molecule as a set of connections, choosing single, double or triple for each one, and check it against two numbers: the outer-shell electrons of the whole molecule, and the shared and lone pairs on the central atom.
You know that an atom with a nearly full outer shell will take electrons, and that a metal with a nearly empty one will let them go — and that when those two meet, the electrons cross over and the ions that are left attract. What this lesson asks is what happens when both atoms are the taking kind, so neither will let go and there is nothing to transfer.
| Term | What it means |
|---|---|
| Covalent bond | A pair of electrons shared between two atoms. |
| Bonding pair | A shared pair. |
| Lone pair | A pair on one atom that is not being shared. |
| Double bond | Two shared pairs between the same two atoms. |
| Triple bond | Three shared pairs between the same two atoms. |
| Molecule | A group of atoms held together by covalent bonds; a real particle. |
| Lewis structure | A drawing that shows where every outer-shell electron has gone. |
Two non-metal atoms meet. Each has a nearly full outer shell and each would take an electron if it could, but neither can take one from the other — they pull about equally hard.
So they share. One electron from each atom forms a pair that sits between the two nuclei, and both nuclei attract it. Count the shell of either atom and the shared pair counts toward it, so both atoms end up with a fuller shell than they had. That is the bargain, and the attraction of two nuclei for one pair between them is the bond.
How many bonds an atom makes follows directly:
Sometimes the two bonds an atom needs go to the same partner. Oxygen gas is two oxygen atoms sharing two pairs — a double bond. Nitrogen gas is two nitrogen atoms sharing three pairs — a triple bond, and one of the hardest bonds in ordinary chemistry to break, which is why the air is mostly an unreactive gas.
Another way: picture
Two people each holding one end of the same rope. Neither of them owns the rope and neither can walk off with it; while both are holding on, they cannot get away from each other either. A double bond is the same two people holding two ropes, and it takes more to pull them apart.
Another way: steps
To work out the structure of a simple molecule:
Total the outer-shell electrons. Add every atom's group number. That fixed total is the budget.
Find how many bonds each atom makes. Eight less its outer-shell count, or one for hydrogen.
Choose the central atom. The one that makes the most bonds goes in the middle.
Join the others to it. One single bond to each partner to begin with.
Raise the bond order if needed. If the central atom still needs more bonds than it has partners, turn single bonds into double or triple ones.
Fill in the lone pairs. Each atom's electrons not in bonds sit as lone pairs.
Check the work. Does every atom other than hydrogen end with eight electrons around it, counting its shared pairs and lone pairs? Does each atom's own count hold — shared pairs plus twice its lone pairs equals its group number? And does the whole structure use exactly the total you started with?
Counting bonds as the electrons an atom is short is allowed because each shared pair adds one electron to an atom's own count. An atom two short of eight needs two shared pairs to reach eight, and then it is full.
Putting the atom with most bonds in the middle is allowed because an atom that makes only one bond can join only one partner, so it cannot sit between two. Hydrogen and the halogens are always on the outside of a simple molecule.
Raising a bond to double or triple is allowed because two atoms can share more than one pair. Carbon needs four bonds but has only two oxygen partners in carbon dioxide, so it shares two pairs with each.
Checking the total is allowed because bonding moves electrons into pairs but never creates or destroys one. The structure must place every outer-shell electron the atoms brought, no more and no fewer.
Every structure in this lesson can be checked with two numbers, and the check is worth doing every time.
The total is every outer-shell electron in the molecule. Water is one oxygen (six) and two hydrogens (one each): eight. Methane is one carbon (four) and four hydrogens: eight. Carbon dioxide is one carbon (four) and two oxygens (six each): sixteen.
The central atom's own count is the other. Whatever it started with, it must still have: each shared pair it is in uses one of its electrons, and each lone pair uses two. So
$$\text{outer-shell electrons} = \text{shared pairs} + 2 \times \text{lone pairs}$$
| Molecule | Central atom | Its electrons | Shared pairs | Lone pairs |
|---|---|---|---|---|
| $\mathrm{CH_4}$ | carbon | 4 | 4 | 0 |
| $\mathrm{NH_3}$ | nitrogen | 5 | 3 | 1 |
| $\mathrm{H_2O}$ | oxygen | 6 | 2 | 2 |
| $\mathrm{HCl}$ | chlorine | 7 | 1 | 3 |
| $\mathrm{CO_2}$ | carbon | 4 | 4 | 0 |
Read the middle three rows downwards and the pattern is the whole of the next lesson in advance: as an atom keeps more pairs to itself, it makes fewer bonds — and those lone pairs are about to decide what shape the molecule has.
