Chem 4.4-4.6 objectives

How to know if atom has a full octet

When it has 8 valence electrons(noble gases)

How to know if Lewis dot structure has a full octet

When the bond structures amount of electrons is 8

Lewis structure-single bond amount of valence electrons

2 valence electrons

Lewis structure-double bond amount of valence electrons

4 valence electrons

Lewis structure-triple bond amount of valence electrons

6 valence electrons

Lewis structure-lone pair amount of valence electrons

2 valence electrons

How to know if atom shares electrons

If it’s covalent, metal+metal

How to find amount of valence electrons

• Look at what group it’s in, the number is the amount of valence electrons (ex:group 18, 8 valence electrons)

• This doesn’t apply to transition metals, groups 3-12

OH⁻ valence electrons

• Oxygen contributes 6 electrons.

• Hydrogen contributes 1 electron.

• The −1 charge adds 1 electron.

Total:

6+1+1=8 electrons

resonance

Resonance means that electrons can be spread out over multiple atoms instead of staying in just one bond.

Think of it this way:

• Sometimes, you can draw more than one correct Lewis structure for the same compound.

formal charge

• when there is more than one correct way to draw an electron-dot structure without moving the atoms

• It helps you decide which of several possible Lewis structures is the best one.

• An atom's formal charge is not necessarily its actual charge in the molecule.

formal charge formula

FC=V-N-B/2 or FC=V-D-B

• V= valence electrons in the neutral atom

• N = nonbonding electrons (dots)

• B= bonding electrons (all electrons in bonds)

resonance hybrid

a hybrid of all the drawn structures

Octet exceptions: H and Be can be stable with ___ electrons(duet)

2

Octet expections:B is stable with ___ electrons

6

Octet exceptions: elements where n ≥ ___ can exceed (hypervalent)

3

what does “elements where n ≥ 3 can exceed (hypervalent)” mean

Elements where the principal quantum number Elements where the principal quantum number n ≥ 3 (elements in Period 3 and below

period

horizontal

groups

vertical

VSEPR stands for

Valence Shell Electron Pair Repulsion Theory

Valence Shell Electron Pair Repulsion Theory

• Predicts the shapes of a number of molecules and polyatomic ions

• Bonds in an atom will arrange themselves to be as far apart as possible

• Lone pairs of electrons will maximize the distance from each other

2 bonds or lone pairs

around the central atom is

linear

3 bonds or lone pairs

around the central atom is

trigonal planar

4 bonds or lone pairs

around the central atom is

tetrahedral

5 bonds or lone pairs

around the central atom is

trigonal bipyramidal

6 bonds or lone pairs

around the central atom is

octahedral

When there is a lone pair instead of a bond, the molecule geometry (shape) is different than the electron geometry

• This is because lone pairs are not connected to atoms

• Lone pairs take up more space than bonds and will slightly compress bond angles

Electron geometry

describes the three-dimensional arrangement of all electron groups (both bonding pairs and non-bonding lone pairs) around a central atom

Molecular structure

(or molecular shape) describes the arrangement of only the bonded atoms in a molecule, ignoring the physical space taken up by lone pairs.

the higher electronegativity

the higher (bond) polarity

polar molecules always have ______ _______

dipole moment

symmetry

looks identical or corresponding when you fold, flip, rotate, or slide them

how to determine if molecular structure is nonpolar

• there’s a electronegativity difference of 0-0.4

• it’s one of the symmetrical shapes (linear, tetrahedral)

• the pull is symmetrical

symmetry cancels out

polarity

how to determine symmetry

need to draw out the molecular structure (shape) to determine if it’s symmetrical

When is there a dipole moment?

When the atom,molecule is polar/not cancelled by symmetry