Transcript Unit 1 Lesson 4 - Bohr Diagrams

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