Bonding

BondingYear 11 DP Chemistry

What is a bond?

Ionic Bonds result from electrostatic attraction between oppositely charged ions

Covalent Bonds result from electrostatic attractions involving electrons and positively charged nuclei

Metallic Bonds result from electrostatic attractions between delocalised electrons and a lattice of positively charged nuclei

A chemical bond is a force that holds atoms together making a new substance

What type of bond?Ionic Bond: As a rule of thumb, we say that the difference between the electronegativity values needs to be high (i.e. greater than 1.7) to be ionic. They form between cations on the left and anions on the right of the Periodic Table.

Covalent Bond: If the difference between the electronegativity values of two highly electronegative atoms is low, a covalent bond is formed. They tend to form between non-metals, but sometimes metals are involved (eg Al2Cl6)Metallic Bond: If the difference between the electronegativity values of two highly electropositive atoms is low, a metallic bond is formed. These form between metals of the same or different type of atom

ElectronegativityThe relative tendency of an atom to attract bonding electrons to itself on the Pauling Scale

Formation of ionsGroups 1,2,3Lose electrons Group 1 – loses

one e- to gain a full valence shell

Group 2 – loses 2 e- to gain a full valence shell

Group 3 – loses ??

Groups 5,6,7Gain electrons Group 5 – gains 3

e- to gain a full valence shell

Group 6 – gains 2 e- to gain a full outer shell

Group 7 - ??

Transition ions Ions of the transition elements can form more

than one ion This is due to s and d orbitals having similar

energy levels

For example – Fe forms two ions[Ar]4s23d6

Can you deduce which two ions and why?

Fe(II) – losing two 4s electronsF(III) – losing two 4s e- and one 3d e- to give a half-filled d

Exercises Using a Periodic Table, determine which

ions are formed from the following elements? Na, Al, O, Ca, F, N

Describe the size of each of the above ions relative to their original atoms.

Ionic BondingOppositely charged ions are formed by electron transfer due to a large electronegativity difference(> 1.7 difference)

Na has a low electronegativity relative to Cl, so ions are formed by a transfer of an electron to achieve a full valence shell for both atoms. These oppositely charged ions then form a bond.

Sodium Chloride Lattice Ionic compounds form

a repeating crystal lattice of positive and negative ions. The compound is neutral

The electrostatic attraction is very strong so ionic compounds form solids at room temperature

This shows a model of a NaCl lattice with alternating positive and negative ions

Ionic formulas The formulas for ionic compounds are

found by balancing the overall charges to zero (neutral) First determine the charge on each ion Second determine the number of each ion

needed to make the compound neutral Write the positive ion first indicating the

number of each using subscripted numbers

Examples:Li+ F- LiF Mg2+ Cl- MgCl2

Polyatomic ionsSome ions contain more than one element and the charge on the ion is spread (delocalised) over the entire ion. They have specific names and act as a single unit.

Important ones to know:NH4

+ (ammonium)NO3

- (nitrate)OH- (hydroxide)SO4

2- (sulfate)CO3

2- (carbonate)PO4

3- (phosphate)HCO3

- (bicarbonate or hydrogen carbonate)

Ionic compounds form in the same way with polyatomic ions. Here we see Na2SO4. Notice the sulfate did not change formula.

Exercises Write the formulas formed by the

following pairs of elements: (Ca, S); (K, N); (Ca, P); (Fe(II), O); (Fe(III), O)

Write the formulas for the following compounds: potassium hydroxide, magnesium nitrate, sodium hydrogen sulfate

Use electronegativity values to determine the type of bonding between Al and O; Al and Br.

Covalent bondingElectrons are shared between two atoms. These atoms are most commonly non-metals.

The shared electrons are held in place by the positively charged nuclei that are sharing them.

Single bond = 1 shared pairDouble bond = 2 shared pairsTriple bond = 3 shared pairs

How many bonds?Single bonding:

Multiple bonding:

Multiple bondingThe more shared pairs the stronger the bond and the shorter the bond. Note the trend in C-C bonds.

Note: bond energy is the amount of energy required to break a bond

Coordinate (dative) bondsIn this type of covalent bond, the difference is that one of the atoms in the pair donates both of the electrons in the bond.

Examples: CO, NH4+,

H3O+

Electron Dot Diagrams (Lewis)

The dots represent the valence electrons for each element

The Octet RuleChemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.

Fluorine molecule

Hydrogen Chloride

Here, the octet rule is satisfied for Cl, but is irrelevant for H which can only hold 2 e-.

Double Bonds - ethene

Two pairs of shared electrons

Draw the Lewis structure for Acetylene (ethyne) – C2H2

Formation of water

Notice the double bond in O2. No other configuration will satisfy the octet rule. Why is H4O not formed?

