Atoms

The atom is the atomic unit of chemistry (and of AsciiChem). An atom captures:

Field	Source position	Example
element	element symbol	C, He, Fe
isotope	prefix	^14C
subscript	suffix (after _)	H_2
charge	suffix	Ca^2+
oxidation_state	suffix	Fe^(III)

Element symbols

A capital letter optionally followed by a lowercase letter.

Source

H

$\mathrm{H}$

He

$\mathrm{He}$

Uue

$\mathrm{Uu}\text{e}$

Notes

Two-letter symbols (He, Uue) are captured as a single element, not as H followed by e. The grammar requires capital-then-lowercase for the two-letter form.

Subscripts

A subscript multiplicity follows _ directly. The subscript binds to the immediately preceding atom.

H_2

${\mathrm{H}}_2$

O_3

${\mathrm{O}}_3$

H_2O

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

For multi-character subscripts, use braces:

H_{2a}

${\mathrm{H}}_{\mathrm{\{2a\}}}$

Charges

Charges are suffix superscripts in IUPAC number-then-sign form:

Ca^2+

${\mathrm{Ca}}^{2+}$

Cl^-

${\mathrm{Cl}}^{-}$

SO_4^2-

$\mathrm{S}{\mathrm{O}}_4^{2-}$

Canonicalisation

AsciiChem canonicalises charge signs. ^+2 becomes ^2+ on round-trip per IUPAC convention.

Oxidation states

Oxidation states are Roman numerals wrapped in parens:

Fe^(II)

${\mathrm{Fe}}^{\mathrm{II}}$

Fe^(III)

${\mathrm{Fe}}^{\mathrm{III}}$

Mn^(VII)

${\mathrm{Mn}}^{\mathrm{VII}}$

The model captures these as Atom#oxidation_state (“II”, “III”, “VII”), distinct from a generic superscript.

The prefix isotope — the headline semantic fix

The defining difference between AsciiChem and AsciiMath lives here.

^14C

${}^{14}\mathrm{C}$

^131I

${}^{131}\mathrm{I}$

^238U

${}^{238}\mathrm{U}$

In AsciiChem, the ^14 prefix binds to the following atom. The parse produces one Atom with isotope: "14". The MathML output is:

<msup><mi>C</mi><mn>14</mn></msup>

In AsciiMath, the same input would force a phantom carrier:

<msup><mi></mi><mn>14</mn></msup><mi>C</mi>

— an empty base followed by a sibling atom. The semantic binding (isotope 14 belongs to Carbon) is lost.

Stoichiometric coefficients

Leading digits on a molecule are the coefficient, distinct from the leading digit of a subscript. The grammar uses positive lookahead to disambiguate.

2H_2O

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

3H_2

$3{\mathrm{H}}_2$

The 2 in 2H_2O is the molecule’s coefficient; the 2 in H_2 is the atom’s subscript. Both are captured in the right place.