Compare ionic and covalent bonding with valence electrons, predict formulas of binary compounds, and name binary ionic and covalent compounds

Learning goal: Describe how ionic and covalent bonds form in terms of valence electrons; compare ionic compounds (three-dimensional lattices of ions) with covalent substances (molecules or, sometimes, lattices); predict from positions on the periodic table whether two elements form an ionic or a covalent compound; predict the formula and name of an ionic compound from elements in groups 1, 2 and 17, hydrogen and oxygen; and name or write formulas for binary covalent molecules using the Greek prefixes one to ten.

Why atoms bond. In middle school a compound was two or more elements joined chemically. This course explains the joining with valence electrons. Atoms bond by losing, gaining or sharing valence electrons, and a useful guide is that each atom usually ends up with a full outer shell, as a noble gas has. It is a pattern with exceptions (the sulfur atom in SF₆ and the nitrogen atom in NO₂, two molecules named later in this lesson, do not fit it), and explaining them is left for chemistry. There are two extreme models of how that happens, and real bonds fall on a continuum between them.

Ionic bonding. When a metal atom meets a nonmetal atom, the metal, with few valence electrons, gives them up, and the nonmetal, with a nearly full shell, takes them. The metal becomes a cation, the nonmetal an anion, and the ionic bond is the attraction between the two oppositely charged ions. Because an ion attracts oppositely charged ions from every direction, the ions do not pair off into separate units. They stack into a repeating three-dimensional lattice, each cation surrounded by anions and each anion by cations, extending in all directions through the crystal. There is no "molecule" of sodium chloride; the formula NaCl states the ratio of ions in the lattice, one Na⁺ for each Cl⁻, not a separate two-atom particle. The lattice explains ionic compounds' typical properties: hard, brittle crystalline solids with high melting points, which conduct electricity when melted or dissolved (the ions can then move) but not as solids.

Covalent bonding. When two nonmetal atoms meet, neither gives up electrons easily, so they share pairs of valence electrons, and each shared pair is a covalent bond. Sharing usually produces small, separate molecules with a fixed number of atoms: two hydrogen atoms share one pair in H₂; one oxygen atom shares one pair with each of two hydrogen atoms in H₂O; one carbon shares two pairs with each of two oxygens in CO₂. The formula of a covalent compound describes the actual molecule. Covalent substances are usually gases, liquids or low-melting solids and usually do not conduct electricity, because there are no free charges; graphite, a form of carbon, is a notable exception that conducts. Sharing can also build a lattice: in diamond every carbon atom is covalently bonded to four neighbors in a network that continues through the whole crystal, which is why diamond is so hard and has so high a melting point. So "ionic means lattice, covalent means molecule" is a useful first rule, but the diamond example shows the condition: covalent bonding can form either molecules or a three-dimensional lattice.

Predicting the type from the table. A metal (left side) with a nonmetal (right side) is predicted to form an ionic compound: sodium and chlorine, magnesium and oxygen, calcium and bromine. Two nonmetals are predicted to form a covalent compound: carbon and oxygen, nitrogen and hydrogen, sulfur and fluorine. Prediction from electronegativity numbers, polar covalent bonds and compounds with polyatomic ions or transition metals are left for chemistry; here the two-class rule based on position is the tool.

Predicting ionic formulas. An ionic compound has no net charge, so the ions combine in whatever ratio makes the positive and negative charges cancel. Using the group charges from the previous lesson (group 1: 1+, group 2: 2+, group 17: 1−, hydrogen 1+ with nonmetals, oxygen 2−), the ratio follows. Calcium (2+) and chlorine (1−): two chloride ions are needed for each calcium ion, so the formula is CaCl₂. Sodium (1+) and oxygen (2−): two sodium ions per oxide ion, Na₂O. Magnesium (2+) and oxygen (2−): one of each, MgO. Potassium (1+) and bromine (1−): KBr. The subscripts give the smallest whole-number ratio, so MgO is written, never Mg₂O₂. A quick check: multiply each subscript by its ion's charge and confirm the positives equal the negatives.

Naming. A standard system is needed because a name must point to exactly one substance for anyone, anywhere, in any language, and an ingredient list, a safety sheet or a prescription must not be ambiguous. Binary ionic compounds are named with the metal first, unchanged, then the nonmetal with its ending changed to -ide: CaCl₂ is calcium chloride, Na₂O is sodium oxide, MgO is magnesium oxide, KBr is potassium bromide. No prefixes are used, because the charges already fix the ratio. Binary covalent compounds need prefixes, because the same two nonmetals can form several different molecules. The Greek prefixes for one to ten are mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and deca-. Name the first element with a prefix (omit mono- on the first element), then the second element with a prefix and -ide: CO is carbon monoxide, CO₂ is carbon dioxide, N₂O₄ is dinitrogen tetroxide, SF₆ is sulfur hexafluoride, P₄O₁₀ is tetraphosphorus decoxide. The final vowel of a prefix is usually dropped before "oxide" (monoxide, tetroxide, decoxide). Going the other way, the name gives the formula directly: dichlorine heptoxide is Cl₂O₇, carbon tetrachloride is CCl₄. Water and ammonia keep their common names. Organic molecules, and compounds with polyatomic ions or transition metals, are named in chemistry, not here.

