Chemistry Unit 3

Molecular Orbital Theory

Because electrons are waves, when they combine constructively, bonding molecular orbitals form and they have lower energy than the atomic orbitals and become more stable.

When atomic orbitals combine destructively, antibonding molecular orbits form and they have a higher energy and are less stable

Both bonding and antibonding orbitals are in the same space, just different energies

How to fill out a MO table

Number of valence electrons are placed into the orbitals starting from the lowest bonding orbital to the antibonding orbital

When will a bond form in molecular orbital theory

A bond will form if only the bonding molecular orbital is filled or there is one antibonding orbital that will not bond

If both he bonding and antibonding orbitals are full, then the bond will cancel out and they will not bond

Why do two H atoms form a bond

It has one bond between the 2 H atoms because two electrons (one from each H) fill only the bonding orbital it can bond

When they come together they only have 1 orbital in each and combine to have 1 molecular orbitals form

Why do 2 heliums not form a bond

They have two electrons and two molecular orbitals so t goes to the bonding molecular orbital, but then also goes to the anti bonding orbital and it cancels out so He2 cannot form

How to break a hydrogen bond/ what happens

To break the bond, enough energy has to be added to raise an electron to the anti bonding orbital so they cancel out and this is done with a high temperature

n Atomic Orbitals yield n Molecular orbitals

Write out electron configuration and how many letters/ orbitals per letters= that is how many orbitals

Bonding in metals

As the number of molecular orbitals increases, the energy gap between them decreases

Metals have a lot and they have almost continuous energy so electrons can just move freely and lots of different photons at any wavelength can be accepted to move electrons up and down

Why are metal so hard to break/ high boiling and melting points

They have a sea of electrons where they are delocalized and attracted to many atomic cores and they flow freely up and down all energy levels too and this them flexible and ductile but really hard to break

Why is metal shiny

An absorption of a photon will promote an electron to a higher energy and the it immediately falls down and emits a photon so it interacts with many wavelengths of absorbing and emitting photons and electrons go up and down

Why are metals malleable/ ductile

There is a sea of electrons and they all move with respect to one another

Why an metals conduct electricity

Because electrons can move around freely and in metals, the bonding molecular orbital/ valence band overlaps with the antibonding MOs/ conduction band so electrons flow everywhere

In semimetals, there is a small gap so you need energy to move the electrons

What happens from melting to boiling

It absorbs energy that comes from the surroundings and the attraction overcome is between particles

Condensing to Freezing

Energy is released into the surroundings and an attraction is formed between particles

Discrete Materials

Exist as separate atoms or molecules like any noble gases; usually lower melting and boiling points

Continuous Materials

Exist as extended networks of atoms connected to each other like metals, diamonds, graphite; higher melting and boiling points because they are in a network and it is hard to break them up

Covalent Bond

Very strong, require a lot of energy to break (if it is a metal and has a high idling point, then it is metallic but if it is a non meta and high boiling point, covalent)

Caused by attraction of electrons from one atom to nucleus of other atoms

Present only when atomic orbitals interact constructively

Present within molecules or networks

London Dispersion Forces

Relatively we bonds and cause by fluctuating charges and present within all molecular species

Valence Bond Theory

Atomic orbitals overlap to form bond, greater the overlap the stronger the bond and electron pairs form orbitals; electrons are localized and explains why different orbitals overlap differently

Hybridization occurs but mainly sigma (diamond) and pi (graphite)

Explains why the same element can form different things due to orbital hybridization where an atom’s atomic orbitals mix to form new hybrid orbitals

Bonding for diamonds

Each carbon atom forms four bonds to 4 identical carbon atoms and it is called a tetrahedral

Valence bond theory says that for each bond carbon needs a singly occupied orbital pointing in the direction of the bond so atomic orbitals hybridize to form bonding orbitals and then combine with orbitals. From other atom to form a bond

One s, 3 p orbitals= sigma bond which is end-to-end

Why do diamonds have high boiling and melting point

Covalent bonds aer extremely strong so you cannot melt a diamond because that needs too much energy because the complexes are so strong

Why are diamonds hard

Covalent bonds are really strong

Why do diamonds not conduct electricity

To conduct electricity, electrons need to move and be delocalized but in the diamond they do not move around and have a huge gap in orbital/ anti bonding orbital

Sigma bonding

End to end overlap of atomic orbitals and it can rotate so the molecules can have different shapes

Where are pi bonds found

In graphite

In double and triple carbon bonds, one for each leftover p orbital and they cannot rotate

Where are sigma bonds

In diamonds and graphite

In all bonds for every sp there is and they cannot rotate

Pi bonding

Side to side overlap of atomic orbitals which prevent rotation

Graphite bonds

1 s and 2 p orbitals so three sp² orbitals (there is a p orbital leftover) and its called a trigonal planar

sp² hybridized orbitals overlap for a localized sigma bond framework and the unhybridized p orbitals form pi ones

Formed in sheets and then the pi orbitals form over sheet and electrons are free

Why can graphite conduct electricity

Because electrons can move freely over the entire sheet within its delocalized pi molecular orbitals

