How do you determine the number of electrons that go into the Lewis structure of a molecule? A polyatomic ion?

Understanding the role of valence electrons is crucial when determining the Lewis structure of a molecule or polyatomic ion. Valence electrons are the outermost electrons of an atom and are vital in the formation of chemical bonds. For example, carbon, which is in group 14 of the periodic table, has four valence electrons. To find the total valence electrons in a molecule, you simply add up the valence electrons from each atom in the molecule. This count forms the foundation for creating Lewis structures.

Consider water (H₂O) as an example. Oxygen has six valence electrons, and each hydrogen atom has one. Thus, for the water molecule, the total count is \( 6 + (1 \times 2) = 8 \) valence electrons. For a polyatomic ion, this process includes an additional step where you add electrons equal to the negative charge or subtract them if the ion is positively charged. Therefore, the Lewis structure accounts for every valence electron, ensuring they are all used to either form bonds or as non-bonding pairs, known as lone pairs.

Polyatomic ions are charged entities composed of two or more atoms covalently bonded together. When constructing the Lewis structure for a polyatomic ion, it is important to consider the overall charge. This charge affects the total number of valence electrons that participate in bonding.

For instance, let's take the sulfate ion (SO₄²⁻). Sulfur has six valence electrons, and each oxygen has six as well. Adding these up gives us \((6 + (6 \times 4))\) valence electrons. However, because the sulfate ion carries a 2− charge, we add two more electrons for the correct total, which would be \(30\) valence electrons to use in our Lewis structure. A correct Lewis structure will also visually represent the net charge of the ion, often by placing brackets around the structure and indicating the charge at the upper right outside the brackets.

The octet rule is a chemical rule of thumb that reflects the observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but hydrogen is an exception, which instead follows the duet rule and is stable with two electrons.

When applying the octet rule to Lewis structures, after initial single bonds have been formed, remaining valence electrons are distributed as lone pairs to fulfill the octet. This is done by placing them around the outer atoms first and then on the central atom if there are electrons left. If necessary, atoms can share electrons through double or triple bonds to achieve a filled outer shell. The adherence to the octet rule is a key factor in verifying the validity of a Lewis structure, although there are exceptions where certain molecules or ions do not follow the rule strictly.