A Lewis structure is a statement about where the electrons are: which pairs are shared, which are not, and how the atoms are connected. It is bookkeeping, and it is checkable.
It is not a picture of the molecule's shape. $\mathrm{H_2O}$ is usually drawn with the two hydrogens on either side of the oxygen, in a straight line across the page, and the molecule is not straight at all — it is bent, at about $104.5^\circ$. The drawing is flat because paper is flat.
So the honest reading of a structure is: this atom is joined to that one, by this many shared pairs, and these pairs are left over. Where the atoms actually sit in space is a separate question with a separate answer, and it is the next lesson's.
This is also why the structures in this lesson are built as connections rather than drawn. What is being asked is exactly what a structure asserts and exactly what can be marked right or wrong: which atom is joined to which, and with how many shared pairs.
Corn in Iowa needs nitrogen, and the air is nearly 80 percent nitrogen, yet plants cannot use a single molecule of it. The reason is the triple bond in $\mathrm{N \equiv N}$: three shared pairs between two nitrogen atoms, one of the strongest bonds in ordinary chemistry. Breaking it takes about 945 kilojoules for every mole of molecules, far more than a plant can supply.
So fertilizer plants in Kansas, Louisiana and Oklahoma break it for them. The Haber-Bosch process forces nitrogen and hydrogen together at about 400 degrees Celsius and 200 times atmospheric pressure, over an iron catalyst, to make ammonia, $\mathrm{NH_3}$. In ammonia, nitrogen makes three single bonds to three hydrogens and keeps one lone pair — exactly the structure this lesson's bookkeeping predicts.
The United States makes more than 15 million tons of ammonia a year, and most of it ends up on fields. The bookkeeping matters there too: the lone pair on nitrogen is what lets ammonia take a hydrogen ion and become the ammonium ion in fertilizer, and the three single bonds are far easier for soil bacteria and plant roots to deal with than the triple bond they replaced.
The fizz in a soda bottled in Atlanta is carbon dioxide, $\mathrm{O=C=O}$, dissolved under pressure. Its two double bonds hold its sixteen outer-shell electrons, and because the molecule is unreactive, it can sit dissolved in the drink for months.
The bond is described as a transfer. The chlorine takes an electron from the hydrogen. If it did, you would have $\mathrm{H^+}$ and $\mathrm{Cl^-}$, which is what happens when hydrogen chloride dissolves in water and not what holds the gas molecule together. In the molecule the pair is shared.
The two nuclei are said to attract each other. They are both positive; they repel. What holds the atoms together is that both nuclei attract the same pair of electrons sitting between them.
A double bond is read as two single bonds side by side. It is two shared pairs between the same two atoms, and it behaves as one bond that is shorter and stronger than a single one — and, unlike a single bond, it will not let the two ends twist round.
Lone pairs are forgotten. They are easy to leave out because nothing is joined to them, and they are the electrons that decide the shape of the molecule. An oxygen atom drawn with two bonds and nothing else is missing four of its six electrons.
The bonds inside a molecule are taken to be what boils. Boiling water breaks nothing inside a water molecule; it separates whole molecules from each other. The bonds survive, which is why steam is still water.
Total the outer-shell electrons.
$5 + 3 \times 1 = 8$
The total is fixed before any bond is drawn.
Find nitrogen's bonds.
$8 - 5 = 3$
An atom makes as many bonds as its shell is short.
Join the three hydrogens.
$\text{three N–H single bonds}$
Nitrogen goes in the middle.
Find nitrogen's lone pairs.
$(5 - 3) \div 2 = 1$
Its electrons not in bonds pair up.
Check the total.
$3 \times 2 + 2 = 8$
Three shared pairs and one lone pair place all eight.
Total the outer-shell electrons.
$4 + 2 \times 6 = 16$
Every atom in the formula contributes.
Find each atom's bonds.
