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An atom consists of an extremely small and positively charged nucleus surrounded by a cloud of negatively charged electrons. Although the nucleus is less than one ten-thousandth the size of an atom, it contains more than 99.9% of the mass of the atom. The nuclei consists of positively charged protons and electrically neutral neutrons that are held together by the so-called strong force. This force is much stronger than the electrostatic force that binds electrons to the nucleus but its range is limited to much smaller distances, generally only a few 10-15 meters. Both protons and neutrons can be grouped together under the label nucleon.
The number of protons in the nucleus is called the atomic number or Z. This determines the chemical element of the atom. The number of neutrons in the nucleus is denoted by N. N is not the same for all atoms of a given element as different values of N denote different isotopes. The atomic mass of each nucleus, A, is equal to Z + N.
In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. The electric charges of the proton and the electron are +1 and -1 respectively making a net charge of zero.
The International Union of Pure and Applied Chemistry has declared 118 known elements which range from the lightest, Hydrogen, to element 118, Oganesson, a super-heavy element that is not naturally found on Earth. All elements heavier than Uranium are man made. Among the elements there are approximately 250 stable isotopes and more than 2000 unstable isotopes.
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