Resonance When more than one possible Lewis structure

can be drawn, it indicates resonance The actual structure is an average of the

resonance structures

Notice that the bond lengths would be different in benzene unless there is a resonance structure like the one represented above

The Octet Rule2nd row elements C, N, O, F observe the octet rule.2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive.3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

Lewis Diagrams Determine the number of valence e- for each

element Determine the number of bonds for each element

(usually the number of e- needed to fill the valence shell – exceptions AHL)

Central atom is the one with the most bonds Join the atoms so that all have the correct number of

bonds Each bond has one e- from each atom (unless dative

bond), so a pair makes a bond Once all bonds are made, place the remainder of e-

around the appropriate atoms as non-bonding pairs Check that the octet rule has been satisfied for each

element

Completing a Lewis Structure -CH3Cl

Valence e- (C = 4, H = (3)(1), Cl = 7 Total = 14 ) Number of bonds (C = 4, H = 1, Cl = 1) Carbon is the central atom (most bonds) Bonds are arranged and extra e- are added around

Cl as non-bonding pairs.

..

..H C

H

H

Cl

..

....

.. ..

All octets (ignoring H) have been satisfied.

Exercises1. N2 has a triple bond. Using dot diagrams, show why a

single or double bond is incorrect. 2. Illustrate how CO and H3O+ contain dative bonds.3. Draw a Lewis diagram for HCN4. Draw the two resonance structure of ozone O35. Use a drawing to show how many resonance structures

are possible for the nitrate ion (NO3-)

6. Compare the bond lengths and strengths of the two C,O bonds in the carboxyl group below.

Octet rule exceptionsThere are 3 ways the octet rule breaks down:

1. Molecules with odd number of electrons

2. Molecules where an atom has < octet3. Molecules where an atom has > octet

Odd number of electronsNO (nitrous oxide) Total e- = 6+5=11 Octet can be

achieved around the O with a single bond between

Remaining e- around the N

What about a double or triple bond? Try it.

Less than an octetThis mostly occurs with H, B and BeBF3 (boron trifluoride) Total e- = (3x7)+3 = 24

Octets to outer atoms

Extra e- (24-24=0) to BTherefore no extra electrons to add. B has only 6 e- (< octet). What about double bonds??? see next…

Less than a octet (cont’d)Add a double bond to BF3 for a possible octet…

This would give 3 resonance structures. What would they look like?

The above structure would lead to a δ+ on F and a δ- on B. Is this likely considering the electronegativites?

Because B has only 6 valence electrons, BF3 reacts strongly with compounds that have unshared pairs of electrons

NO

Greater than an octetThis is the most likely exception to the octet rule.

PCl5

Starting in period 3, expanded valence shells are possible.

The octet rule is based upon valence orbitals containing an s and p orbital. This gives 2 + 6 = 8 e- (an octet). In the third shell (n=3) d orbitals become available. P is below. A 3s can be exited to the 3d, which allows for 5 valence shell bonding electrons.

Promote one e-

Greater than an octet (cont’d)Sulfur (also in group III) can expand it’s octet to have more than 8 electrons as well. Sulfur can form SF2, SF4, SF6

Go to this website to see an animation on expanded octets in sulfur:http://www.saskschools.ca/curr_content/chem20/covmolec/exceptns.html

Greater than an octet (cont’d)Other notable expanded octets…

PF6- (12

e-)XeF4

(12 e-)

Polarity Polar covalent bonds

Electrons are unequally shared by atoms Electronegativity difference is 0.5>__<1.7

Nonpolar covalent bonds Electrons are equally shared Electronegativity difference is <0.5

Polar covalent bondsSome bonds are not purely ionic, but they still have a significant difference in electronegativity that leads to one atom pulling the electrons more strongly than the other.

PolarityThis electronegativity difference leads to a partially positive end δ+ of a molecule and a partially negative end δ- (note: δ is the Greek letter delta and means partial)

F is more electronegative, so the electrons spend more time around the F nucleus

Non-polar covalentSmall or no difference in electronegativity values leads to non-polar substances.

C and H have very little difference in electronegativity, so methane is non-polar

Cl is a highly electronegative element, but there is no difference when one Cl atom bonds to another Cl atom

Non-polar compoundsSome compounds contain polar bonds, but the polarity is cancelled out due to the structure.

O is more electronegative than C meaning each bond is polar towards the O atom, but due to its linear shape, these polarities cancel each other resulting in a non-polar molecule.

Exercise1. Show how S can form 3 compounds with F2. Predict if these substances contain polar bonds. H2,

CCl4, H20 3. Use arrows to show the overall polarity of the

compounds below?

Molecular geometry VSEPRTo determine the shape of covalent

molecules, we use the Valence Shell Electron Pair Repulsion Theory (VSEPR) which states:

“The geometric arrangement of atoms around a central atom is determined by the repulsion between electron pairs in the valence shell of the central atom.”

In other words… e- e-

Stay away! I

am repulsed by you e-

e-

Ditto!!

What shape?So, VSEPR theory says that molecular

geometry is determined by the shape that keeps e- pairs as far apart as possibleConsider CO2

We know C forms four bonds and O forms 2 bonds

What arrangement will allow the valence e- around the central atom (C) to be as far apart as possible?

