Difficulty: Introductory | Prerequisites: Basic understanding of elements, periodic table (nonmetals)
Up to this point, the compounds covered in Lab 9 have all been ionic: a metal transfers electrons to a nonmetal. Molecular (covalent) compounds are different. They form when two nonmetals share electrons rather than transferring them. Because there are no ions involved, the naming system is entirely different: instead of using metal names and charge balancing, molecular compound names use Greek prefixes to specify exactly how many atoms of each element are present. This is the other half of the naming toolkit you need for general chemistry.
Molecular compounds are made of two nonmetals sharing electrons. Name them using Greek prefixes (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-) to indicate the number of each atom. The second element always gets the "-ide" ending. No charges, no criss-crossing, no Roman numerals.
Molecular compound (covalent compound)
A compound formed when two or more nonmetals share electrons through covalent bonds. No ions are formed.
In simple terms, instead of one atom stealing electrons from another, they share. Water (H₂O) is the most familiar example.
Covalent bond
A chemical bond formed by the sharing of one or more pairs of electrons between two atoms.
Think of it as: two atoms holding onto the same electrons at the same time.
Greek prefixes (numerical prefixes)
The naming system for molecular compounds uses Greek-derived prefixes to state the number of atoms of each element:
mono- = 1 (often dropped for the first element)
di- = 2
tri- = 3
tetra- = 4
penta- = 5
hexa- = 6
hepta- = 7
octa- = 8
nona- = 9
deca- = 10
In simple terms, the prefix is just the Greek word for the number of atoms.
Name the first element using a Greek prefix only if there is more than one atom of it. (Mono- is dropped for the first element.)
Name the second element with a Greek prefix (always, including mono- if needed) and change its ending to "-ide."
When a prefix ends in "a" or "o" and the element name begins with "a" or "o," drop the trailing vowel from the prefix for easier pronunciation (e.g. "monoxide" not "monooxide").
Dinitrogen pentoxide: N₂O₅ (di- = 2 nitrogen, penta- = 5 oxygen)
Silicon tetrachloride: SiCl₄ (one silicon, tetra- = 4 chlorine)
Phosphorus tribromide: PBr₃ (one phosphorus, tri- = 3 bromine)
Dinitrogen trisulfide: N₂S₃
Oxygen difluoride: OF₂
Iodine heptafluoride: IF₇
ClF₅: Chlorine pentafluoride
SF₆: Sulfur hexafluoride
CS₂: Carbon disulfide
N₂O₃: Dinitrogen trioxide
PCl₅: Phosphorus pentachloride
SeF₆: Selenium hexafluoride
Water (H₂O) is a representative molecular compound: colourless liquid, density 1.0 g/mL, melting point 0 °C.
Molecular compounds generally have much lower melting and boiling points than ionic compounds because the intermolecular forces holding molecules together are weaker than the ionic bonds in a crystal lattice. Many are gases or liquids at room temperature.
Molecular compounds are everywhere. Carbon dioxide (CO₂) is a greenhouse gas central to climate science. Silicon tetrachloride (SiCl₄) is used in the production of high-purity silicon for semiconductors. Phosphorus tribromide (PBr₃) is a reagent in organic chemistry for converting alcohols to alkyl bromides. Understanding the naming system lets you read chemical labels, safety data sheets, and journal articles without guessing.
Students sometimes try to use ionic naming rules (no prefixes, charge balancing) for molecular compounds. If both elements are nonmetals, use Greek prefixes.
Students often forget to drop "mono-" from the first element. Write "carbon dioxide," not "monocarbon dioxide."
The vowel-dropping rule trips people up. It is "monoxide" (not "monooxide") and "pentoxide" (not "pentaoxide"), but "dioxide" keeps both vowels because "di-" does not end in "o."
Students sometimes confuse molecular and ionic compounds. A quick check: if the compound contains a metal, it is almost certainly ionic; if both elements are nonmetals, it is molecular.
⚠️ Know the Greek prefixes from mono- through deca-.
⚠️ Given a molecular formula, write the correct name (and vice versa).
⚠️ Know when to drop mono- from the first element.
⚠️ Know the vowel-dropping convention (pentoxide, monoxide).
⚠️ Be able to distinguish ionic from molecular compounds at a glance (metal + nonmetal = ionic; nonmetal + nonmetal = molecular).
True or False: N₂O₅ is named nitrogen pentoxide. (False. It is dinitrogen pentoxide, because there are two nitrogen atoms.)
Fill in the blank: The prefix for 4 is ___. (tetra-)
True or False: Molecular compounds use Roman numerals in their names. (False. Roman numerals are for ionic compounds with variable-charge metals. Molecular compounds use Greek prefixes.)
Fill in the blank: SF₆ is named ___. (Sulfur hexafluoride)
True or False: Molecular compounds generally have higher melting points than ionic compounds. (False. They generally have lower melting points.)
Q: Name the compound PCl₅.
A: Phosphorus pentachloride. (One phosphorus, so no prefix; five chlorines, so penta-.)
Q: Write the formula for dinitrogen trioxide.
A: N₂O₃. (Di- = 2 nitrogen; tri- = 3 oxygen.)
Q: A compound is made of selenium and fluorine with the formula SeF₆. Name it.
A: Selenium hexafluoride.
Q: How can you tell at a glance whether a compound is ionic or molecular?
A: Check whether a metal is present. Metal + nonmetal = ionic. Two nonmetals = molecular (covalent). Use Greek prefixes for molecular, and either plain naming or Roman numerals for ionic.
Q: Why is it "carbon monoxide" and not "monocarbon monoxide"?
A: The prefix mono- is dropped for the first element by convention. It is kept for the second element to distinguish CO (carbon monoxide) from CO₂ (carbon dioxide).
Molecular compounds lead directly into Lewis structures and molecular geometry (VSEPR theory), which are typically the next major unit in general chemistry. The concept of shared electrons here is the same one used to explain why water is bent, why CO₂ is linear, and how polarity arises. Understanding molecular vs. ionic bonding also connects to intermolecular forces, which explain differences in boiling points, solubility, and physical states.
Molecular compound, covalent compound, covalent bond, electron sharing, Greek prefixes, numerical prefixes, mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, nonmetal bonding, binary molecular compound, naming molecular compounds, water, carbon dioxide, sulfur hexafluoride, phosphorus pentachloride, dinitrogen pentoxide, ionic vs. molecular, Chemistry Lab 9