Two non-metal atoms fill their outer shells by sharing electron pairs. The shared bond is strong, but the pull between separate molecules is weak, which sets the properties of simple molecular substances.
Last week ended on a question the club could not answer. Sodium handed its electron to chlorine because one side wanted to lose and the other to gain. Put two chlorine atoms together and nobody wants to lose. Both are one electron short. Neither will be the giver, and yet chlorine gas exists, and so does the water that got into the connector in the first place. The way out is not to move an electron across but to share it, so that a single electron does two jobs at once.
A hydrogen atom has one electron and needs two to fill its first shell. Two hydrogen atoms each push one electron into the space between them, and from then on both count that pair as their own. A shared pair like this is a covalent bond. Draw it dot-and-cross the way you did last week: one atom's electron is a dot, the other's is a cross, and the pair sits in the overlap where the two rings cross. Two chlorine atoms do the same. Each is 2,8,7, each puts in one, and each ends up counting eight.
For a non-metal in groups 4 to 7, the number of bonds it makes is 8 minus its group number, since that is its shortfall. Group 7 makes one, group 6 two, group 5 three, group 4 four. Hydrogen is the exception: its shell is full at 2, so it makes one. Oxygen is 2,6, so it makes two bonds, which is why water is H₂O. Oxygen brings six outer electrons, two go into those bonds, and the four left over sit as two pairs that bond nothing, called lone pairs. Carbon is 2,4 and makes four, which is why methane is CH₄.
Two oxygen atoms break the pattern. Each one is two electrons short, and a single shared pair only fixes one of those. So the two atoms share two pairs instead, putting four electrons into the overlap, and both then count eight. Chemists call that a double bond and draw it as two lines rather than one. A double bond is shorter and stronger than a single bond. Carbon dioxide does the same thing twice over: the carbon shares two pairs with one oxygen and two pairs with the other.
This is the part most people get backwards. The covalent bond inside a molecule is strong. The pull between one whole molecule and the next is weak. Boiling water at 100 degrees drags the molecules away from each other; it does not break a single bond inside any of them. A substance made of small separate molecules like these is called simple molecular, and that weak between-molecule pull is why it melts and boils so much lower than sodium chloride, which needs 801 degrees. Since a molecule carries no charge and holds no free electrons, nothing can carry a current, solid, liquid or gas.
Two non-metals cannot both take, so they share instead. One pair of electrons in the overlap is a covalent bond, and both atoms count it. For a non-metal in groups 4 to 7, the number of bonds it makes is 8 minus its group number. When one pair is not enough, atoms share two and make a double bond. The bond inside a molecule is strong, the pull between molecules is weak, and that split explains both the low melting points and the fact that nothing conducts.
The salt crust from last week is still on the bench, and so is the water that made it. Salt was a transfer. Water is not, and neither is the chlorine in the cylinder.
Two chlorine atoms, both 2,8,7. Which one hands an electron to the other?
Jobs that lean on this skill. Follow one to see everything it is built on.
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.