Step 1 of 18 of the SMILES tutorial. A SMILES string is a sequence of atom symbols. A bare C is one carbon, and because SMILES never spells out the hydrogens of a common element, that single letter already means methane. The ten elements of the organic subset (B, C, N, O, P, S, F, Cl, Br, I) may be written without brackets; anything else needs a bracket atom such as [Fe]. Replace the C below with O or N and watch water and ammonia appear.
It opens on C, which you can edit in place: the structure follows every keystroke, and so does the notation when you draw instead.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.