Atoms, Molecules, and Ions

This is Unit Two of Chemistry, Part Three: bonding and naming. Two bond types open the part, and the distinction is sharper than it sounds. An ionic bond is electrostatic attraction between two ions that already carry opposite charges. A covalent bond is attraction between two nuclei and a pair of electrons sitting between them. One is not a weaker version of the other. Predicting which you get has a guideline — metal with nonmetal gives ionic, two nonmetals give molecular — and the book states plainly that the guideline has real exceptions. Aluminium chloride pairs a metal with a nonmetal and behaves as a molecular compound. Then charges. Group number predicts an ion's charge well at the table's edges and poorly toward its centre, where many metals form more than one stable charge. That ambiguity is what the whole Roman-numeral naming system exists to resolve — iron chloride names two different compounds until you say which iron. The rest is naming: ionic compounds, molecular compounds by prefix, hydrates, acids, and the handful of common names that override every rule. The capstone then runs an unknown solid all the way to a complete name. By the end you will be able to do five things. One: tell the two bond types apart. Two: say when group number predicts a charge. Three: write a formula by charge balance. Four: say when a name needs a numeral. Five: name an acid from its anion.

Three entries. Two bond definitions and a procedure for predicting which one a compound has. Watch that the procedure's own warning names a counterexample — the book does not present this as a rule.

Opposite charges.

Chem 0288 Ionic Bond

An ionic bond is the electrostatic attraction holding a cation and an anion of opposite charge together within a compound — in sodium chloride each sodium ion is held to surrounding chloride ions purely by that attraction. An ionic bond is not a shared electron pair; the attraction is between two ions that already formed.

A shared pair.

Chem 0291 Covalent Bond

A covalent bond is the attractive force between two positively charged nuclei and a shared pair of electrons positioned between them — in a hydrogen molecule both nuclei are held by one shared pair. Shared does not mean split evenly in every case; how unevenly two atoms can share a pair comes later.

Which one forms.

Chem 0294 Predicting Ionic vs. Molecular Bonding from the Periodic Table

The procedure checks whether a compound's elements combine a metal with a nonmetal, or two nonmetals with each other. Potassium iodide pairs a metal with a nonmetal, so it is predicted ionic. Hydrogen peroxide pairs two nonmetals, so molecular. Lithium carbonate pairs a metal with a polyatomic ion, so ionic again, as does magnesium oxide — matching its high-melting, crystalline behaviour. The metal-plus-nonmetal guideline is a simplification with real exceptions: aluminium chloride pairs a metal with a nonmetal and behaves as a molecular compound.

Three entries. The table predicts charges reliably at its edges, stops predicting them toward the middle, and some ions are not single atoms at all. The middle entry is why half the naming system exists.

From the group.

Chem 0297 Predicting Monatomic Ion Charge from Group Number

Main-group metals tend to lose electrons to match the nearest preceding noble gas, and nonmetals gain electrons to match the nearest following one — calcium in group two loses two, bromine in group seventeen gains one. The trend weakens toward the table's centre, where transition metals often form charges the pattern cannot predict.

More than one charge.

Chem 0300 Variable-Charge Metals

Many transition metals and some main-group metals form more than one stable cation charge, unpredictable from periodic-table position alone — iron forms two different charges and copper two more, each stable in different compounds. A name like iron chloride is genuinely ambiguous here, since two different compounds answer to it.

A charged group.

Chem 0303 Polyatomic Ion

A polyatomic ion is a group of covalently bonded atoms carrying an overall charge and acting as one unit — sulfate moves through a formula and a reaction as a bonded group, never splitting into loose oxygen atoms. Parentheses around such an ion multiply the whole bonded group, not the atom nearest the subscript.

Two procedures. The first names a family of related anions by relative oxygen count, and the second uses ions of known charge to build a formula that balances. The first entry's warning is one of the most useful in the unit.

A family of anions.

Chem 0306 The -ate/-ite/per-/hypo- Oxyanion Naming Pattern

The pattern uses one suffix for the most common member of an oxyanion series, another for one fewer oxygen, and two prefixes for one more or two fewer still. Chlorine's four run from perchlorate at the top through chlorate, chlorite and hypochlorite at the bottom. The same suffix pair applies to sulfate and sulfite, or nitrate and nitrite. The counts are relative, not absolute — assuming a suffix always means four oxygens because chlorate's series works that way fails immediately on sulfate or nitrate.

Making them cancel.

Chem 0309 Writing Formulas for Ionic Compounds by Charge Balance

Writing an ionic formula means choosing whole-number counts of each ion so total positive charge exactly cancels total negative. Aluminium at three plus with oxide at two minus needs two aluminium against three oxide, six charges each way. Calcium at two plus with phosphate at three minus needs three calcium against two phosphate, with parentheses marking phosphate as a repeated unit. Sodium with sulfide needs two sodium and no parentheses, since sulfide is a single atom. Omitting parentheses where an ion repeats obscures which atoms belong to it.

Four entries. A base rule, the addition that variable-charge metals force, a retired system that still turns up on labels, and hydrates. The third is a twin entry — the retired form is not a valid alternative, though it is still printed.

The base rule.

Chem 0313 Naming Binary Ionic Compounds with Fixed-Charge Cations

A binary ionic compound with a fixed-charge cation is named by stating the metal's name, then the nonmetal's name with its ending changed to -ide — sodium and chlorine give sodium chloride. This works only when the metal's charge is fixed and unambiguous; variable-charge metals need a charge marker added.

Adding the charge.

