What is an atom?

What, exactly, is an atom? It’s one of the questions that the quantum theory of atoms in molecules (QTAIM) has claimed to answer. Devised by the late Richard Bader over several decades, the theory attempts to develop a rigorous, complete description of chemical bonding, which characterises bonds in terms of the topology of the continuous electron density surface prescribed by quantum mechanics. Specifically, two nuclear centres are bonded if the electron density between them contains a saddle point: a kind of col with maxima in two directions and a minimum in the third. This definition of an atom, as ‘the union of an attractor and its associated basin’, looks very different from the one students generally learn – and from the one most chemists mentally picture as they build and analyse molecules.