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.