Different forms of the same chemical element that have different arrangements of atoms, like how carbon can be diamond, graphite, or buckminsterfullerene.
From Greek 'allo' (other) + 'tropos' (turning/way). Coined in the 1840s by chemists to describe elements that could exist in multiple physical forms despite being chemically the same.
Diamond and graphite are both pure carbon, but diamonds are hard and sparkly while graphite is soft and dark—they're allotropes! This shows how the same atoms arranged differently create completely different properties, which is wild when you think about it.
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