一些元素的原子有時會結合成不同的結構，稱為同素異形體。例如，地球大氣層中的氧氣以兩種形式存在——穩定的雙原子氧（ O_2 ）和臭氧（ O_3 ）。臭氧是一種不穩定的分子，當雙原子氧吸收來自太陽的紫外線時形成。

固體碳有三種常見的同素異形體。鉆石由碳原子以四面體晶格結合而成，而石墨由碳原子以六面體結合的平板組成。富勒烯是由碳原子組成的球體（"巴基球"）或管道，包括足球狀的分子 C_{60} 。

一種元素的同素異形體可以有非常不同的物理和化學性質。鉆石是自然界中已知的最硬的礦物，因為每個碳原子在四面體中與其他四個碳原子剛性結合；石墨則相對較軟，因為平坦的片狀物結合得很弱，可以相互滑動。正常的雙原子氧形成一種無色無味的氣體，而臭氧是一種淡藍色的氣體，具有刺激性氣味。

The atoms of some elements sometimes bind together into different structures called allotropes. For example, oxygen in the Earth's atmosphere exists as two allotropes-stable diatomic oxygen( O_2 ) and ozone( O_3 ). Ozone is an unstable molecule that forms when diatomic oxygen absorbs ultraviolet light from the Sun.

Solid carbon has three common allotropes. Diamond consists of carbon atoms bonded in a tetrahedral lattice, while graphite consists of flat sheets of carbon atoms bound in hexagons. Fullerenes are spheres ('buckyballs') or tubes of carbon atoms, including the soccer-ball-shaped molecule C_{60} .

Allotropes of an element can have very different physical and chemical properties. Diamond is the hardest known mineral in nature because each carbon atom is bonded rigidly to four other carbons in a tetrahedron; graphite is relatively soft because the flat sheets are weakly bonded and can slide over each other. While normal diatomic oxygen forms a colourless, odourless gas, ozone is a pale blue gas with a pungent smell.