Lewis Structure; only using Valence Electrons. The only electrons that have major chemical significance are the valence electrons Lewis structures represent the type and quantity of atoms in a molecule using dashes to indicate covalent bonds and dots to represent unshared electrons (or lone pairs). Basically, why worry about all of the electrons when only the outer ones are important for bonding! Interesting Lewis fact, he ran the US chemical warfare service in WWI
Lewis structures for ionic Dots aren’t shown by cation because it loses electrons to drop to previous noble gas 8 dots surround anion because it gains electrons to fill its valance shell and become the next noble gas. KBr K+K+ [ Br ] -
Covalent Bonds Both polar and non-polar covalent bonds are sharing. Hydrogen follows the duet rule (next slide) Most others follow the octet rule (next slide) Noble gasses do NOT form bonds or share because they already have a full valence shell
OCTET RULE atoms in compounds tend to have the electron configuration of a noble gas. Remember Lewis structures represent the type and quantity of atoms in a molecule using dashes to indicate covalent bonds and dots to represent unshared electrons (or lone pairs). ****exception: hydrogen tend to have only 2 electrons (duet rule) (because the first noble gas has 2)
Octet Rule; Fluorine F F F F The shared electrons, or bond, is shown as a line F F Two F with 7 V electrons share to form F 2 Bond Remaining dots are “lone pair”
Rules for Lewis 1.ALL valence electrons from all atoms must be included. 2.Atoms that are bonded to each other share one or more pairs of electrons 3.The electrons are arranged so that each atom is surrounded by enough to create a FULL valence shell for that atom.
Figuring out what to share Add valence electrons “around the clock” not in pairs. The free electrons are the ones that will form pairs. This lets you know how many bonds each atom must form. HCO Will form 1 bond Will form 4 bond Will form 2 bond
You can form double and triple bonds too! If more than one pair is shared, then a double or triple bond is shared. CO 2 OCOOCO Do a count, all have 8 electrons! OC O
Bonds for polyatomic ions? Just like before, but one atom in the group will NOT have the full valence shell. You must add that electron, and then show it is an ion (like our first example with Br - ) NO 2 - ONOONO [ O N O ] - or
One last thing before we practice When you have multiple atoms in a compound like CO 2 or NO 2 - the general rule is the singular atom is the one in the center. Also, the Octet rule is not always observed and there are a few exceptions.