Metal atoms hand their outer electrons to non-metal atoms, and the ions that result pull on each other in a giant lattice. Work out ionic charges and formulas from the group numbers.
A phone comes out of the crate with a white crust on its connector, left behind after seawater got in. The crust is salt. Last week the club could say what an atom does on its own: a group 1 metal hands its outer electron over, a group 7 non-metal takes one in. It never said where that electron ends up. Put the two in the same beaker and the answer is obvious. The metal's spare electron goes to the non-metal, which needed exactly one. Both end with full outer shells, and both end up charged.
Sodium is 2,8,1 and chlorine is 2,8,7. Sodium hands its single outer electron to chlorine. Sodium is left as 2,8, with 11 protons against 10 electrons, so it carries 1+. Chlorine becomes 2,8,8, with 17 protons against 18 electrons, so it carries 1-. Chemists draw this as a dot-and-cross diagram: each atom's shells are rings round its symbol, one atom's electrons are dots and the other's are crosses, and each ion's charge is written outside its ring. After the transfer the chloride's outer ring shows seven crosses of its own and one dot from sodium.
You never have to work a charge out atom by atom. The group number gives it. Group 1 loses one, so 1+. Group 2 loses two, so 2+. Group 3 loses three, so 3+. Group 7 gains one, so 1-. Group 6 gains two, so 2-. Group 5 gains three, so 3-. Written down they are Na+, Mg2+ and O2-, with the charge outside the symbol. Metals go positive because they lose and non-metals go negative because they gain. A non-metal ion takes an -ide name, so chlorine gives chloride and nitrogen gives nitride.
A compound carries no overall charge, so the charges have to cancel. Sodium chloride is the easy case: 1+ and 1- cancel one for one, giving NaCl. Magnesium has two electrons to give away, so watch where both of them go. In magnesium chloride one goes to each of two chlorine atoms, giving MgCl₂. In magnesium oxide both go to the same oxygen, which needed exactly two, giving MgO. Aluminium oxide is harder again. Aluminium is 3+ and oxide is 2-, so two aluminium ions at 6+ meet three oxide ions at 6-, and the formula is Al₂O₃.
The ions do not pair off and drift away. Opposite charges pull on each other, and the bigger the two charges the harder that pull, so every positive ion attracts every negative ion around it and the whole thing locks into a giant regular lattice. Melting it means beating all those attractions at once, which is why sodium chloride stays solid until 801 degrees. In the solid the ions are pinned in place, so nothing charged can move and no current flows. Melt it, which chemists call making it molten, or dissolve it in water, and the ions come free to move. Only then does it conduct.
A metal hands its outer electrons to a non-metal, and both end with full outer shells and opposite charges. The group number gives the size of each charge. The formula is whatever ratio makes the charges cancel. The ions then lock into a giant lattice, which is why an ionic solid melts so high and only conducts once melted or dissolved.
The white crust on the phone connector is still on the bench. Last week the club could say what each atom wants on its own. Today it has to say where the electron actually goes.
Sodium is 2,8,1 and chlorine is 2,8,7. Which way does the electron travel?
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.