Bonding & Molecular Shape
Jan 05, 2016
Bonding & Molecular Shape
Groups on the Periodic Table
Alkali Metals Alkaline Earth
Metals Transition Metals Boron Group Carbon Group Nitrogen Group
Oxygen Group (Chalcogens)
Halogens Noble Gases Actinide Series Lanthanide
Series
Why Are There Patterns? Elements have physical and
chemical properties based upon their valence electrons.
These valence electrons are the electrons in the outermost energy level (s & p orbitals)
The number of valence electrons may be determined by using the periodic table
Be like the Noble Gases Bonding is when atoms tend to
gain, lose, or share electrons in order to gain a full set of valence electrons.
Metals will usually lose electrons which is why they form positive ions. Nonmetals will gain electrons and usually form negative ions.
Take a look at the number of electrons in the outer shell of each of the atoms below. Notice Sodium (Na) has 1 in its outer shell. It is easier to remove 1 than gain 7, so notice that the electron is leaving Sodium and heading to the Chlorine atom.
Lewis Dot Structures
Dots representing valence electrons are written outside of the chemical symbol.
Lewis dot structures are a quick and easy way to see how many valence electrons an atom has.
The Group Number on the Periodic Chart is the same as the valence electrons.
To the left is the electron configuration of the second period of elements. Notice that there are never more than 2 electrons per side.
Chemical Bonding Any chemical reaction is a
rearrangement of valence electrons to get a filled outer energy level. Chemical bonds are formed.
HOWEVER, while there are different types of bonds, they are all very similar.
The Nature of Bonding
All bonds are comprised of electrons (always in pairs) between the two atoms that are being bonded.
The are classified differently based on how even or uneven the sharing of electrons is between the atoms.
The measure of how much an atom pulls on shared electrons is called Electronegativity.
Electronegativity This idea was developed by Linus
Pauling – 2 time Nobel Prize winner. It is a measure of how strongly an
atom grabs onto shared electrons. Every atom has its own value. Fluorine is the highest at 4.0 We can use these values to classify
which type of bond we are working with.
The symbol for it is En
Electronegativity # 2 Here is how the type of bond is
determined.• No matter how big the atom, it can be
broken down to individual pairs of atoms being bonded together.
• Look up the En for each atom off a chart.
• Subtract the values so it comes out to be a positive number or zero.
• This value is the key to learning more about the bond in question.
Ionic Bonds In an ionic bond electrons are
‘transferred’ from one element to another.
The cation (+ ion) is attracted to the anion (- ion); a bond is formed.
Ions in an ionic bonds may be positive or negative.
Ionic Bonds Continued
Keep in mind that the electron can’t be totally transferred. There must always be a connection between both atoms and the shared electrons. If not, then they are just floating ions, like they are when in a solution.
Covalent Bonds Sometimes electrons are shared
more evenly between molecules. While the atom that is more
electronegative is winning the “tug of war for the shared electrons”, the difference is small and the electrons show balance between the two atoms.
Bond Lengths
Not all bond lengths are the same. H-H bond is the shortest at (0.074
nm) As you move down a group the
bond length becomes longer. Multiple bonds are shorter than
single bonds C-C 0.154 nm C=C 0.134 nm
Bond LengthBond Length
Bonding & Electronegativity The electronegativity difference
(ΔEn) between two atoms determines type of bond.
Non-polar covalent is when the ΔEn is ≤ .4
Polar covalent is when the .4 < ΔEn < 1.67
Ionic is when the ΔEn is ≥ 1.67
Practice
What kind of bond is formed between •Fluorine and Cesium
•Sodium and Iodine
•Phosphorus and Sulfur
Answers to the last slideAnswers to the last slide
Ionic bond because the difference in Ionic bond because the difference in Electronegativity is greater than 1.67Electronegativity is greater than 1.67
It is also polar covalent because the It is also polar covalent because the difference is 1.6 just slightly less than difference is 1.6 just slightly less than 1.671.67
This would be a non-polar covalent This would be a non-polar covalent bond because the difference in Ebond because the difference in Enn is .4is .4
FormulasFormulas Models – Show which Models – Show which
atoms are bonded to atoms are bonded to which.which.
Molecular Shape – Molecular Shape – Gives the angles of Gives the angles of the bondthe bond
Formulas Ball and Stick –
Ball represents the inner atom, the sticks represent bonds
Space Filling – shows a more life-like view of the molecule
How Do We Know This? VSEPR Theory – Valence-shell
electron pair repulsion theory In a small molecule, pairs of
valence electrons are arranged as far apart from each other as possible.
