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In class the professor went over lewis dot structures and did some sort of calculation to calculate how many valence electrons were needed for the diagram, but his explanation confused me. How do you calculate how many electrons to depict in the dot structure?
You have to look at the periodic table and see which group the element is in and based off of which group, they have specific numbers of valence electrons. For instance, group one has 1 valence electron, while 2 has 2 and so on. Then based off of how many of that element is in the compound, you multiply it. So for a diatomic oxygen O2, you’d multiply the 6 valence electrons by 2 because there are two.
^^ And after that you can compare the separate molecules and their charges. For example, SO42- will have 4×6=24 O electrons (4 O and each one has 6 e-) and then S will have 6 e-, for a combined total of 30 e-. Then, since the molecule SO42- has a charge of -2, that means there are 2 extra electrons as well making it more negative, so you add the 2 extra and get a grand total of 32e- for the whole molecule.
Also, if the molecule is charged, you need to account for those electrons. For example, if the molecule has a charge of +1, you need to subtract one electron, or if the molecule has a charge of -2, you need to add two electrons.
Basically, add up all the valence electrons of the atoms present (taking into consideration the number of each atom in a molecule) and add or subtract electrons if given an ion and according to the charge of that ion. (an ion with a negative charge means adding electrons and an ion with a positive charge means subtracting electrons)
First, you should reference the periodic table to identify how many valence electrons this element has. Valence electrons are the element in the highest energy orbital of that atom and are the electrons that are used in chemical bonding. After counting the number of valence electrons for each atom, add them together. Lastly, in the case of ionic bonds you must consider the additional elements that have been added or taken away. Cations will be labeled with + (an number of electrons) that must be subtracted from the total elctrons you found when considering the neutral atoms in the periodic table. Anions are the opposite, needing you to add that number of electrons to represent the electrons that were added to the neutral form of the atom.
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