Post on 01-Jan-2016
Unit 9 Bonding
The attractive forces between atoms leads to chemical bonds that result in chemical compounds.
CH4 methane gas molecule
Why do atoms form bonds?
Atoms form bonds due to the need to have the most stable configuration for its electrons. Atoms lose, gain, or share valence electrons in order to achieve a lower energy state (stable).
How atoms bond with each other depends on:
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ElectronegativityIonization Energy# Valence Electrons
Metallic bonding
http://www.launc.tased.edu.au/online/sciences/PhysSci/pschem/metals/Metals.htm
Metallic bonding
Metallic bonding is the strong attraction between closely packed positive metal ions and a 'sea' of delocalized electrons.
http://www.bbc.co.uk/schools/gcsebitesize
What are the properties of metals?
How does metallic bonding affect the properties of metals?
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What property of metals is illustrated above?
Metallic bonds
Ionic Bonding
• an electrostatic force – Electrostatic refers to the attraction between
opposite charges
• Stronger than metallic bonds because of the opposite charges
Ionic bond
• Formed by the transfer of electrons between a metal and a nonmetal
• # of e- lost by metal = # of e- gained by nonmetal
Charge
• The number of electrons that need to be lost or gained by an atom so it has the same electronic configuration as a noble gas.
• When an atom gains or loses electrons, it is called an ION.– + ions are cations– - ions are anions
• The charge of the ion is called the OXIDATION NUMBER.
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Charge based on “copy cat” principleSulfur wants to be like Argon (1s22s22p63s23p6)
Sulfur Atom 1s22s22p63s23p4
16 Protons 16(+)16 Electrons 16 (-)
0 No charge
Sulfur has 6 valence e- and will gain 2 more to complete an octet. The result is a:Sulfur Anion 1s22s22p63s23p6
16 Protons 16 (+)18 Electrons 18 (-)
2- Charge
Ionic Bonds
Strong electrostatic (positive-negative) force in ionic compounds makes a strong ionic bond.
How does the strong ionic bond affect the properties of ionic compounds?
Formula unit – smallest unit of an ionic compound; lowest whole number ratio of ions represented in an ionic compound
• The greater the difference between eN values of 2 atoms is, the more ionic the bond will be. We call this “ionic character”.
0.8- 4.0 = 3.2 very ionic
Ionic Bond
http://www.chm.bris.ac.uk/pt/harvey/gcse/ionic.htm
Na+ Cl-
Writing Lewis Dot Structures
• Element symbol represents the kernel (core) of the atom (nucleus and inner e-)
• Dots represent the valence e-
www.meta-synthesis.com
• Metals tend to lose e- while nonmetals tend to gain electrons
Writing Lewis Dot Structures - Ionic
hyperphysics.phy-astr.gsu.eduIonic bonds
• Metals tend to lose e- while nonmetals tend to gain electrons
Writing Lewis Dot Structures – Ionic Bonds
chemistry58.wikispaces.com
Ionic bonds
Properties of ionic compounds (all related to the strong attraction between
the + and – charges)• high melting points and
boiling points• hard solids• good conductors – in
aqueous solutions and when molten
• have a crystal lattice structure Ions are
here
Really, we don’t hate you.
Covalent Bond
• Covalent Bond –formed when two nonmetals share pairs of valence electrons in order to obtain the electron configuration of a noble gas
• Molecule - formed when two or more atoms bond covalently. –(A molecule is to a covalent bond as a formula unit is to an ionic bond.)
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How covalent atoms bond
Diatomic Molecules• HOFBrINCl
Share electrons when they bond together23
Polyatomic Ions
• covalently bonded group of atoms, with a charge
• Watch this
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They are listed on your STAAR chart. You will not have a bad time.
Properties of Covalent Molecules
• Can exist as gases, liquids, or solids depending on molecular mass and polarity
• Usually have lower MP and BP than ionic compounds of the same mass
• Do not usually dissociate (break apart into ions) in water
• Do not conduct electricity
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How to draw Lewis dot structures for covalent molecules.
1. Write the formula for the compound.2. Count the total number of valence electrons. 3. Predict the location of the atoms:
a) Hydrogen is NEVER the central atom.b) If carbon is present, it is ALWAYS the central atom.c) If there is only 1 atom of an element, it is the central atom.d) The least electronegative atom is generally the central atom.