Lone pairs of electrons repel more than electrons in bonds
Double bonds count as one pair!!
Effective Pairs
Covalent bonds count as 1 effective pair.
Double bonds and triple bonds count as 1 pair !!
Lone pairs count as 1 pair.
Shapes of Small Molecules
Linear – atoms connected form a straight line.
Bond forms an angle of 180o
Carbon dioxide and acetylene are examples.
Shapes of Small Molecules
Trigonal Planar – 3 atoms connected to a central atom.
Bond forms an angle of 120o
Boron trichloride & carbonate ion
Shapes of Small Molecules
Tetrahedral – 4 atoms connected to a central atom.
Bond forms an angle of 109.5o
Methane & carbon tetrachloride
Shapes of Small Molecules
Pyramidal – 3 sided pyramid
Lone electron pair not involved in bonding.
Bond forms an angle of 107.3o
Ammonia, Phosphorous Trichloride
Shapes of Small Molecules Bent – 3 atoms
not in a straight line
There are lone electron pairs not involved in bonding.
Bond forms an angle of 104.5o
Water & Nitrite Ion
Beyond Tetrahedral
Bromine Pentafluoride (square pyramidal BrF5)
Bromine Trifluoride (T-shaped BrF3)
Beyond Tetrahedral
Phosphorus Pentachloride (trigonal bipyramidal PCl5)
Sulfur Tetrafluoride (see-saw SF4)
Beyond Tetrahedral
Sulfur Hexafluoride (octahedral SF6)
Tetrachloroiodate Ion (square planar ICl4-1)
Practice
What shape is formed from
•Nitrogen trichloride NCl3•Formaldehyde HCHO
• Oxygen Difluoride OF2
How many effective pairs in each ? There is a tutorial on Shapes of
Molecules on the website.
Answers Answers
4 effective pairs with 3 of them 4 effective pairs with 3 of them bonding. Pyramidalbonding. Pyramidal
3 effective pairs with all 3 of them 3 effective pairs with all 3 of them bonding. Trigonal Planarbonding. Trigonal Planar
4 effective pairs with 2 of them 4 effective pairs with 2 of them bonding. Angularbonding. Angular
Hybrid Orbitals In bonding the electron orbitals
“mix” together forming hybrids. The hybrid orbital had characteristics
of both. Many types of hybrid orbitals are
possible. The number of hybrid orbitals
created must always equal the number of atomic orbitals that are started with.
sp Hybrid Orbitalssp Hybrid Orbitals
sp hybrid orbitals sp hybrid orbitals combine a 1s combine a 1s and 1p orbital.and 1p orbital.
Forms linear Forms linear molecules like molecules like BeFBeF22..
spsp22 Hybrid Orbitals Hybrid Orbitals
spsp22 hybrid hybrid orbitals combine orbitals combine 1s and 2p 1s and 2p orbitals.orbitals.
Forms trigonal Forms trigonal planar molecules planar molecules like BFlike BF33..
sp3 Hybrid Orbitals sp3 hybrid orbitals
combine 1s and 3p orbitals.
Forms tetrahedral molecules like CH4.
Other hybrid orbitals form from the d orbital.
Polarity
Molecules as well as bonds may be polar or nonpolar.
Polar molecules are also called dipoles.
Polarity is determined by the shape of the molecule as well as the polarity of each of the bonds.
Polarity Bond polarity is represented by
vectors. Arrows point to the more
electronegative atom. The polarity of the molecule is the sum
of all the vectors.
Practice
Is it a polar molecule? •Chloroform (CHCl3)
•Formaldehyde•Methane
•Chloroform and methane are non-polar.
•Formaldehyde is a polar molecule.
Hydrogen Bonds The weak intermolecular forces
between polar molecules creates Hydrogen bonds.
H bonds have 5% of the strength of a covalent bond.
This might not seem like much but there are so many of them that they require a lot of energy to break.
This gives water unique properties like its high boiling pt.
Polarity Determines Physical Properties
Polar molecules will align in an electrical field.
The more polar a molecule, then the higher the boiling point.
Higher polarity gives a high surface tension.
Polarity Determines Physical Properties
Polar molecules will dissolve in polar solvents Hydrophobic - “water fearing” Hydrophilic – “water loving” Soap is an amphipathic molecule
Organic Chemistry Organic chemistry is the
study of carbon compounds. It is the largest branch of
Chemistry. Biochemists are organic chemists.
There is a large number of organic compounds; gasoline, proteins, etc.
The structure to the right is a fullerene. Look it up ! It is a new type of molecule and appears to have a lot of promise in many applications.