Why is graphite shiny

Because it can absorb/emit photons of many wavelengths

Why is graphite slippery

Sheets canal side overlap each other and only held together by LDFs

Emergent properties

Properties of the compounds with many atoms act different than just the atoms alone

How many bonds does carbon have

4 bonds= 4 hybrid orbitals= 4 atomic orbitals mix together; carbon usually has sp³ (one s, three p)

Hybrid orbitals

Formed by mixing two or more atomic orbitals on the same atom which make it better and easier to form strong covalent bonds and it allows fro stronger bonds due to more overlap

3D model rules

Two bonds from each central atom can be in the same plane which is a straight line and then one is a dotted line going behind and one is a little ledge that is coming towards us

Lewis Dot Structure rules

1. Count total valence electrons in the formula to see how many you have to work with (use group numbers), watch out for ions!

2. Write skeleton structure- see how many bonds each element can have

3. Use 2 electrons for each bond and each element except for B and H should have eight electrons around it

4. If there are not enough electrons= multiple bonds

How many bonds can each element make?

What do Lewis dot structures lack?

They are only 2D and it helps more if they are 3D because they show the overlap

Isomers

Number if each type of atom/element us the same and the same chemical formula

BUT, different connectivity and properties

Draw a line with how many carbon chains there are and that will tell you if they are the same or if they are isomers

CH4 vs C2H6

When molecular substance boils, forces between molecules are overcome like the LDFs

C2H6 has more atoms so therefore, more electrons and stronger LDFs so C2H6 is floppier and has a higher melting and boiling points

Alkane

Single carbon-carbon bonds with only sigma bonds so they can rotate

Alkene

Contain double C-C bonds and it has one sigma bond, one pi bond

Each is a sp² and p bond and the pi bond cannot rotate because it is a double bond but it can only break if you put energy into it and that changes the structure

Alkyne

At least one C-C triple bond that consists of one sigma bond (sp) and two pi bonds

Formal charge

The charge that is lowest and closest to 0 is the most stable and this is how we tell which elements are the most stable

Formal Charge = valence electrons of atom - # bonds to central atom - # non bonded electrons/ dots on atoms

When we have a formal charge, it removes rules so you bond to whatever satisfies the other element

VSEPR

Valence Shell Electron Pair Repulsion

Helps us figure out shapes of molecules from the Lewis structure because we assume all electron centers want to get farthest from each other

What happens with 2 electron centers

Hybridization= sp

Electron centers geometry= linear

Shape= linear (if it does not have any lone pairs or it is bent)

What happens with 3 electron centers

Hybridization= Sp²

Electron centers geometry= trigonal planar

Shape= trigonal planar or if there is a lone pairs of electrons, it will be bent

What happens with 4 electron centers

Hybridization= sp³

Electron Center Geometry= Tetrahedral

Shape= Tetrahedral, trigonal planar for 1 pair of lone electrons, bent for 2 lone electron pairs

Difference between electron center geometry and shape

They are not the same!

Electron center geometry is how many electron centers are coming out of the center atom

Shape= dis includes the lone pairs and only counts the atoms and it is a 3D shape and we can use it to tell polarity

Trigonal planar

A 2D shape with 3 atoms coming out of it and no lone pairs

Trigonal pyramid

A 3D shape where there is three atoms and one lone pair combined out of it

Linear

Shape and geometry when there is only 2 things without any lone pairs

Bent

For trigonal planar (3) and one of the three is a lone pair

Tetrahedral and when two of them are lone pairs

So essentially when there are only two elements connected and lone pairs

Electronegativity

An atom that has a high effective nuclear charge (protons- core electrons/most previous noble gas) attracts its own valence electrons but also attracts electrons to itself in a bond from other atoms in bonds

Which element is most electronegative and which do not bond

Fluorine is the most electronegative

Electronegativity only relates to bonding atoms, Neon atoms do not bond and therefore they do not count

Electronegative trends

Across group= increase

Down columns= decrease (bigger electron cloud= not hold electrons tightly)

Polar bonds

When two atoms of different electronegativity ex bond and the electrons are not shared equally= they form a dipole

When the polar arrow has nothing to cancel it out= electrons go to one side (more electronegative side)

Non polar bonds

Where dipoles do not form and the arrows cancel each other out with something as or more powerful; or, when they are all going different directions

CH is always non polar

Which are stronger, IMFs or bonding

BONDING by a long shot but IMFs to least to most powerful= LDFs are for non polars only, then dipole dipole, then hydrogen bonding interaction

LDFs

Present in all solid and liquid molecules; this is what non polars molecules overcome when melted and they have a low boiling/melting point because they have temporary dipoles that fluctuate

Dipole- Dipole

When molecules are polar this is their strongest thing they overcome; garunteed partial charges so solid dipoles so they have a giver melting/boiling point and always mark the partial positive and partial negative charges because that is what they are attracted

Hydrogen bonding interactions

Strongest polar IMFs because the partial charges are between the most partial positive and partial negative; the H has to be connected to an O, N, or F and they have to have a lone pair of electrons on them

For picture= you have to connect the O and the H together