4. Place one electron PAIR between the central atom and each ligand (side atom) to “hook” the atoms together.
5. Dot the remaining electrons in pairs around the compound to complete the octet. Start with the ligands.
6. Check that each atom has an octet. (H only needs a pair, not an octet.)7. Watch This
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Lewis Structures for Molecules
• Draw the Lewis dot structure for these molecules:– Hydrogen + Bromine (HBr)
– Carbon + Chlorine (CCl4)
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Bonding e- PairsLone Pairs(nonbonding electrons)
Covalent bonds
Writing Lewis Dot Structures - Covalent Bonds
Number of bonds• Single Bonds - when one pair of e- is shared
between atoms
• Double bond – when atoms share 2 pairs of valence electrons; ex. O2
• Triple bond – when atoms share 3 pairs of valence electrons; ex. N2
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Describing bonds
• Sigma bond - the first bond between 2 atoms– A single bond is a sigma bond.
• Pi bond - the second bond between 2 atoms– A double bond consists of a sigma bond and a pi
bond. – A triple bond consists of a sigma bond and two pi
bonds.
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Carbon can form single, double and triple bonds with itself.
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• The shape of a molecule plays a very important role in determining its properties.
Why are molecular shapes important?
• Properties such as smell, taste, and proper targeting (of drugs) are all the result of molecular shape.
VSEPR Theory also called electron geometry
• Electron groups around the central atom will be most stable when they are as far apart as possible. We call this valence shell electron pair repulsion theory.– Because electrons are negatively charged, they
should be most stable when they are separated as much as possible.
• The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule.
TO DETERMINE VSEPR SHAPE (electron geometry)
1) Draw the Lewis dot structure for the molecule2) Identify the central atom3) Count the number of electron groups around the
central atom.4) Look up the VSEPR shape on the chart.**shapes with no lone pairs are symmetrical**shapes with lone pairs are assymmetrical
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O N O ••••
••
••
•••• There are three electron groups on N•One lone pair•One single bond•One double bond
Electron Groups• Each lone pair of electrons = one electron
group on a central atom.• Each bond = one electron group on a central
atom (regardless of whether it is single, double, or triple.)
Two Electron Groups: Linear Electron Geometry
• When there are two electron groups around the central atom, they will occupy positions on opposite sides of the central atom.
• This results in the electron groups taking a linear shape.
Linear Geometry
Three Electron Groups: Trigonal Planar Electron Geometry
• When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom.
• This results in the electron groups taking a trigonal planar shape.
Trigonal Planar Geometry
Four Electron Groups: Tetrahedral Electron Geometry
• When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom.
• This results in the electron groups taking a tetrahedral shape.
Tetrahedral Geometry
Molecular Geometry• The actual geometry of the molecule may be
different from the VSEPR shape.• Lone pairs repel bonded atoms which distorts the
expected shape.
Bond Angle Distortion from Lone Pairs
Electron Geometry: Tetrahedral Tetrahedral TetrahedralMolecular Geometry: Tetrahedral Trigonal Pyramidal Bent
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Predicting the Shapes around Central Atoms
1. Draw the Lewis structure.2. Determine the number of electron groups
around the central atom.3. Classify each electron group as a bonding or
lone pair, and count each type.– Remember, multiple bonds count as one group.
4. Look it up
Practice:
• Determine the shape.
1. NF3
2. SiCl4
3. H2O
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Types of Bonds:• Nonpolar covalent equal sharing of
electrons between atoms; occurs between the atoms in a diatomic molecule (HOFBrINCl) and between C and H; ex. CH4
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Polar Covalentunequal sharing of electrons between atoms; occurs between two nonmetals or a nonmetal and a metalloid; ex. H2O
Electrons
Ionic
• complete transfer of electrons
• occurs between– m/nm (ex. NaCl)– m/PAI– PAI/nm– PAI/PAI
THIS IS A CONTINUUM. IT DESCRIBES THE “IONIC CHARACTER” OF THE BOND.
Bond type
Non-Polar Covalent Polar Covalent Ionic NPC PC I
• Difference in electronegativity values
• Distance between atoms on the periodic table
Small medium big
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Practice:
• What type of bond exists in each of the following?
1. HCl2. CaO
3. H2O
4.Br2
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Water is a POLAR molecule
The more electronegative atom will have a slight negative charge, the area around the least electronegative atom will have a slight positive charge. 52
Symmetric molecules tend to be nonpolarAsymmetric molecules with polar bonds are polar
53Symmetric means there are no lone pair around the central atom.
Type of Compound
Elements involve in bonding
(metal/non metal)
Valence electrons
are…
Melting /Boiling
point
Electrical conductivity
Other properties
Metallic
Ionic
Covalent
Type of Compound
Elements involve in bonding
(metal/non metal)
Valence electrons
are…
Melting /Boiling
point
Electrical conductivity
Other properties
Metallic Metal-metal delocalized high Conductor Malleable, ductile, shiny
Ionic Metal - nonmetal
Lost/gained high Conducts in solutions or molten
Brittle, solid at room temperature
Covalent Nonmetal - nonmetal
shared low Non-conductor
Mostly liquid or gas at room temperature
Bonding determines some physical properties