In the double-slit experiment with single atoms, the behavior of the atoms is similar to that of other quantum particles like electrons and photons. The concept of "going through both slits" or "spreading out" is a consequence of the wave-particle duality exhibited by quantum objects.
When an atom is sent through the double-slit apparatus one at a time, it behaves as a quantum wave, spreading out and interfering with itself. This means that the atom doesn't follow a single definite path through one slit or the other, but rather it exhibits wave-like characteristics and simultaneously explores multiple paths.
The interference pattern that emerges on the screen behind the double slits is the result of the constructive and destructive interference of the atom's wavefunction. The wavefunction describes the probability distribution of finding the atom at different locations. As the wave-like atom passes through the slits, it interferes with itself, leading to regions of enhanced and diminished probabilities on the screen.
However, once the atom interacts with a detector or a measurement device, its wavefunction collapses, and it is found at a specific position corresponding to one of the possible paths. This collapse of the wavefunction results in a particle-like behavior, and the atom is observed at a definite location, either passing through one slit or the other.
So, in summary, the atom behaves as a wave and exhibits interference patterns, suggesting it explores multiple paths simultaneously, but upon measurement, it appears as a particle at a specific location.