Everything in nature — solid, liquid, or gas — is made up of one or more elements. An atom is the smallest portion of an element that can exist, while retaining the characteristics of that element. The lightest atoms are those of the element hydrogen and the heaviest atoms occurring naturally in significant quantity are those of uranium.

Atoms, although extremely small, have an internal structure. Every atom consists of a central nucleus, carrying nearly all the mass of the atom, surrounded by a number of negatively charged electrons. The nucleus of an atom has a positive electrical charge which is balanced by the negative charge of the electrons. Consequently, in its normal state, the atom as a whole is electrically neutral.

All atomic nuclei contain even smaller particles called protons and all except one form of hydrogen also contain neutrons. The protons have a positive electric charge, and the neutrons have no charge. The protons are thus responsible for the electric charge of the nucleus. Each atomic species is characterized by the number of protons and neutrons in the nucleus.

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