Unit 9 Bonding

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The attractive forces between atoms leads to chemical bonds that result in chemical compounds.

CH4 methane gas molecule

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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). 3

How atoms bond with each other depends on:

eee

I want

an

electron

e

ElectronegativityIonization Energy# Valence Electrons

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Metallic bonding

http://www.launc.tased.edu.au/online/sciences/PhysSci/pschem/metals/Metals.htm

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Metallic bonds

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

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Ionic Bonding

• an electrostatic force • Electrostatic refers to the attraction between

opposite charges– Stronger than metallic bonds because of the

opposite charges

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Ionic bond

–Formed by the transfer of electrons between a metal and a nonmetal

–# of e- lost by metal = # of e- gained by nonmetal

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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.

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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

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Charge of Ions• Related to the number of electrons that are

lost or gained when an atom becomes an ion; called OXIDATION NUMBER

Group2

Group 1

Group 13

Group 14

Group 15

Group 16

Group 17

Group 18

+1

+2

+3

+4

-3

-2

-1

0

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Ionic Bonds

Strong electrostatic force (positive-negative) 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

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• The greater the difference between eN values of 2 atoms the more ionic the bond will be.

0.8- 4.0 = 3.2 very ionic15

Ionic Bond

http://www.chm.bris.ac.uk/pt/harvey/gcse/ionic.htm

Na+ Cl-

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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 

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• Recall that metals tend to lose e- while nonmetals tend to gain electrons

Writing Lewis Dot Structures - Ionic

hyperphysics.phy-astr.gsu.eduIonic bonds 18

• Recall that metals tend to lose e- while nonmetals tend to gain electrons

Writing Lewis Dot Structures – Ionic Bonds

chemistry58.wikispaces.com

Ionic bonds 19

Properties of ionic compounds

• high melting points and boiling points

• hard solids• good conductors – in

aqueous solutions and when molten

• have a crystal lattice structure Ions are

here

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Really, we don’t hate you.

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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

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What is a 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

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Diatomic Molecules• HOFBrINCl

Share electrons when they bond together25

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: Write everything

1. Write the formula for the compound.2. Count the total number of valence electrons. 3. Predict the location of the atoms:

a) If there is only 1 atom of an element, it is the central atom.b) If carbon is present, it is ALWAYS the central atom.c) The least electronegative atom is generally the central atom.d) Hydrogen is NEVER 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

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Exceptions to the octet rule:

• Molecules that have an odd # of valence electrons; ex. NO2 has 17 total valence electrons and can’t form an exact # of pairs

• Molecules with fewer than 8 electrons present; ex. BH3 where B only has and only needs 6 electrons

• Molecules with an expanded octet; ex. PCl5 where P forms 5 bonds and SF6 where S forms 6 bonds

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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.

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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.

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TO DETERMINE VSEPR SHAPE also known as electron

geometry write everything• Use VSEPR (valence shell electron pair repulsion)

rules: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:•Three lone pair•One single bond•One double bond

Electron Groups• Each lone pair of electrons constitutes one

electron group on a central atom.• Each bond constitutes one electron group on a

central atom, regardless of whether it is single, double, or triple.

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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.

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Linear Geometry

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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.

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Trigonal Planar Geometry

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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.

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Tetrahedral Geometry

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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.

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Bond Angle Distortion from Lone Pairs

Electron Geometry Tetrahedral Tetrahedral TetrahedralMolecularGeometry Tetrahedral Trigonal Pyramidal Bent 46

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Predicting the Shapes around Central Atoms write everything

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

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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

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Ioniccomplete transfer of electrons; occurs between m/nm, m/PAI, PAI/nm or PAI/PAI; ex. NaCl

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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. 55

Symmetric molecules tend to be nonpolarAsymmetric molecules with polar bonds are polar

56Symmetric means there are no lone pair around the central atom.