wave–particle duality
Think of two completely different things from everyday life: a marble, which is a tiny lump you can hold and locate exactly, and a ripple on a pond, which spreads out, bends around obstacles, and can overlap with other ripples to make bright and dark patterns. In the everyday world nothing is both. But at the scale of light and electrons, the same object behaves like a marble in some experiments and like a ripple in others. That two-faced behaviour is wave–particle duality.
More precisely, wave–particle duality is the principle that every quantum object — light, electrons, even whole atoms — carries both wave-like and particle-like properties, and which face it shows depends on how you look. Shine light through two narrow slits and it interferes like a wave, making stripes; shine it on a metal and it knocks out electrons one packet at a time, like a hail of particles. Neither picture alone is complete; both are needed.
The honest point is that 'wave' and 'particle' are human pictures borrowed from large-scale life, and a quantum object is really neither — it is its own kind of thing, fully described only by its wavefunction. Duality is not a contradiction the object suffers; it is a sign that our two everyday pictures are each just a partial shadow of a deeper reality. For chemistry, the wave side of the electron is what gives atoms their orbital shapes.
Fire electrons one at a time at a screen with two slits. Each electron lands as a single dot, like a particle. But after thousands of dots accumulate, they pile up into bright and dark bands — an interference pattern that only a wave can make. Each electron somehow went through both slits as a wave, then arrived as a particle.
One slit experiment, two natures: travels as a wave, lands as a particle.
Duality is not about the object switching back and forth in time. The same single experiment cannot show both faces fully at once; trying to detect which slit an electron uses destroys the interference, because measuring forces the particle-like description.