CHEM 320 - Chapter 3: Lewis dot structures & Formal charges

Lewis dot structure

approximation for structure of molecules and ions

depicts electron distribution

bond pairs

shared electrons

lone pairs

unshared electrons

localized

implies that the electrons are confined to a particular bond/atom

2

1 bond = __ electrons

octet rule

most main group atoms will end up with an electron configuration of ns2np6 when in a molecule

aka most main group elements need 8 electrons total when bonded in a molecule

followed by C, N, O, F among others

NOT

octet rule does ___ always apply to inorganic compound

nonbonding

___ pairs, aka lone pairs, contribute to the reactivity and shape of the molecule, but not necessarily to the bonding

covalent

implies the sharing of electrons between atoms

valence

Only __ electrons are involved in covalent bonding as the core electrons are too low energy, aka they have a higher effective nuclear charge

core

FILLED nd orbitals are considered __ electrons and thus do not take part in bonding, like most full orbitals

Lewis methodology

1. calculate the # of electron needed for the ideal/most stable configuration of the atom

2. calculate the number of valence electrons available, DO NOT FORGET TO INCLUDE CHARGE OF MOLECULE.

3. difference between # of electrons needed and # available is the number of bonding electrons. Half of that is the # of bonds.

4. sketch molecule

5. fill in electron pairs around atoms

6. draw all possible resonance structures

bending

lone pairs take up space, thus ___ molecules when they are on the central atom, aka they give molecule a bent shape

larger

the ___ the atom the lone pairs are on, the farther away from the nucleus they are, thus the more space they take, and the more compressed the bond angle is.

smaller

larger atom that the lone pairs are on = ___ bond angle

VSEPR

Valence Shell Electron Pair Repulsion theory

tool to approximate structures of molecules

MINIMIZES the electrostatic repulsion between regions of high electron density (lon epairs, single/double/triple bonds)

VSEPR methodology

1. write the electron dot structure for molecule and count the # of charge clouds surrounding atom of interest

2. predict geometric arrangement of charge clouds by assuming that they are oriented in space as far away from each other as possible

formal charge

`apparent electronic charge of each atom in a molecule based on Lewis dot structure

can be used to determine the preferred Lewis structure

valence, unshared, bonds

formal charge = (# of __ electrons in free atom) - (# of __ electrons in molecules) - (# of __ in molecules)

LOWER

the ___ the formal charge of the molecule and each atom overall, the better the Lewis structure