(20 14? According to the calculation of quantum mechanics, there are five structures of carbon nitride, one of which is harder than diamond crystal and becomes the first one.

A. According to the information given in the title, silicon nitride is a new superhard material, which conforms to the typical physical properties of atomic crystals, so A is wrong;

B. Because carbon nitride is an atomic crystal and has no molecules, it has no molecular formula, so B is wrong;

C. According to the structure in the figure, there are four valence bonds around each carbon atom, three valence bonds around each nitrogen atom, four electrons in the outermost layer of carbon atom, forming an eight-electron stable structure, and five electrons in the outermost layer of nitrogen atom. After forming three valence bonds, it is also an eight-electron stable structure, so C is correct;

D according to the diagram, each carbon atom is connected with four nitrogen atoms, and each nitrogen atom is connected with three carbon atoms, so d is correct;

So choose CD.