This pack of purple atoms are used typically to represent phosphorus atoms and is for extending your existing set or for replacing lost atoms. Each pack contains 10 balls with four holes in each ball arranged in a tetrahedral array. Each atom is 23 mm diameter.
Why would I need these? You could use them to represent a tetrahedral phosphorus atom in the phosphate anion or with the fourth hole filled by an orbital shape (available separately) to represent a phosphine such as trimethyl phosphine.
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