$\text{carbon } 4; \ \text{each oxygen } 2$
Eight less the outer-shell count.
Count carbon's partners.
$2 \text{ oxygens for } 4 \text{ bonds}$
Count the partners before deciding the bond order.
Raise the bond order.
$\mathrm{O=C=O}$
Two bonds to each oxygen: two double bonds.
Fill in the lone pairs.
$\text{carbon } 0; \ \text{each oxygen } 2$
Each oxygen keeps four of its six electrons as two lone pairs.
Check the total.
$4 \times 2 + 4 \times 2 = 16$
Four shared pairs and four lone pairs place all sixteen.
Total the outer-shell electrons.
$5 + 5 = 10$
Two nitrogen atoms.
Find each atom's bonds.
$8 - 5 = 3$
Each nitrogen is three short.
Count the partners.
$\text{one partner each}$
All three bonds must go to the same atom.
Raise the bond order.
$\mathrm{N \equiv N}$
Three shared pairs: a triple bond.
Find each lone pair.
$(5 - 3) \div 2 = 1$
One lone pair on each nitrogen.
Check the total.
$3 \times 2 + 2 \times 2 = 10$
Every electron placed.
Check each atom's eight.
$3 \times 2 + 2 = 8$
Six shared electrons and its own lone pair.
Total the outer-shell electrons.
$6 + 2 = 8$
Group 16 means six outer-shell electrons.
Find sulfur's bonds.
Find sulfur's lone pairs.
In tetrafluoromethane, $\mathrm{CF_4}$, each bond joins a carbon atom to a fluorine atom. What is that bond?
Complete the worked solution: a molecule has a central atom from group $7$ bonded only to hydrogen atoms. Find how many bonds the central atom makes, how many lone pairs it keeps, and how many outer-shell electrons the whole molecule holds.
Find the bonds.
$\text{eight} - (\text{group}) =$ b
An atom bonds as many times as its shell is short.
Find the lone pairs.
$((\text{group}) - (\text{bonds})) \div \text{two} =$ l
Its electrons not in bonds pair up on the atom.
Find the molecule's total.
$(\text{group}) + (\text{bonds}) =$ t
The central atom's electrons plus one from each hydrogen.
One molecule of carbon dioxide, $\mathrm{CO_2}$, has a central carbon atom with two oxygen atoms bonded to it. Join the atoms, and choose the right kind of bond for each join.
This task has no paper form; do it on a device.
One molecule of methane, $\mathrm{CH_4}$, has a central carbon atom with four hydrogen atoms bonded to it. Join the atoms, and choose the right kind of bond for each join.
This task has no paper form; do it on a device.
The safety sheet at a natural gas pipeline in Pennsylvania prints a structure for methane, $\mathrm{CH_4}$: one carbon with four outer electrons and four hydrogens with one each. How many outer-shell electrons must any correct structure for one molecule account for?
The answer: a.
A cylinder delivered to a workshop carries a printed structure for its contents, $\mathrm{CS_2}$, and somebody has to check it before it is used. Two numbers settle whether the printed structure can be right at all.
A structure for $\mathrm{CS_2}$ must account for a outer-shell electrons in all, and it must leave the central carbon atom in b shared pairs.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Do the electron bookkeeping for chlorine, $\mathrm{Cl_2}$. Count the outer-shell electrons the central chlorine atom brings, then how many shared pairs and how many lone pairs it ends up in, and finally how many outer-shell electrons the whole molecule has to account for.
| count | |
|---|---|
| outer-shell electrons on one chlorine atom | |
| shared pairs the chlorine atom is in | |
| lone pairs on the chlorine atom | |
| outer-shell electrons in the whole molecule |
You can build and check the structure of a small molecule. Say out loud why nitrogen gas has a triple bond and oxygen gas only a double one, and what the two lone pairs on a water molecule's oxygen are doing there. Next: those lone pairs turn out to decide the shape of the molecule, and the shape decides a great deal else.
15. Your turn: the structure of hydrogen sulfide, made of one sulfur and two hydrogens, step 2
$8 - 6 = 2$
One to each hydrogen.
15. Your turn: the structure of hydrogen sulfide, made of one sulfur and two hydrogens, step 3
$(6 - 2) \div 2 = 2$
Two shared pairs and two lone pairs.