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How are we expected to know the charges of certain elements when drawing out the Lewis structure? During lecture, Lavelle went over the Lewis structure for Ammonium Sulfate --> (NH4)2SO4. How are we suppose to know that NH4 has a charge of +1 and SO4 has a charge of -2? Is there a trend on the periodic table or are we suppose to memorize these charges? Thanks in advance.
As Lavelle was saying, you use the octet rules as guideline for transferring electrons. Likewise, you can use the same guideline to know the charges of the elements when doing Lewis structures. For example, Oxygen likes gain two electrons to make an octet, so the ion will generally have a charge of -2. Since there are 4 atoms in the sulfate ion, Oxygen overall has a charge of -8. Sulfur likes forming 4 bonds, so it has a charge of +4. Generally, the elements with higher electronegativity have preference over charges when the molecule has a total charge of 0 or the charge is given. The transition elements can often have more than one common ion, so in that case the charge will be dependent on the other elements in the molecule. You don't have to memorize all the ion charges, but I've found that it helps of memorize the overall charge of common ions and the more-common elements that don't follow the octet rule.
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