atom: Atomic Weight and Number

The atomic number of an atom is simply the number of protons in its nucleus. The atomic weight of an atom is given in most cases by the mass number of the atom, equal to the total number of protons and neutrons combined. An atom may be conveniently symbolized by its chemical symbol with the atomic number and mass number written as subscript and superscript, respectively. For example, the symbol for uranium is U (atomic number 92); the isotopes of uranium with atomic weights 235 and 238 are indicated by U23592 and U23892.

Sections in this article:

• Introduction
• Contemporary Studies of the Atom
• Discovery of the Atom's Structure
• From Dalton to the Periodic Table
• Early Atomic Theory
• Development of Atomic Theory
• Atomic Weight and Number
• The Electrons
• The Nucleus
• Structure of the Atom
• Bibliography

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