1) Atoms have a nucleus surrounded by electron shells. The number of electrons equals the atomic number.
2) Electron shells can hold a certain number of electrons and fill from the innermost shell outward. The outermost shell electrons are called valence electrons.
3) To draw a Bohr diagram, find the element and atomic number, determine the number of electron shells based on its period, draw the nucleus and shells, and add the correct number of electrons in allowed shells moving counter-clockwise.
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