Chem 0316 Naming Ionic Compounds with Variable-Charge Cations (Stock System)

A variable-charge metal's compound is named by adding its charge as a Roman numeral in parentheses right after the metal's name — iron two chloride and iron three chloride distinguishing two otherwise identical-sounding compounds. The numeral states the metal ion's own charge, never the compound's subscript count and never the nonmetal's charge.

The older form.

Chem 0317 The Retired -ic/-ous Naming System

An older system used one suffix for a metal's higher charge and another for its lower charge instead of a Roman numeral — stannous fluoride on a toothpaste tube names what current nomenclature calls tin two fluoride. The system still appears on commercial labels, but it is retired practice rather than a valid alternative to Roman-numeral naming.

Water included.

Chem 0320 Naming Ionic Hydrates

A hydrate's name adds a Greek numerical prefix plus hydrate to the anhydrous compound's name, stating how many water molecules are bound per formula unit — copper two sulfate with five waters is copper two sulfate pentahydrate, written with a dot. Anhydrous names the compound with the water removed, which is the reference point the hydrate's name is built from.

Three entries, and the system changes entirely here. Ionic names infer subscripts from fixed charges; molecular names state them with prefixes. Then a handful of compounds that answer to neither system.

The prefixes.

Chem 0323 Numerical Prefixes for Molecular Compound Names

Naming a molecular compound's atom counts uses Greek prefixes on each element, with mono- usually dropped from the first element only. Carbon dioxide drops mono- from carbon and keeps di- on oxygen; carbon monoxide keeps mono- on the second element. Dinitrogen pentoxide keeps di- on nitrogen because nitrogen is not the element being left unmarked. Tetraphosphorus decaoxide keeps both. Dropping mono- from the second element instead, or everywhere out of habit, turns carbon monoxide into a name that no longer distinguishes it from carbon dioxide.

The order.

Chem 0327 Naming Binary Molecular Compounds

A binary molecular compound is named with the more metallic element first and the nonmetallic element second ending in -ide, each carrying a prefix — sulfur hexafluoride puts sulfur first with no prefix needed and fluorine second with its own. Unlike ionic naming, the subscripts here are stated through prefixes rather than inferred from fixed ion charges.

Rules overridden.

Chem 0330 Common Names That Override Systematic Rules

A small set of molecular compounds keep a traditional common name instead of the one their own systematic rules would predict — nitrous oxide is called that in practice, where the prefix rules alone would give something else. Water is the most extreme case: nobody uses its systematic name outside a joke about the name itself.

Two entries, and which pattern you use depends on whether the acid carries oxygen. Both carry a limit worth keeping: one applies only in water, and the other has accepted irregulars.

Without oxygen.

Chem 0333 Naming Binary Acids

A binary acid — hydrogen dissolved in water with one other nonmetal — is named with a hydro- prefix, the nonmetal's root, and an -ic acid ending, so hydrogen chloride dissolved in water is hydrochloric acid. The hydro- prefix applies only to the acid in water; the same compound as a gas keeps its ordinary name.

With oxygen.

Chem 0336 Naming Oxyacids

An oxyacid's name drops hydrogen, takes the parent oxyanion's root, changes the -ate ending to -ic or the -ite ending to -ous, and adds acid — carbonate becomes carbonic acid. Sulfuric and sulfurous acid are named irregularly rather than following the root exactly, and are among a handful of accepted exceptions.

One capstone taking an unlabelled solid to a complete name, using nearly every entry in the part. Its warning is about a shortcut: assuming a one-to-one ratio out of habit produces a formula whose charges do not balance.

The capstone. An unknown solid.

Chem 0340 Capstone: Classifying and Naming an Unknown Ionic Compound

A chemist finds an unlabelled crystalline solid containing chromium, sulfur, oxygen and water. The sulfur and oxygen in a one-to-four ratio identify the sulfate ion. Chromium is a transition metal, so its charge is not predictable from position. Analysis gives a two-to-three cation-to-anion ratio: three sulfates carry six negative charges, so two chromium ions carry six positive, making each chromium three plus. That gives the anhydrous formula with parentheses around sulfate, and the name chromium three sulfate. Six bound waters make it the hexahydrate. Guessing a one-to-one ratio out of habit gives a formula whose charges do not balance at all.

Five things to carry out of Part Three, and out of Unit Two. An ionic bond is electrostatic attraction between two ions already carrying opposite charges. A covalent bond is attraction between two nuclei and an electron pair shared between them. To predict which a compound has, check whether it pairs a metal with a nonmetal, giving ionic, or two nonmetals, giving molecular — a guideline with real exceptions rather than a rule. Group number predicts ion charge at the table's edges, where main-group metals lose electrons to match the preceding noble gas and nonmetals gain them to match the following one. It fails toward the centre, where many metals form more than one stable charge — which is exactly the ambiguity Roman-numeral naming exists to resolve. To write an ionic formula, choose whole-number counts of each ion so the total positive charge exactly cancels the total negative. Where a polyatomic ion repeats, put it in parentheses so it is clear which atoms belong to the repeated unit and which do not. A name needs a Roman numeral when the metal's charge is variable, and the numeral gives that ion's charge rather than any subscript. A molecular compound instead takes Greek prefixes stating each element's atom count, with mono- dropped from the first element only. And an acid is named from its parent anion. Without oxygen, use the hydro- prefix and an -ic acid ending, and only for the acid in water. With oxygen, drop hydrogen, take the anion's root, and turn -ate into -ic or -ite into -ous — with sulfuric and sulfurous accepted as irregular. Unit Three turns to composition, solutions, and stoichiometry.