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The constituent of elements - the smallest building block which can combine chemically with other atoms and therefore form compounds.. The atom consists of an electron cloud surrounding a nucleus. The nucleus contains positively charged protons and neutral neutrons, whereas the electron cloud is made up of negatively charged electrons. The number of protons (Z) determines the chemical element and the number of neutrons determines the isotope of the element.

Physics is really nothing more than a search for ultimate simplicity, but so far all we have is a kind of elegant messiness

– Bill Bryson