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Science 20S Unit #1 - Chemistry in Action Lesson #4 - Bohr Diagrams Bohr Diagrams • Used to represent the arrangement of electrons in atoms. • Each orbital or shell can accommodate a specific number of electrons. – 2,8,8,18,18,32,32 • The outermost shell that contains electron is known as the valence shell. – Electrons in this shell are known as valence electrons. Drawing Bohr Diagrams • First draw the nucleus including information about the number of protons and neutrons. – The number of protons is the same as the atomic number found on the periodic table. – The number of neutrons can be determined by taking the atomic mass and subtracting the atomic number. • # of Neutrons = Atomic mass - atomic number • Eg. Oxygen • Next start drawing the electrons starting at the innermost orbit – The number of electrons is the same as the atomic number – Keep in mind the number of electrons each orbit can accommodate • Eg. Oxygen – Inner orbit – Valence orbit • Try Lithium, Sulfur and Calcium Ions • All atoms are trying to have their outer shell full of electrons. – This is known as the Octet Rule. • To get a full outer shell an atom must gain or lose electrons. • An atom that has gained or lost electrons has an electrical charge – It is no longer neutral. • Atoms that have a charge are known as Ions. • Positively charged ions are called cations. • Negatively charged ions are called anions. • Determining whether electrons will be gained or lost depends on the number of electrons in the valence shell. – 3 or less will lose their electrons. • Metals tend to lose their electrons. – 5 or more will gain electrons. • Non-metals tend to gain electrons. – Atoms with 4 electrons in their valence shell can either lose or gain electrons. Bohr Diagrams for Ions • First draw the Bohr diagram for the atom – Eg. Lithium Ion • Second, decide whether electrons should be added or removed to make a full outer shell and do so. – In this case it is easier to remove 1 electron rather than add 7 electrons • Third, draw square brackets, determine the charge and include it in the diagram – Charge = # of protons - # of electrons – 3 p+ - 2 e- = 1+ • Eg. Sulfur Ion •Original Bohr Diagram • Decide whether to add or remove electrons – In this case it is easier to add 2 electrons than remove 6 electrons • Determine the charge – 16 p+ - 18 e- = 2- • Try Magnesium, Nitrogen and Aluminum