रासायनिक तत्त्व ऐसे उन शुद्ध पदार्थों को कहते हैं जो केवल एक ही तरह के परमाणुओं से बने होते हैं। या जो ऐसे परमाणुओं से बने होते हैं जिनके नाभिक में समान संख्या में प्रोटॉन होते हैं। सभी रासायनिक पदार्थ तत्त्वों से ही मिलकर बने होते हैं।

A chemical element is a chemical substance that cannot be broken down into other substances by chemical reactions. The basic particle that constitutes a chemical element is the atom. Chemical elements are identified by the number of protons in the nuclei of their atoms, known as the element's atomic number. For example, oxygen has an atomic number of 8, meaning that each oxygen atom has 8 protons in its nucleus. Atoms of the same element can have different numbers of neutrons in their nuclei, known as isotopes of the element. Two or more atoms can combine to form molecules. Chemical compounds are molecules made of atoms of different elements, while mixtures contain atoms of different elements not necessarily combined as molecules. Atoms can be transformed into different elements in nuclear reactions, which change an atom's atomic number.