Compounds

Most elements are written as a bare symbol, one atom at a time: sodium is Na, copper is Cu, argon is Ar. Seven are never found that way. In nature they come in pairs, and the pair is what you write.

H₂   N₂   O₂   F₂   Cl₂   Br₂   I₂

Learn these seven. They sit in a rough 7 on the table, with hydrogen tacked on. Write them as pairs whenever you mean the pure element, so chlorine gas is Cl₂ and not Cl.

A few elements go further and bond in larger groups. Sulfur comes as a ring of eight, S₈, and white phosphorus as a cluster of four, P₄.

This is the trap in the book's first question of the section: O and O₂ are not the same thing on paper. O is the element, and one atom of it. O₂ is a molecule with two atoms in it. Sulfur tempts you into writing S₂ by the same pattern, and sulfur is S₈.

• The seven diatomics, with a drill

Sucrose is the reminder that plenty of formulas are already in lowest terms. Nothing but 1 goes into 12, 22 and 11 together, so the two formulas are the same, and that is a real answer.

Going the other way is not possible on its own. CH₂O is the empirical formula of glucose, C₆H₁₂O₆, and also of acetic acid, C₂H₄O₂. The ratio does not tell you which molecule you have.

Ionic compounds only ever get written as the empirical formula, because there is no molecule to count. NaCl means one sodium for every chlorine, not a pair sitting together.

Structural isomers are joined up differently. Propanoic acid and methyl ethanoate are one pair. C₂H₆O is another: ethanol, with its oxygen on the end, and dimethyl ether, with the oxygen in the middle.

Spatial isomers are joined up the same way and sit differently in space. L‑alanine and D‑alanine are mirror images that cannot be laid on top of each other, the way your hands cannot.

Not every formula has an isomer. Two carbons and six hydrogens can only be put together one way, so C₂H₆ is ethane and nothing else.

• Isomers, drawn and compared

A cation is positive, made by losing electrons, and keeps the element's name: the calcium ion. An anion is negative, made by gaining them, and takes an -ide ending: the chloride ion.

Metals are on the left and have few electrons to shed, so they go positive. Nonmetals are on the right and are close to a full count already, so they go negative. Group 8A is already there and forms no ions worth naming.

This is a main group rule only. The transition metals in the B groups do not have one predictable charge, which is exactly why their names carry a Roman numeral.

• Polyatomic ions, where the charge sits on a group of atoms

Opposite charges pull on each other, and that pull is the ionic bond. It does not join two ions into a private pair. Every sodium ion in table salt is surrounded by chloride ions, every chloride by sodium ions, and the pattern repeats in all three directions for as far as the crystal goes.

So there is no molecule of NaCl to point at. What the formula gives is a formula unit: the smallest whole number ratio of the ions, and enough of each to make the charge come out to zero.

Say formula unit for ionic compounds and molecule for covalent ones. The exam has asked which word belongs.

The subscript is a count, not a charge. The 2 outside the bracket in Sr(H₂PO₄)₂ says there are two dihydrogen phosphate ions. Each one still carries 1−. Writing H₂PO₄²⁻ because of that 2 is the mistake the question is set to catch.

Work it from the side you know. Strontium is group 2, so it is Sr²⁺; the compound is neutral, so the two anions have to carry 2− between them, which is 1− each.

An ion made of one atom, like Ba²⁺ or O²⁻, is a monatomic ion. One built from a central atom with oxygens around it, like CO₃²⁻ or SO₄²⁻, is an oxyanion, and that is the family the -ate and -ite endings name.

That water is not held tightly. Heat the crystal and it leaves as steam, and what is left is the anhydrous compound. Copper(II) sulfate is the one everybody remembers: blue with its five waters, white without them.

The prefixes are the same ones covalent names use, mono through deca, and here the mono is kept: a compound with one water is a monohydrate.

Wednesday's lab is the dehydration of a hydrate, where the water driven off is weighed to find how many went with each formula unit. Worth having the dot straight before then.

• Lab 4, dehydration of hydrates

Two nonmetals have no reason to hand electrons over, because neither one is inclined to give them up. Instead they share a pair, and the shared pair is the covalent bond. What it makes is a real molecule, a fixed group of atoms with a size and a shape.

H₂O   CH₄   CO   CO₂

Because nothing has to cancel, the formula cannot be worked out from charges. There is no rule that says carbon and oxygen must meet one to one, and so CO and CO₂ both exist and are different gases.

That is why covalent names carry prefixes and ionic names do not. The prefix is carrying the information that the charges would have carried.

Conducting is the careful one. A solid ionic compound does not conduct: the ions are locked in the lattice and nothing can move. Melt it, or dissolve it, and it conducts well, because now the ions can travel.

So the flat sentence "ionic compounds conduct electricity" is marked wrong. The one that is right says molten or dissolved.

Covalent compounds have no ions to move at all, so they generally do not conduct in any state. These are strong tendencies rather than laws, and the book hedges them the same way.

A full d subshell is hard to break into. Taking a further electron out of silver's or zinc's 4d or 3d costs far more energy than any bond gives back, so there is no second charge to choose between.

Aluminum is fixed for the plainer reason: it is a main group metal with three outer electrons, and losing all three leaves it with neon's electron count. That is the same rule that fixes sodium at 1+ and magnesium at 2+.

Cadmium sits directly under zinc, [Kr]4d¹⁰5s² down to [Kr]4d¹⁰, and behaves the same way. It is not one of the three to memorise, but its name takes no numeral either.