Bonds in detail

AsciiChem supports eight bond kinds in linear notation. The bond grammar uses longest-first alternation so multi-character tokens (like ## and >-) are recognised before their single-character prefixes.

The eight kinds

Kind	ASCII	MathML	Use
single	-	-	Single bond
double	=	=	Double bond
triple	#	≡	Triple bond
quadruple	##	≣	Quadruple bond (organometallic)
wedge	>-	↑	Stereo wedge (out of page)
hash	-<	↓	Stereo hash (into page)
dative	~>	→	Coordinate / dative bond
wavy	~~	∼	Resonance / delocalised

Examples by chemistry family

Organic chains and multiple bonds

H-O-H

$\mathrm{H}-\mathrm{O}-\mathrm{H}$

H_2C=CH_2

${\mathrm{H}}_2\mathrm{C}=\mathrm{C}{\mathrm{H}}_2$

HC#CH

$\mathrm{H}\mathrm{C}\equiv \mathrm{C}\mathrm{H}$

Organometallic quadruple bonds

Re##Re

$\mathrm{Re}≣\mathrm{Re}$

Cr##Cr

$\mathrm{Cr}≣\mathrm{Cr}$

Mo##Mo

$\mathrm{Mo}≣\mathrm{Mo}$

The ## spelling keeps the family of #-based tokens; longest-first alternation in the grammar prevents # from shadowing ##.

Stereochemistry

C>-H

$\mathrm{C}\uparrow \mathrm{H}$

C-<H

$\mathrm{C}\downarrow \mathrm{H}$

>- evokes a wedge pointing right (toward the viewer); -< is its mirror (away from the viewer). For full (R) / (S) / (E) / (Z) stereo markers, see Atoms.

Lewis acid-base complexes

NH_3~>BF_3

$\mathrm{N}{\mathrm{H}}_3\to \mathrm{B}{\mathrm{F}}_3$

H_3N~>Ag^+

${\mathrm{H}}_3\mathrm{N}\to {\mathrm{Ag}}^{+}$

The dative bond uses ~> rather than -> because -> is already the reaction arrow. ~> (wave + arrow) conveys electron-pair flow and round-trips cleanly.

Resonance structures

A~~B

$\mathrm{A}\sim \mathrm{B}$

The wavy bond ~~ distinguishes from ~> (dative) and from regular text. Useful for delocalised systems where a single Lewis structure is insufficient.

Round-trip

Every bond kind round-trips exactly. The Text formatter is the canonicaliser — it always emits the spelling listed above regardless of how the bond was constructed.

AsciiChem.parse("Re##Re").to_text   # => "Re##Re"
AsciiChem.parse("NH_3~>BF_3").to_text  # => "NH_3~>BF_3"

Why these spellings?

The design constraints:

1. Plain ASCII. No Unicode in source.
2. No conflict with -> (the reaction arrow). This forces a different spelling for dative — ~> was chosen for its electron-pair-flow connotation.
3. Longest-first alternation. ## must beat #; >- must beat -. The grammar rule order is the contract.
4. Visual mnemonics. >- looks like a wedge; ~~ looks wavy; # and ## extend naturally.

What’s not here

• Aromatic bonds (the partial bond inside an aromatic ring) — v0.2 doesn’t have a distinct spelling; use =/- alternation or the planned ring syntax (TODO 18, 2D structural).
• Hydrogen bonds — typically denoted ... in literature; AsciiChem v0.2 doesn’t have a dedicated kind. Tracked as a possible TODO.
• Ionic “bonds” — not really bonds; represented via charge markers on atoms (Na^+, Cl^-).