A worked model: two compounds side by side. Sodium (group 1 metal) and fluorine (group 17 nonmetal) are predicted ionic. Sodium's one valence electron is transferred to fluorine's seven, giving Na⁺ and F⁻, charges 1+ and 1−, ratio one to one: NaF, sodium fluoride, a lattice of alternating ions found in toothpaste ingredient lists. Nitrogen and oxygen are both nonmetals, so they are predicted covalent and can form several molecules; the one with formula NO₂ is named nitrogen dioxide, and the one with formula N₂O is dinitrogen monoxide. Notice the difference in what the formula means: NaF is a ratio in a crystal that has no smallest particle, while NO₂ is a three-atom molecule you could count.

Investigation: sort compounds by property and match to bond type. Materials: small labeled samples of table salt (sodium chloride), sugar (a covalent molecular solid), paraffin wax, a magnesium oxide tablet or powder, distilled water, a low-voltage conductivity tester with two probes, a heat-resistant spoon and a hot plate on low, goggles. Steps: test each dry solid for conductivity; dissolve a small amount of each in 20 mL of distilled water where it will dissolve, and test the solution; place a pea-sized amount of salt, sugar and wax on separate spoons over the low hot plate for two minutes and record which melts. Record: a table of substance, conducts as solid, conducts in solution, melts easily. A result that supports the claim: the salt does not conduct as a solid, conducts when dissolved and does not melt at low heat, matching an ionic lattice; sugar and wax do not conduct in either state and the wax melts easily, matching molecular covalent substances. Safety: goggles; do not heat above what melts wax, never heat sugar until it smokes, and keep probes out of the hot samples. What the model cannot show: these tests suggest bond type but do not prove it, and they cannot place a compound on the continuum between the two models; magnesium oxide is ionic but so poorly soluble that the solution test gives little evidence either way, a limit worth stating.

Worked example 1 — predict the type and formula. Barium and iodine: barium is a group 2 metal, iodine a group 17 nonmetal, so the compound is predicted ionic. Ba²⁺ and I⁻ need two iodide ions per barium ion to cancel: BaI₂, barium iodide. Check: (+2) + 2 × (−1) = 0. Sulfur and oxygen: both nonmetals, predicted covalent, so the formula is not fixed by charges; the molecule with one sulfur and three oxygens is SO₃, sulfur trioxide, and the one with two oxygens is SO₂, sulfur dioxide.

Worked example 2 — name from formulas. Li₂O: lithium is a metal, so ionic; lithium oxide, no prefixes. CCl₄: two nonmetals, covalent; carbon tetrachloride (mono- omitted on the first element). N₂O₅: dinitrogen pentoxide, with the a of penta- dropped before oxide. CaBr₂: calcium bromide. H₂O keeps its common name, water, but the naming rule would give dihydrogen monoxide. The first decision is always metal-plus-nonmetal (no prefixes) versus nonmetal-plus-nonmetal (prefixes).

Worked example 3 — formulas from names. Potassium oxide: K⁺ and O²⁻, two potassium per oxide, K₂O. Magnesium chloride: Mg²⁺ and Cl⁻, MgCl₂. Diphosphorus pentoxide: the prefixes give it directly, P₂O₅. Hydrogen chloride: hydrogen behaves as 1+ with chlorine's 1−, HCl. Sulfur hexafluoride: SF₆. Writing the charges first for ionic names, and reading prefixes straight off for covalent names, prevents the common error of adding prefixes to an ionic compound (there is no "calcium dichloride").

Worked example 4 — lattice or molecule, and why it matters. Sodium chloride and carbon dioxide are both binary compounds, but NaCl is a lattice of Na⁺ and Cl⁻ ions in a one-to-one ratio with no separate NaCl particle, while CO₂ exists as separate three-atom molecules. That is why salt is a hard solid that melts at a high temperature and conducts only when molten or dissolved, and why carbon dioxide is a gas at room temperature and never conducts. Diamond is the reminder that covalent bonds can also form a lattice: each carbon atom shares electrons with four neighbors in an endless network, so diamond is a very hard solid rather than a gas of small molecules.

Representation task: two particle models. Draw a particle model of a small piece of the sodium chloride lattice (at least a 3 by 3 layer of alternating Na⁺ and Cl⁻, with a note that it continues in all three directions) and a particle model of three separate water molecules, showing shared electron pairs as short lines. Under each drawing write the bond type, how the valence electrons were handled (transferred or shared), and what the formula means (a ratio in a lattice, or a count in a molecule).

Independent performance: naming and formula portfolio. Build a table of twelve compounds: six binary ionic compounds using only elements from groups 1, 2, 17, hydrogen and oxygen, and six binary covalent molecules. For each, give the two elements, the predicted bond type with the reason from the table, the formula, and the name. Success criteria: every ionic formula is charge-balanced and shown with its check sum; every covalent name uses the correct Greek prefix and drops mono- on the first element; no prefixes appear in ionic names; at least two of the twelve are found on a real ingredient label, packaging or safety sheet you have seen, with the product named; and a closing paragraph explains why a shared naming system matters, using an example of a name that would be ambiguous without it.