4. Ionic and Covalent Compounds

Water molecules are polar: each has a slightly negative oxygen end and slightly positive hydrogen ends. They crowd round the ions at the surface of the crystal, and the pull of many water molecules is enough to pry them away one at a time. Each ion is then surrounded by water and moves independently. This is the subject of the Aqueous Reactions unit.

Sugar is held together by covalent bonds, which are not broken by water. The molecules separate from each other and spread out through the water, but each stays whole and neutral. Dissolving and ionizing are different things.

HCl is a covalent molecule, but a very polar one, and water is able to pull it apart into H⁺ and Cl⁻. So it is a molecular compound that becomes ionic when it dissolves. Pure HCl gas does not conduct; the solution does. The weaker acetic acid in vinegar splits only a little, so it conducts only a little.

No. Metals conduct because their outer electrons are free to move through the whole piece, not because of ions moving. A metal conducts when solid, which ionic compounds do not. That is one of the ways to tell the two kinds apart.

In a crystal of salt each Na⁺ is surrounded by six Cl⁻ and each Cl⁻ by six Na⁺, repeating in every direction. There is no pair to point to. The formula NaCl is a formula unit: the ratio of ions in the crystal.

For the main groups, it comes from the nearest noble-gas configuration, which you saw in the last unit: group 1 gives 1+, group 2 gives 2+, group 13 gives 3+, group 17 gives 1−, group 16 gives 2−, and group 15 gives 3−. Transition metals vary, which is why their names carry a Roman numeral.

The bracket is only needed when you want more than one of a polyatomic ion, so that the subscript applies to the whole ion. CaSO₄ has one sulfate; Ca(NO₃)₂ has two nitrates. Writing CaNO₃₂ would mean something different.

NH₄⁺ is a polyatomic cation with a charge of 1+. It behaves like sodium in a formula. Try it with sulfate: (NH₄)₂SO₄, two ammonium for one sulfate.

No. Melting separates molecules from each other, and the bonds inside each molecule stay put: it is still H₂O afterwards. The forces between neutral molecules are much weaker than those between ions, which is why molecular solids melt so readily. These forces are the subject of the last unit.

Water molecules are polar and pull on each other more strongly than methane's nonpolar molecules do. Even between molecules the strength varies a lot. But the whole molecular range, from −182 to +80 °C here, is small beside the ionic range.

Coulomb's law says the force between charges grows with the product of the charges. A 2+ and a 2− attract four times harder than a 1+ and a 1− at the same distance. The experiment after next makes this quantitative with a lattice energy.

They are read by a script from Wikipedia's data for each compound, which cites the CRC Handbook of Chemistry and Physics and other references. Sodium chloride is 800.7 °C there, for example.

Ions on the surface of the crystal have fewer neighbours than ions inside. In a small crystal most ions are on the surface, so the average is low. As the crystal grows, the fraction on the surface shrinks, and the average approaches the value for an ion in the middle of an endless crystal: 1.7476 pairs-worth.

Round one ion there are 6 opposite charges at distance 1, 12 like charges at distance √2, 8 opposite at √3, and so on, alternating. The terms get smaller only slowly, so the running total swings above and below the answer before settling. Evjen's trick of counting ions on the edge of the block as a fraction makes it converge quickly: the test suite checks it reaches 1.7476.

It is half the length of the edge of the NaCl unit cell, 564 pm, from a table of unit-cell sizes in Wikipedia's article on ionic radius. It is the centre-to-centre distance between neighbouring Na⁺ and Cl⁻ ions.

It is nearly all of it. The lattice energy is the energy released when gaseous ions form the crystal, and this Coulomb sum is the main part. A small correction (the ions push back when pressed too close, the Born repulsion) trims about 12%: that gives −753 kJ/mol for NaCl, within 4% of the value from experiment, −786.

The planes of the lattice that line up when you slide by one ion are the planes where like charges meet. Real crystals of salt cleave along flat faces, which is why a grain of salt is a tiny cube.

No. They are hard and have high melting points because it is very hard to pull ions apart or push them together. Brittle describes only what happens when they are sheared. A crystal can be strong and brittle at once.

By adding up Coulomb's law for every pair of one ion from the top block and one from the bottom, for blocks two ions deep and nine wide. The meter shows the sum, and the numbers, like those in the previous experiment, are a model, in units of e²/(4πε₀r₀).

You cannot gather gaseous ions and watch them fall into a crystal. So chemists measure everything else in the cycle (the enthalpy of formation, the energies to make atoms, ionization energy and electron affinity) and use Hess's law to find the one missing piece. That is the Born–Haber cycle, from 1919.

Ionization energies and electron affinities are from the table of elements used throughout this course. The energies to make gaseous atoms from the elements in their standard states, and the enthalpies of formation of the salts, were read by a script from the NIST Chemistry WebBook (the Cox, Wagman et al. CODATA values). The twelve lattice energies are from Wikipedia's lattice-energy article, citing Shriver and Atkins.

Chlorine releases 349 kJ/mol when it gains an electron, but removing an electron from sodium costs 496 kJ/mol. Each step alone is not in the salt's favour. It is the lattice, with +1 and −1 charges strongly attracted at short range, that tips the balance.

Every compound has its own accounting. A hypothetical MgCl would cost less to make (one fewer electron) but its lattice would be much weaker than MgCl₂, where Mg²⁺ pays a larger ionization cost and gets back a much bigger lattice energy. Real compounds are the ones where the net energy comes out lowest.

Sodium and chlorine can only make NaCl, because the ion charges fix the ratio. Carbon and oxygen can share electrons in different numbers, so they make both CO and CO₂, and nitrogen and oxygen make N₂O, NO, NO₂, N₂O₄ and N₂O₅. The prefix says which.

A few molecules kept their old names: H₂O is water, NH₃ is ammonia, CH₄ is methane. They are not formed by the rules, so learn them as exceptions. By the rules H₂O would be "dihydrogen monoxide".

Chlorine makes four oxyanions with different amounts of oxygen: ClO⁻ hypochlorite, ClO₂⁻ chlorite, ClO₃⁻ chlorate, ClO₄⁻ perchlorate. The acids are hypochlorous, chlorous, chloric and perchloric acid. The name tracks the number of oxygens, in that order.

As a gas it is a molecule and is named hydrogen chloride. Dissolved in water it releases H⁺ and is called hydrochloric acid. The name changes with the state, which is why the conductivity of HCl in water was so different from pure HCl.