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.

एकाच प्रकारच्या अणूंचा बनलेला मूलभूत रासायनिक पदार्थ म्हणजे मूलद्रव्य. मूलद्रव्यांचा अणुक्रमांक त्यांच्या अणूंच्या गाभ्यात असलेल्या प्रोटॉनच्या संख्येएवढा असतो.