Post on 31-Mar-2015
UNIT 7: BONDING
Why Bond?
Elements bond in order to get a full valence shell
Elements want to have the same number of electrons as the noble gases The Octet Rule: in forming compounds,
atoms tend to achieve the electron configuration of a noble gas
**** only VALENCE electrons are involved in bonding
Types of Bonds
Ionic Covalent Metallic
Review of Metals
Metals lose electrons to form positive ions (cations)
Ionic radius is smaller than atomic radius
Group 1: Alkali metals Lose 1 electron
Group 2: Alkaline Earth Lose 2 electrons
Review of Nonmetals
Gain electrons to form negative ions (anions)
Ionic radius is larger than atomic radius
Group 17: Halogens Gain 1 electron
Group 16: Chalcogens Gain 2 electrons
Ionic Bonds
The force of attraction that holds ions of opposite charge together This attraction is between a metal and a
nonmetal Forms from the TRANSFER of electrons
Electrons are transferred from METAL NONMETAL
Large difference in electronegativity Overall charge of compound is neutral (+
and – cancel out) Ex) NaCl
Animation
http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/bom1s2_11.swf
Writing Lewis Dot Structures for Ionic Compounds
Lewis Dot diagrams help us visualize what is happening to valence electrons when a bond forms
Remember- Follow the octet rule!! Ex) Li and F
Steps: 1) Draw all elements and their individual Lewis Dot diagrams. 2) Draw an arrow indicating the transfer of electrons3) Redraw your bonded compound with appropriate charges
More Practice…
Draw the Lewis Dot Diagrams for the following ionic compounds:
BaO
K2S
CaCl2
What if you are not given the formula?
Draw the Lewis Dot diagram for the ionic bonding between sodium and sulfur.
Ionic Compounds
An ionic compound exists as a collection of positively and negatively charged ions arranged in repeating patterns
A formula unit is the lowest whole-number ratio of ions in an ionic compound NaCl (1:1 ratio) , MgCl2 (1:2 ratio)
Properties of Ionic Compounds (salts)
Ionic bonds are the strongest bonds, so all are solids
Hard High melting points and boiling points Soluble in water Cannot conduct electricity as a solid Can conduct electricity as a liquid or in
an aqueous solution http://www.youtube.com/watch?v=NfNIn4R8tg4
They are electrolytes- substances that conduct electricity when dissolved in water
Covalent Bonds
A bond between two nonmetals that involves a sharing of electrons (“tug of war”) Can have EQUAL or UNEQUAL
sharing Small or no difference in
electronegativity Overall charge of a covalent
compound is neutral Covalent compounds are called
molecules Ex) O2, CO, CCl4, H2
Types of Covalent Bonds
There are 3 types of covalent bonds
Covalent Bonds
Non-Polar Covalent
Polar covalen
t
Coordinate Covalent
Non-Polar Covalent Bond
Electrons are shared equally Same atoms- same
electronegativity All diatomics have non-polar bonds! Remember: HOFBrINCl
Two of the same non-metal
Ex) Br2
Polar Covalent Bond
Electrons are shared unequally Different atoms with different
electronegativity values Ex) HF
The more electronegative atom attracts electrons more strongly and gains a slightly (-) charge
The less electronegative atom has a slightly (+) charge
Bond Polarity
• Bond Polarity: refers to a separation of charge in a bond
Ex) HF : partial (+) charge on H and partial (-) charge on F
• This separation of charge is often called a dipole
• ***The greater the difference in electronegativity, the greater the polarity.
Challenge Question
Which of the following covalent compounds has the greatest degree of polarity?
Choice 1) CO
Choice 2) HCl
Choice 3) NO
Choice 4) HBr
Drawing Lewis Structures for Covalent Compounds
Remember! After drawing your diagram, all atoms MUST have 8 valence electrons (except for hydrogen, which should have 2 electrons)
Ex) F2
What if we have more than two non-metals?
