Elements and compounds in 3D – molecular models, formulae and molecular mass

What it shows

An element is made of only one kind of atom, while a compound contains atoms of two or more elements chemically joined. In a molecule such as H₂O the formula counts the atoms: the small number after a symbol tells how many atoms of that element there are. Adding the atomic masses gives the molecular mass, for example 2 × 1 + 16 = 18 amu for water. Sodium chloride is different: it is a giant lattice of Na⁺ and Cl⁻ ions, and its formula NaCl shows the 1 : 1 ratio of ions rather than a single molecule.

How to use

Choose a substance and a model type, then drag to rotate and scroll or pinch to zoom. Press Separate atoms to pull the atoms apart for counting, and Show symbols or Hide symbols to toggle the labels. Compare O₂ with CO₂, or H₂ with HCl, and read the table of atom counts and masses below. Set Mode to Quiz, decide Element or Compound for each model, then press Next question.

Parameters you can change

• Substance H₂ – hydrogen, O₂ – oxygen, N₂ – nitrogen, Cl₂ – chlorine, H₂O – water, CO₂ – carbon dioxide, CH₄ – methane, NH₃ – ammonia, HCl – hydrogen chloride, NaCl – sodium chloride (ionic crystal)
• Model type Ball-and-stick, Space-filling
• Mode Explore, Quiz: element or compound?
• Show atom symbols
• Rotate the model automatically

Questions to explore

1. Why is O₂ an element while CO₂ is a compound, even though both contain oxygen atoms?
2. How many atoms are in one CH₄ molecule, and what is its molecular mass?
3. How is a crystal of table salt different from a water molecule, and what does the formula NaCl tell us?