The Lewis structure looks like this:

Linear

Linear (1800)Linear molecules have two areas of

high electron density around the central atom.

Other examples of linear molecules :

Ethene (C2H4)Ethyne (acetylene)

C2H2

Molecular chlorine (Cl2)

3 pairs around the central atom

3 e- pairs around the central atom leads to a trigonal planar shape as in BF3

Trigonal planar - Angles are1200

Bent – angles are less than 1200 due to lone pairs taking up more space than bonding pairs. These angles are 1170

If one of those pairs is a non-bonding or lone pair of electrons, the shape is described as bent or v-shaped as in SO2

4 pairsIf we look at the Lewis structure for CCl4, we might assume a flat structure with 900 bond angles

However, in 3-D space, it is possible to allow the electrons to be further apart using a tetrahedral shape with bond angles of 109.50

4 pairs (cont’d)

Tetrahedral:No lone pairs

109.50Methane

Trigonal Pyramidal:One lone

pair1070

Ammonia

Bent:Two lone

pairs1040Water

5 pairs (AHL)

Trigonal bipyramidal(0 lone pairs)

See saw(1 lone pair)

Angles = 900 & 1200

5 pairs (cont’d)

T-shaped2 lone pairs

Linear3 lone pairs

6 pairs (AHL)

Octahedral (0 lone pairs)

Square planar (2 lone pairs)

Square pyramid (1 lone pair)

Justify the shape of XeF4. Why are the lone pairs at 1800?What other two shapes are possible with 6 pairs?

Summary of geometries

Allotropes of CarbonEach carbon atom in graphite is bonded to 3 other carbon atoms forming flat sheets of carbon rings. These layers are loosely bonded to each other making graphite soft

In diamond, each carbon is bonded to 4 other carbons in a giant repeating lattice. This lattice is non-polar and very strong, making diamond the hardest mineral on Earth. It’s m.p. is over 35000C!

The fullerene contains 60 carbons arranged like a soccer ball with alternating 5 and 6-member rings.

GraphiteCarbon bonded to three other carbon atoms leaves one valence electron per carbon atom. These electrons are delocalised allowing graphite to conduct electricity

The individual layers contain strong covalent bonds, but are only loosely bonded to other layers. This allows them to easily slide over one another making graphite useful in pencils and as a solid lubricant

Fullerenes and nanotubesThe third allotrope of carbon was discovered in 1985 and includes some weird and wonderful shapes. The first was known as a “Bucky Ball” which has a structure like a soccer ball and contains 60 carbons- C60

There are other fullerenes that contain more or less than 60 carbons. These have been detected in space leading scientists to suggest that they could be the origin of life in the universe

Nanotubes are another area of current research by materials scientists. These tubes have high tensile strength and are able to conduct electricity. These have potential medical applications by attaching to resistant bacteria or cancer cells. They are also being researched for the proposed “space elevator” to be used as cables.

Bonding in Silicon

Silicon atoms contain 4 valence electrons and form regular repeating covalent bonds. It s chemically similar to C. The picture below show it’s crystalline form

Silicon dioxide (SiO2), which is the chemical component of sand. has a structure similar to diamond with a repeating giant lattice of tetrahedral shapes. This makes it very hard. However, it has polar bonds allowing it to be dissolved slowly by some alkaline solutions and it also has a much lower m.p. than diamond (17700C)

This shows a computer generated model of the complicated giant lattice of silicon dioxide.

Metallic bonding

Metallic bonding can be described as

A repeating lattice of positive metal ions in a sea of delocalised electrons

Delocalised electrons are not attached to any particular metallic nucleus and are free to move about the lattice.

Metallic PropertiesConductivityThis is due to the electrons being able to move freely through the lattice.

This means the electrons can act as charge carriers for conducting electricity and energy carriers for conducting heat

Alkali Metals

Alkali Earth metals Transition Metals Group

III

0.108 0.313    0.210 0.226   0.3770.139 0.298 0.017 0.023 0.048 0.077 0.006 0.093 0.172 0.143 0.596 0.166 0.0670.077 0.076 0.016 0.023 0.069 0.187 0.067 0.137 0.211 0.095 0.630 0.138 0.116

Fe CuElectrical Conductivity of Metals

Metallic PropertiesMalleability and Ductility The delocalised electrons in the 'sea' of electrons in the metallic bond, enable the metal atoms to roll over each other when a stress is applied.

ReferencesSource of animations and diagrams:http://intro.chem.okstate.edu/1314f00/lecture/chapter10/vsepr.htmlwww.google.com (images search)Sundin, C.; (n.d.); found at

http://www.uwplatt.edu/ ; accessed 04/2011Dynamic Science; (n.d.); found at

http://www.dynamicscience.com.au/tester/solutions/chemistry/chemistry%20index.htm ; accessed 04/2011

Courtland, R.; New Scientist (online); 28/10/2010; found at http://www.newscientist.com/blogs/shortsharpscience/2010/10/buckyballs-abound-in-space.html ; accessed 04/2011