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Ionization Energy Trends

ElectronegativityIncreasing Electronegativity

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Lewis Structure Assumptions

• Only valance electrons are involved in bonding.• Atoms react to form molecules, so to achieve

stable noble gas electron configurations.• Atoms in molecules want eight valance electrons

(octet rule) except for hydrogen which wants two electrons (duet rule).

• In covalent compounds atoms share electrons to form bonds in order to achieve stable noble gas electron configurations. In ionic compounds electrons are transferred from one atom to another to achieve stable noble gas electron configurations.

Rules for Drawing Lewis Structures

Step 1: Count the total number of valance electrons.

Step 2: Identify the central atom (the first atom written unless that atom is hydrogen). Place all terminal atoms around that atom. Hydrogen atoms NEVER have more than one bond.

Step 3: Complete the octet for all atoms in the Lewis structure with lone pairs of electrons (except hydrogen).

Step 4: Check your structure by counting the number of valance electrons used (they will match step 1 if the structure is correct). If your valance electrons don’t match you will need to tweak your structure as follows.

Tweaking Lewis Structures

Too Many Electrons Initially: Redraw the

Lewis structure from step 2 adding a double

bond. If you still have too many electrons

add another multiple bond and repeat.

Always add double bonds before triple

bonds. Every double bonds effectively

remove two electrons from the structure while

triple bonds effectively remove 4 electrons.

“Exceptions” to the Octect

Two Few Electrons: If you only have four or

six valance electrons initially you can’t

possibly fill the octet rule (usually BeH2 or

BH3). Just place hydrogens around central

atoms and call it done.

“Exceptions” to the Octect

Odd number of electrons: One atom will

have to have less than eight electrons.

Draw the Lewis structure as if it had one

more valance electrons than it actually does.

Then subtract one electron from the least

electronegative element (often the central

atom).

“Exceptions” to the Octect

Exceeding the Octet Rule: If the central atom is inthe third row or below it may be possible to exceedeight electrons around the central atom. But only ifyou absolutely have no choice. If you can giveeverything eight and only eight do that first. Whenyou must break the octet rule draw the structure asyou would in steps 1-4 and the place the extraelectrons around the central atom. Even whenbreaking the octet rule no atom will ever havemore than 12 electrons.

Example Total # of Things

Lone Pairs

Geo. Shape 3-D Lewis

Structure

Bond Angles

Hybrid Orbitals

CO2 2 0 Linear Linear 180 sp

CH2S 3 0 Trig.Planar Trig. Planar 120 sp2

SeO2 3 1 Trig.Planar Bent 120 sp2

SiCl4 4 0 Tetra. Tetra. 109.5 sp3

PCl3 4 1 Tetra. Trig. Pyramid. 109.5 sp3

H2S 4 2 Tetra. Bent 109.5 sp3

HI 4 3 Tetra. Linear 180 sp3

Example Total # of Things

Lone Pairs

Geo. Shape 3-D Lewis

Structure

Bond Angles

Hybrid Orbitals

PF5 5 0 Trig. bipyramid

Trig. bipyramid

120, 90, 180

dsp3

SeBr4 5 1 Trig. bipyramid

See-saw 120, 90, 180

dsp3

BrCl3 5 2 Trig. bipyramid

T-shaped 90, 180 dsp3

XeCl2 5 3 Trig. bipyramid

linear 180 dsp3

TeF6 6 0 Octahed. Octahed. 90, 180 d2sp3

IF5 6 1 Octahed. Square pyramid

90, 180 d2sp3

RnBr4 6 2 Octahed. Square planar

90, 180 d2sp3

Organic Lewis Structure Rules

• Based on formal charge arguments

Atom Bonds Formed in Lewis Structure

Lone Pairs on Atom in Lewis Structure

C 4 0

N 3 1

O 2 2

F, Cl, Br, I 1 3

p-orbitals are 90 degrees from each other

1 s + 3 p = 4 sp3 orbitals

1 s + 2 p = 3 sp2 orbitals

1 s + 1 p = 2 sp orbitals

Unhybridized p-orbitals delocalize the -electrons

ALL atoms are sp2 hybridized in NO31-.

There are NO sp3 hybridized orbitals.