Molecular Compounds and Binary Molecular Nomenclature

Molecular Compounds (Covalent Compounds)

Definition: Covalent (molecular) compounds are composed of discrete, neutral molecules formed by sharing electrons (not transferring them).
Bonding focus: Covalent bonding is a major concept treated in detail later in the course.
Properties: Often exist as gases, low-boiling liquids, and low-melting solids; many exceptions exist.
Composition: Usually formed by nonmetals; periodic table helps identify covalent vs ionic in many cases, but the distinction has gray areas.
Ionic vs covalent: Ionic compounds typically form from metals and nonmetals; covalent compounds form from nonmetals; simple rules have exceptions. A simplistic periodic-table approach to predict ionic vs covalent is imperfect.
Naming difference: Nomenclature for molecular compounds differs from ionic compounds because covalent bonding allows variable composition ratios.

When two nonmetallic elements form a molecular compound, multiple combinations (e.g., CO and CO$_2$) are possible; they cannot share the same name.
Naming approach uses prefixes to specify the exact numbers of atoms of each element.
Order rule: the name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is given first.
The name of the more nonmetallic element (the one farther to the right and/or top) follows, and its ending is changed to the suffix $-ide$.
Number specification: Greek prefixes indicate the numbers of atoms. Common prefixes are:
- mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca
- When only one atom of the first element is present, the prefix mono- is usually omitted.
Vowel rule: When two vowels are adjacent, the "a" in the Greek prefix is usually dropped (e.g., penta- becomes pent- before $-oxide$ such as in pentoxide).
Examples:
- CO is named carbon monoxide.
- CO_2 is named carbon dioxide.
Common vs systematic names (examples):
- NO is often called nitric oxide, but its proper (systematic) name is nitrogen monoxide.
- N_2O is known as nitrous oxide, though the rule would give dinitrogen monoxide.
- H_2O is usually called water, not dihydrogen monoxide.
Quick takeaway: Remember the common names and apply the prefix-based naming for molecular formulas with two nonmetals; the first element is the more metallic one, the second ends with -ide, and prefixes denote quantity.

First element: name the more metallic element (left/bottom) first; second element: more nonmetallic (right/top) second with -ide suffix.
Use Greek prefixes to indicate the number of atoms of each element.
Omit the mono- prefix for the first element if only one atom is present.
If two vowels come together, drop the 'a' in the prefix (e.g., penta- → pent- in pentoxide).