Naming Molecules & Compounds

Chemistry Lab

Learn to tell molecules from compounds, then work every compound in both directions: build it atom by atom, and snap its name together from puzzle pieces.

Molecules vs. Compounds

A molecule is two or more atoms bonded together. That is the whole definition; the atoms can be the same element or different elements. A compound is a substance made of two or more different elements bonded together in a fixed ratio.

The two words overlap, but neither one contains the other. Two examples show why:

$\ce{O2}$ (oxygen gas) is a molecule, because two atoms are bonded together. It is not a compound, because both atoms are the same element.
$\ce{NaCl}$ (table salt) is a compound, because it contains two different elements (sodium and chlorine) in a fixed 1:1 ratio. But solid $\ce{NaCl}$ is not built from individual Na–Cl pairs. It is a giant repeating lattice of ions, so chemists call $\ce{NaCl}$ a formula unit, not a molecule.

That lattice-versus-molecule difference is exactly why this lab's build step works two ways: a covalent compound like $\ce{CO2}$ gets a real atom-and-bond workbench, while an ionic compound like $\ce{MgCl2}$ gets a charge-balancing ion tray instead. There is no single "molecule" of $\ce{MgCl2}$ to build.

One simplification to know about: the workbench connects atoms with single lines, even though some real molecules (like $\ce{CO2}$) hold certain atoms together with double or triple bonds. Connecting the right atoms to the right partners is the grade-8 skill; counting shared electron pairs comes later.

How Naming Works

Chemists name a compound differently depending on the kind of bond that holds it together. This lab covers the two kinds you will build and name: binary covalent compounds (two nonmetals) and binary ionic compounds (a metal and a nonmetal).

Covalent naming: count both elements

Each element's count in the formula becomes a Greek prefix: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca. Three rules control how the prefixes attach:

$\ce{CO}$ → carbon monoxide (carbon: first element, count 1, no prefix; oxygen: mono + ox + ide, with the vowel dropped)
$\ce{CO2}$ → carbon dioxide (two oxygens → "di")
$\ce{N2O}$ → dinitrogen monoxide (the first element takes a prefix too, whenever its count is more than 1)

A few everyday covalent compounds keep older common names instead of their systematic ones: water (not "dihydrogen monoxide"), ammonia (not "nitrogen trihydride"), and methane (not "carbon tetrahydride"). Simple hydrogen compounds also drop their counting prefixes: $\ce{HCl}$ is "hydrogen chloride," never "hydrogen monochloride," because no other compound of just hydrogen and chlorine exists to confuse it with.

Ionic naming: balance the charges, never count them

An ionic compound forms when a metal gives up electrons to a nonmetal. The metal keeps its own name unchanged; the nonmetal's stem takes -ide, just like the covalent suffix. The critical difference: ionic names never use Greek prefixes. The subscripts in an ionic formula come from balancing positive and negative charge to zero, not from counting atoms into the name.

$\ce{MgCl2}$ is "magnesium chloride," never "magnesium dichloride." Magnesium's +2 charge needs two −1 chloride ions to reach zero. The 2 is a consequence of charge balance, and it is never spoken as part of the name.

Choose a Compound

60 compounds in 4 tiers. Every level is open: jump anywhere, or work through a tier in order. A check mark means the level is fully done, both built and named (or named alone, for the 7 levels marked naming-only).

Your compound
Built Named

Step 1 · Build it

Step 2 · Name it

Click a puzzle piece to snap it onto the answer track, in the order the name is spoken. Click a placed piece to send it back. With the keyboard, the Left and Right arrow keys reorder a placed piece and Delete removes it.

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