氯是一种卤族化学元素，化学符号为Cl，原子序数为17。

自然界中游离状态的氯存在于大气层中，是破坏臭氧层的单质之一。氯气受紫外线分解成两个氯原子（自由基）。大多数通常以氯化物(Cl)的形式存在，常见的主要是氯化钠（食盐，NaCl）。

Chlorine is a chemical element with symbol Cl and atomic number 17. It has a relative atomic mass of about 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen, following fluorine. The element is a yellow-green diatomic gas under standard conditions. Chlorine has the highest electron affinity and the third highest electronegativity of all the reactive elements. For this reason, chlorine is a strong oxidizing agent. Free chlorine is rare on Earth, and is usually a result of direct or indirect oxidation by oxygen.