Chemistry students the world over are familiar with covalent bonds and hydrogen bonds. Now a study has revealed a strange variety of bond that acts like a hybrid of the two. Its properties raise questions about how chemical bonds are defined, chemists report in the Jan. 8 Science.

Hydrogen bonds are typically thought of as weak electrical attractions rather than true chemical bonds. Covalent bonds, on the other hand, are strong chemical bonds that hold together atoms within a molecule and result from electrons being shared among atoms. Now, researchers report that an unusually strong variety of hydrogen bond is in fact a hybrid, as it involves shared electrons, blurring the distinction between hydrogen and covalent bonds.

“Our understanding of chemical bonding, the way we teach it, is very much black and white,” says chemist Andrei Tokmakoff of the University of Chicago. The new study shows that “there’s actually a continuum.”

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