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Atoms consist of very small, positively charged nucleus surrounded by negatively charged electrons. In spite of the extremely small size of the nucleus with respect to that of the atom, the nucleus contains more than 99.9% of the total mass. Nuclei formed of positive protons and electrically neutral neutrons held together by the strong or nuclear force. This attracting force is much stronger than the repelling electrostatic force between the protons.
The number of protons in the nucleus (Z) is called the atomic number. This determines what chemical element the atom is. The number of neutrons in the nucleus is denoted by N. The atomic mass of the nucleus (A), is equal to Z + N. A given element can have many different isotopes, which differ from one another by the number of neutrons contained in the nuclei. In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. Since the electric charges of the proton and the electron are +1 and-1 respectively (in units of the proton charge), the net charge of the atom is zero. At present, there are 112 known elements which range from the lightest, hydrogen, to the recently discovered and yet to-be-named element 112. All of the elements heavier than uranium are man made. Among the elements are approximately 270 stable isotopes, and more than 2000 unstable isotopes.