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Nearly one hundred percent of the mass of an atom (99.94%) is contained in the nucleus. The mass of the protons and the mass of the neutrons are almost the same. Electromagnetic force binds the electron cloud to the nucleus. A nearly identical force can allow atoms to bind together, forming molecules. Atoms exist as either electrically neutral, or ions.
A nucleus consists of a bunch of protons and neutrons; these are known as nucleons. Each nucleus can be characterized by two numbers: A, the atomic mass number, which is the total number of nucleons; and Z, the atomic number, representing the number of protons.