Covalent bonding and dot-and-cross for small molecules

Here is a rule that sounds complete: atoms join up until each one has eight electrons in its outer shell. Is that true of every atom in a water molecule? The tempting answer is yes. Eight makes a full outer shell, so every atom in water should count eight. In a correct diagram of water, the oxygen does count eight. Each hydrogen counts only two, and the diagram is still right. Hold on to that two: a single shared pair is what explains it.

A covalent bond is a shared pair of electrons between two atoms. Pair is part of that definition: two electrons, one from each atom. Covalent bonds form between non-metal atoms. There is no metal, no ion, and nothing is handed over. Ionic bonding is the other way: a metal hands electrons to a non-metal, and ions form. In a covalent bond nothing is transferred; the pair is shared. Sharing works because the pair sits where the two outer shells overlap, and both atoms count it at the same time. That is how each atom reaches a full outer shell. The smallest case is hydrogen, H two. Each hydrogen atom brings one electron, the two make one shared pair, and each atom now counts two. Covalent bonding and these diagrams are on both the Foundation and the Higher papers. Three pairs of elements. The first: sodium and chlorine. The second: carbon and hydrogen. The third: magnesium and oxygen. Which of those three pairs bonds covalently? Carbon and hydrogen, two non-metals, so they share. The other two pairs each contain a metal, so their bonding is ionic. One examiner's report on a Foundation paper says: "Occasionally a mark was given for the idea of sharing electrons, but rarely for sharing a pair." In plain words, students said sharing and left out the pair. The fix is to say the whole phrase every time: a shared pair of electrons.

Before drawing anything, find how many bonds each atom makes. An atom makes one covalent bond for each electron it is short of a full outer shell. Hydrogen is one short, so it makes one bond. Chlorine and the other halogens are one short of eight, so they make one bond too. Oxygen is two short, so it makes two bonds. Nitrogen is three short and makes three. Carbon is four short and makes four. One shared pair between two atoms is a single bond. Two shared pairs make a double bond, and three make a triple bond. A molecular formula does not show the bonds. The two in O two counts atoms, not bonds or pairs. Methane is the gas in a kitchen hob, made of carbon and hydrogen. How many hydrogen atoms can bond to one carbon atom in methane? Four. Carbon is four electrons short, so it makes four single bonds, one to each hydrogen. That is why methane is C H four.

Every diagram starts with step zero: read the name in the question, and write its formula down before you draw a single electron. Then three numbers. How many electrons each atom is short. So how many bonds it makes. So how many pairs sit in each overlap. Take oxygen, O two. Each oxygen atom is two, six, so it is two short of eight. It makes two bonds, so its one overlap holds two pairs: a double bond. Draw two outer shells overlapping. One atom's six electrons are dots, the other atom's six are crosses. In the overlap go two dots and two crosses, as two pairs. Each atom keeps its other four electrons outside the overlap, as two pairs of its own. Now the check: go round each atom in turn, overlap included. How many electrons can each oxygen atom claim? Eight each. The left oxygen has four of its own outside, plus the four in the overlap, and the right oxygen is the same. Both shells are full. So oxygen is a molecule of two atoms, held together by a covalent bond. The substance is the molecule; the bond is what holds it. A report on a Foundation paper records that "in the case of oxygen many students stated that it is a covalent bond and is made up of two molecules." Students called the substance a bond, and called one molecule two. What earns credit instead is the sentence shape: a molecule of two atoms, held by a covalent bond.

Water, H two O, is on every list. Step zero, the formula: one oxygen and two hydrogens. Oxygen is two short, so it makes two bonds. Each hydrogen is one short, so it makes one. That means one single bond from oxygen to each hydrogen. Draw oxygen's outer shell with six crosses, overlapping two small hydrogen shells that hold one dot each. Each overlap holds one pair, a dot and a cross. Then the same check as before, going round every atom. How many electrons can the oxygen claim, and how many can each hydrogen? The oxygen claims eight: four of its own in two pairs, plus two in each overlap. Each hydrogen claims two, the pair it shares. That is the diagram from the start of the video. Why are eight and two both correct counts for a full shell? Because hydrogen has only one shell, and that shell is full at two. Oxygen's outer shell is full at eight. The rule is a full outer shell, not always eight. Which molecules you can be asked to draw differs from one exam to another, and the table shows which lists name which. Where no list is printed, practise the method, because any small molecule is fair. Carbon dioxide is on one of those lists, so here it is with the first steps done. Carbon is four short and makes four bonds. Each oxygen is two short and makes two. The carbon atom sits in the middle, with an oxygen atom either side. How many shared pairs go in each carbon to oxygen overlap? Two pairs each, so two double bonds. Count round, and all three atoms claim eight. One report on a Higher paper notes: "Errors included insufficient numbers of electrons involved in the two double covalent bonds, choosing instead to give single bonds between carbon and oxygen." The same report adds: "Centres could perhaps encourage candidates to do a quick check of full shells to ensure their diagrams are feasible." The bonds came out single when they had to be double. Counting round each atom is that check of full shells, and it catches the slip every time.

The count only works if you can tell whose electrons are whose. Two students drew chlorine, C l two. The first drew every electron from both atoms as a cross. The second drew two pairs in the overlap, though the model on the question paper shows a single bond. Look closely at both diagrams, one after the other. What is wrong with each, and which fault could its author never spot? The first needs dots for one atom and crosses for the other. The second needs one pair: chlorine is one short, so it makes one bond. And the first fault is the one its author could never catch. With one symbol for both atoms, there is no way to count whose electrons are whose. An examiner's report on a Higher paper, about drawing nitrogen, puts it plainly: "Some miscounted the electrons on the two N atoms because they used the same symbol for the electrons from both." The same report records that "More than half of students scored both marks" on that diagram, so this is a drawing well within reach. The habit that makes it reachable: dots for one atom, crosses for the other, and then count round every atom.

Your turn to do the counting, starting with the definition itself. What is a covalent bond, in full? A shared pair of electrons between two non-metal atoms. Now a molecule this video has not drawn. Hydrogen chloride: how many shared pairs, and how many electrons can the chlorine claim? One pair, a single bond. Chlorine claims eight: six of its own and the two it shares. Another one that has not been drawn here, the gas that makes up most of the air. Each nitrogen atom is three short. Single, double or triple bond in N two? Triple: three shared pairs in the overlap, and each nitrogen counts eight. One more, and it goes back to that rule about eight. Atoms join until each has eight. What should that rule say instead? Atoms share until each has a full outer shell: eight for oxygen, chlorine and the others here, but two for hydrogen, whose only shell is full at two.

That is the covalent bond and its diagrams, the sharing side of the chapter. If you've got this down, a thumbs up takes it off your list for good. If not, keep it saved. Nobody draws these right first time, and that is completely normal.

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