Molecules

A molecule is an ordered sequence of atoms and groups, with an optional leading stoichiometric coefficient.

Simple molecules

H_2O

${\mathrm{H}}_2\mathrm{O}$

CO_2

$\mathrm{C}{\mathrm{O}}_2$

C_6H_12O_6

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$

Each atom carries its own subscript. The model is Molecule(nodes: [Atom, Atom, ...]).

Stoichiometric coefficients

Leading digits on a molecule are its coefficient:

2H_2O

$2{\mathrm{H}}_2\mathrm{O}$

3H_2

$3{\mathrm{H}}_2$

The Molecule#coefficient field holds the string. The MathML renderer emits it as an <mn> before the atoms.

Groups

Parenthesised sub-formulas are groups. A group can carry an outer multiplicity.

Ca(OH)_2

$\mathrm{Ca}{\left(\mathrm{O}\mathrm{H}\right)}_2$

(NH_4)_2SO_4

${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2\mathrm{S}{\mathrm{O}}_4$

For Ca(OH)_2:

• Atom Ca
• Group OH with multiplicity 2, bracket :paren

For (NH_4)_2SO_4:

• Group NH_4 with multiplicity 2
• Atoms S, O with subscripts 4

Bracket variants

AsciiChem supports three bracket kinds:

Bracket	Open / Close	Use
Paren	( )	Generic grouping (Ca(OH)_2)
Square	[ ]	Coordination complexes ([Fe(CN)_6]^4-)
Brace	{ }	Optional / rare

The bracket kind is preserved through round-trip — (OH)_2 stays as (OH)_2, not [OH]_2.

Coordination complexes

Square brackets wrap coordination complexes. The complex’s charge sits outside the brackets.

[Fe(CN)_6]^4-

$\left[\mathrm{Fe}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]\text{^4-}$

[Co(NH_3)_6]^3+

Install the asciichem gem to see this rendered.

These are parsed as Group(bracket: :square, nodes: [...]) followed by an Atom with the charge. (In v1, the charge attaches to the last atom; v2 may promote it to a complex-level attribute.)