The 2 hole 23mm red atom denotes oxygen and can be used in place of the 4 hole version if you do not intend to use the lone pair electron paddles.
Indigo Instruments has maintained a substantial inventory of genuine Molymod atoms & bonds for over 30 years. These parts are compatible with every set & kit we have sold since day 1. Though quality may appear expensive, no parts support from other vendors costs even more.
Use only these genuine Molymod Atoms & Bonds for full compatibility with any of our Molymod Molecular Model Sets & Molymod Molecular Model Kits.
product as described It is great to have a source of parts for molecular models without buying an entire kit with lots of parts that I don't need.
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Supplying molecular model parts got us started on the WWW. We went live in Oct. of 1994 as indigo.com with a complete offering of all the Molymod components for exactly the reason you mentioned..