Ex) H2O
Ex) NH3
Lewis Structures Continued…
When elements from Group 14 are involved in a covalent bond, they spread their e- out
Carbon tends to form 4 covalent bonds
Ex) CH4
Ex) CCl4
Practice Problem
Draw the Lewis Structure for CH3Br
Can you have more than one bond?
Yes! So far all we have seen are single covalent bonds in which one pair of electrons is shared
Double covalent bond: a bond between two atoms where 2 pairs of e- are shared
Ex) O and O
*** Oxygen tends to form 2 bonds! Triple covalent bond: a bond between two
atoms where 3 pairs of e- are sharedEx) N and N
*** Nitrogen tends to form 3 bonds!
More Practice with Lewis Diagrams..Draw the Lewis diagrams for CO2
Challenge Problem
Draw the Lewis Structure for HCN
Let’s Summarize
C, H, O and N form how many bonds??
Coordinate Covalent Bond
A bond in which both electrons of the shared pair come from the same atom
Ex) NH4+
VSEPR Geometry
Lewis Dot diagrams fail to show the 3-dimensional shapes of molecules
VSEPR Theory (Valence-Shell Electron Pair Repulsion Theory) Repulsion between e- pairs causes
molecular shapes to adjust so that valence e- pairs stay as far apart as possible
http://phet.colorado.edu/en/simulation/molecule-shapes
Molecular Polarity
Molecular polarity is different from bond polarity!!
http://phet.colorado.edu/en/simulation/molecule-polarity In a non-polar molecule, electron
distribution is even (symmetrical) In a polar molecule, electron distribution
is uneven (asymmetrical)
Intermolecular Forces (IMFs)
Only between covalent molecules, never ionic compounds
Weak forces that act between molecules and hold
molecules to each other ****IMFs are not bonds!!!
IMFs occur BETWEEN molecules, bonding occurs WITHIN molecules
IMFs vs. Bonds
Intermolecular Forces of Attraction
• Van der Waals Forces• Dipole interactions• Hydrogen Bonds
London Dispersion Forces
London Dispersion forces: weakest of all molecular interactions; caused by temporary shifts in charge
- Between nonpolar molecules http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml
- The bigger the atom or molecule the greater the strength of dispersion forces the higher the BP
Think about it…
Which has the strongest London dispersion forces?
F2
Cl2
Br2
I2
Dipole-Dipole Interactions
Dipole interactions: attraction between polar molecules
The positive and negative charges of different molecules attract each other
Ex. HCl http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIPOLED.html
Hydrogen bonds: A special case of dipole-dipole interactions
Hydrogen bonds: intermolecular force between the H of one molecule and a highly electronegative atom of another molecule (must be N, O, or F)
Ex. H2O, NH3
***The high b.p. of water is due to hydrogen bonding
Molecule-Ion Attraction
Partial charges on a molecule are attracted to ions
This is what happens when NaCl dissolves in water
Picture: The hydrogens of water align themselves
towards the anion and the oxygen align themselves towards the cation
http://www.kentchemistry.com/links/bonding/LikeDissolveslike.htm
Molecule-Ion Attraction
Properties of Covalent Compounds Soft Low melting points and boiling
points Cannot conduct electricity in any
phase Generally insoluble in water
Except sugars! (C12H22O11)
Network Solids
A special case of covalent bonding Atoms held together in a very strong covalent
network Ex. Carbon (Diamond) and SiO2 Properties:
Hard High m.p. and b.p. Poor conductors
Holds metals together Electrons are mobile and move from one
atom to another, creating (+) charged metal ions
Charged metal ions are immersed in a “sea of mobile electrons”
Metallic Bonding
Bond Energy
Bonds do not break and form spontaneously- an energy change is required
When a bond is broken, energy is ABSORBED (required) Ex) F2 (g) + ENERGY F (g) + F (g) The greater the # of bonds between
atoms, the more energy you need to break them
When a bond is formed, energy is RELEASED (given off) Ex) F(g) + F(g) F2 (g) + ENERGY