Ionic bonding and dot-and-cross diagrams

The old orange street lights glowed because of sodium, and table salt is sodium chloride. Is the sodium in the lamp the same thing as the sodium in salt? The tempting answer is yes. Sodium is sodium, so it must be the same stuff in both. It is the same element: every sodium nucleus, in the lamp and in the salt, holds eleven protons. But the particle is different. The lamp glows with sodium atoms, each holding eleven electrons. The sodium in salt holds only ten, and one missing electron turns a glowing vapour into something you eat. Where that eleventh electron went, and why it went, is what ionic bonding is about.

An ion is an atom that has lost or gained electrons, so its charges no longer balance. An atom has as many protons as electrons. Protons are positive, electrons are negative, so the charges cancel and the atom is neutral. Take one electron away and there is one more proton than electrons. The particle is now a positive ion, with a charge of one plus. Add one electron instead and there is one more electron than protons. That makes a negative ion, with a charge of one minus. The nucleus never changes. The protons stay where they were, so a sodium ion is still sodium: eleven protons, one electron fewer. All of this, and the drawing that follows, is on the Foundation and the Higher papers. Now test that on a particle you can count for yourself. Seventeen protons and eighteen electrons: atom, positive ion or negative ion? A negative ion, charge one minus, because there is one extra electron. Seventeen protons still makes it chlorine, so this is a chloride ion.

Ionic bonding starts with a transfer. A metal atom hands its outer electrons to a non-metal atom, and both end up with a full outer shell, like a noble gas. Sodium is two, eight, one. Its outer shell holds a single electron. Chlorine is two, eight, seven. Its outer shell is one electron short of eight. So the one outer electron leaves the sodium atom and joins the outer shell of the chlorine atom. It leaves one shell and arrives in the other, and it is never in both. Sodium is left as two, eight: a sodium ion, charge one plus. Chlorine becomes two, eight, eight: a chloride ion, charge one minus. The negative ion's name ends in i d e. That transfer is only the first half. It makes the ions, and the ions are not yet the bond. The ionic bond is the strong electrostatic attraction between oppositely charged ions. Electrostatic means the pull between electric charges. The positive sodium ion and the negative chloride ion pull on each other, and that pull is the ionic bond. Three students each wrote one line. The first: electrons shared between two atoms. The second: an electron transferred from a metal to a non-metal. The third: the attraction between oppositely charged ions. Which of those three lines names the ionic bond itself? The third line, the attraction. The transfer is how the ions get made; the bond is what holds them together afterwards. The first line describes a different bond altogether. One examiner's report on a Foundation paper says: "there were just a tiny number who knew that the ionic bond resulted from the attractive force between ions." In plain words, many answers described the transfer, and very few went on to name the attraction. The way to finish an ionic answer is the attraction sentence, every time. That first line, sharing, is covalent bonding, where two non-metal atoms share a pair of electrons. Ionic bonding never shares; its word is transferred. The same report adds: "Many contradicted themselves by talking about sharing the electrons and then talking about transfer." One answer used both words. Pick transferred for ionic, and keep sharing out of it.

Every ion's charge can be read off the periodic table, from the group its element sits in. A group 1 metal has one outer electron to lose, so it forms a one plus ion. A group 2 metal loses two and forms a two plus ion. A group 7 non-metal is one electron short, so it gains one and forms a one minus ion. A group 6 non-metal gains two and forms a two minus ion. Questions on these diagrams mostly use metals from groups 1 and 2 with non-metals from groups 6 and 7. Where another pair turns up, the method is identical. A compound has no overall charge, so the plus charges and the minus charges have to cancel. That decides how many of each ion you need. Now magnesium chloride. Each chlorine atom has room for only one more electron. How many chlorine atoms does one magnesium atom need, and why? Two. Magnesium is in group 2, so it has two outer electrons to hand over, and each chlorine takes one. Two plus is cancelled by two lots of one minus.

A dot-and-cross diagram shows where every outer electron came from. One element's electrons are drawn as dots, and the other element's as crosses. Two rules come before any drawing. Draw only each atom's outer shell, and draw the atoms first, then the ions. First, the atoms. Magnesium is two, eight, two, so its outer shell carries two dots. Each chlorine is two, eight, seven, so each outer shell carries seven crosses. Then the move. One dot leaves magnesium and lands in the first chlorine's shell. The other dot lands in the second chlorine's shell. Magnesium has lost its whole outer shell. It is drawn as its symbol inside square brackets, with the charge, two plus, outside the bracket at the top right. Each chloride ion now has eight in its outer shell: seven crosses and one dot. Each goes in square brackets, with one minus outside. One magnesium ion and two chloride ions, with every electron accounted for. That is magnesium chloride. Every finished diagram gets four checks. Each ion has a full outer shell. Each electron is drawn in one place only. Each ion shows its charge. And the charges add up to zero. One examiner's report on a Higher paper notes: "Few students scored all the marks as they did not draw the atoms of sodium and oxygen before reaction." It goes on: "Students should be aware that electrons cannot be in two places at the same time and should not be drawn as if they are." Students skipped the atoms, and drew a moved electron in both places. The habit that fixes both is to draw the atoms before the ions, every time.

A question can also ask for the transfer in words, with no drawing at all. Calcium and fluorine react. State which one loses electrons and which one gains, how many electrons move altogether, and the charge on each ion. Use the periodic table, and give all four parts of the answer. Try it on paper before I show you the answer. Calcium, a group 2 metal, loses two electrons and becomes a calcium ion, two plus. Each fluorine, in group 7, gains one and becomes a fluoride ion, one minus. So one calcium needs two fluorines, and two electrons move altogether. One report on a Higher paper puts it this way: "They often identified there was a transfer of electrons but did not show which element, metal or non-metal, lost or gained the electrons." The direction was left out. The habit to build is naming the element that loses and the element that gains, in the same sentence. Here is a student's diagram for potassium chloride. The moved electron is drawn in both the potassium shell and the chlorine shell. One ion is labelled proton, and the other is labelled electron. Three separate things are wrong with it. Find all three, and say what each should be. Find all three faults in the diagram. First, the transferred electron has to leave the potassium shell completely; it can only sit in the chloride ion. Second, the positive particle is a potassium ion, a positive metal ion, not a proton. Third, the negative particle is a chloride ion, not an electron. Atom before, ion after, electron for the thing that moved, and the proton never moves.

Four questions to close, and two of them use elements this video has not drawn. What is the ionic bond? Give the full description. It is the strong electrostatic attraction between ions of opposite charge. Now a metal from the very first group of the table. Lithium oxide: how many lithium ions for each oxide ion? Two. Lithium forms a one plus ion and oxygen forms a two minus ion, so it takes two lithium ions to cancel one oxide ion. Here's one that only needs the group number. Sulfur is in group 6. What charge does a sulfide ion carry? Two minus. Sulfur is two electrons short of a full outer shell, so it gains two. One more, back under the orange street light. A sodium atom in the lamp, a sodium ion in salt: what is the same, and what differs? Same eleven protons, so the same element. The ion has given one electron away, ten left, so it carries a charge of one plus.

That one electron is where the ionic side of the chapter begins, with the lattice of these ions still to come. If you own this one, press the thumbs up and it is off your list. If the drawing is wobbly, save it, then draw magnesium chloride out once on paper; doing it by hand is what makes it stick.

Next in the chapter: The ionic lattice. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh four, oh three.

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