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Atoms.   Atoms are made out of organized and coherent clusters of Electrons, Neutrons, and Protons.  The protons and neutrons are concentrated in the center of each atom; collectively this single concentration is called a Nucleus.  (The plural of nucleus is Nuclei.) Both protons and neutrons are also called Nucleons.  Nucleons are held together with the strongest and shortest-range force, which is called (amazingly!) the Nuclear Force.   The most common Hydrogen atoms (H1) have a nucleus of one proton and zero (0) neutrons.  However, a few hydrogen atoms can also have one or two neutrons, in addition to the single proton.  The uncommon hydrogen with one proton and one neutron is also called Deuterium (H2); the one with a proton and two neutrons is also called Tritium (H3).  Atoms with an uncommon number of neutrons are called Isotopes.  Atoms with 2 protons and 2 neutrons are called Helium (He4); atoms with 5 protons and 6 neutrons are called Boron (B11) and so on. (The symbol and name for each element are shown in red in the table below.) The number of protons is called the Atomic number, which determines the name of an atom.  (The atomic number, that is the proton number, for each element is printed in black beneath the red symbol for that element, in the table below.) The number of nucleons determines the Mass number.  hope this helps you
An isotope is a variation of the same element with a different number of neutrons in their nucleus. Isotopes can be classified as either stable or unstable.

An unstable isotope undergoes radioactive decay meaning it looses energy over time while emitting radiation.