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Chemical Bonds
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Chemical BONDING
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electron shells Atomic number = number of Electrons
Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells. Electron shells determine how an atom behaves when it encounters other atoms
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Electrons are placed in shells according to rules:
The 1st shell can hold up to two electrons, and each shell thereafter can hold up to 8 electrons.
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Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons
C would like to N would like to O would like to Gain 4 electrons Gain 3 electrons Gain 2 electrons
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Why are electrons important?
Elements have different electron configurations different electron configurations mean different levels of bonding
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Electron Dot Structures
Symbols of atoms with dots to represent the valence-shell electrons H He: Li Be B C N O : F :Ne : Na Mg Al Si P S :Cl :Ar :
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Learning Check A. X would be the electron dot formula for
A. X would be the electron dot formula for 1) Na 2) K 3) Al B X would be the electron dot formula 1) B 2) N 3) P
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Chemical Bond A bond results from the attraction of nuclei for electrons All atoms trying to achieve a stable octet IN OTHER WORDS the p+ in one nucleus are attracted to the e- of another atom Electronegativity
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Bond Formation exothermic process ENERGY Reactants Energy released
Products
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Breaking Bonds Endothermic reaction
energy must be put into the bond in order to break it ENERGY Products Energy Absorbed Reactants
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Bond Strength Strong, STABLE bonds require lots of energy to be formed or broken weak bonds require little E
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Is a bond forming or breaking? Strong or weak bond?
Reaction Time Energy (KJ) energy absorbed endothermic bond breaking weak unstable bond Products Reactants
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Is a bond forming or breaking? Strong or weak bond?
Reaction Time Energy Energy absorbed endothermic bond breaking strong stable bond Products Reactants
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Is a bond forming or breaking? Strong or weak bond?
Energy (KJ) Reaction Time Energy released exothermic bond formation weak unstable bond Reactants Products
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Is a bond forming or breaking? Strong or weak bond?
Energy (KJ) Reaction Time Reactants Products Energy released exothermic bond formation strong stable bond
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Two Major Types of Bonding
Ionic Bonding forms ionic compounds transfer of e- Covalent Bonding forms molecules sharing e-
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One minor type of bonding
Metallic bonding Occurs between like atoms of a metal in the free state Valence e- are mobile (move freely among all metal atoms) Positive ions in a sea of electrons Metallic characteristics High mp temps, ductile, malleable, shiny Hard substances Good conductors of heat and electricity as (s) and (l)
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how the electrons do not just stay with one ion
Metallic Bond A force of attraction between a positively charged metal ion and the electrons in a metal Many metal ions pass along many electrons Many properties of metals, such as conductivity, ductility, and malleability, result from the freely moving electrons in the metal Usually occurs between atoms of metals Al3+ - - - Notice how the electrons do not just stay with one ion - - - - - - - - - - -
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It’s the mobile electrons that enable me-tals to conduct electricity!!!!!!
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IONic Bonding electrons are transferred between valence shells of atoms ionic compounds are made of ions NOT MOLECULES ionic compounds are called Salts or Crystals
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[METALS ]+ [NON-METALS ]-
IONic bonding Always formed between metals and non-metals [METALS ]+ [NON-METALS ]- Lost e- Gained e-
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IONic Bonding Electronegativity difference > 2.0
Look up e-neg of the atoms in the bond and subtract NaCl CaCl2 Compounds with polyatomic ions NaNO3
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Ionic Bonds: One Big Greedy Thief Dog!
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Anion (-) Cation (+)
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Properties of Ionic Compounds
SALTS Crystals hard 22oC high mp temperatures nonconductors of electricity in solid phase good conductors in liquid phase or dissolved in water (aq)
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Covalent Bonding Pairs of e- are shared between non-metal atoms
molecules Pairs of e- are shared between non-metal atoms electronegativity difference < 2.0 forms polyatomic ions
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Properties of Molecular Substances
Covalent bonding Low m.p. temp and b.p. temps relatively soft solids as compared to ionic compounds nonconductors of electricity in any phase
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when electrons are shared equally
NONPOLAR COVALENT BONDS when electrons are shared equally H2 or Cl2
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2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons.
Oxygen Atom Oxygen Atom Oxygen Molecule (O2)
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when electrons are shared but shared unequally
POLAR COVALENT BONDS when electrons are shared but shared unequally H2O
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Polar Covalent Bonds: Unevenly matched, but willing to share.
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- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.
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Covalent, Ionic, metallic bonding?
NO2 sodium hydride Hg H2S sulfate NH4+ Aluminum phosphate KH KCl HF CO Co Also study your characteristics!
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Drawing ionic compounds using Lewis Dot Structures
Symbol represents the KERNEL of the atom (nucleus and inner e-) dots represent valence e-
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[Na]+ [ Cl ]- NaCl This is the finished Lewis Dot Structure
How did we get here?
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Step 1 after checking that it is IONIC
Determine which atom will be the +ion Determine which atom will be the - ion Step 2 - Write the lewis dot structure for each atom. - Draw an arrow from the electrons that move to where they move. Step 3 Write the symbol for the + ion first. NO DOTS Draw the e- dot diagram for the – ion COMPLETE outer shell Step 4 Enclose both in brackets and show each charge
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What did the atom of fluorine
say to the atom of sodium? You complete me.
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Draw the Lewis Diagrams
LiF MgO CaCl2 K2S
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Drawing molecules using Lewis Dot Structures
Symbol represents the KERNEL of the atom (nucleus and inner e-) dots represent valence e-
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Always remember atoms are trying to complete their outer shell!
The number of electrons the atoms needs is the total number of bonds they can make. Ex. … H? O? F? N? Cl? C? one two one three one four
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Methane CH4 This is the finished Lewis dot structure
How did we get here?
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Step 1 count total valence e- involved Step 2 connect the central atom (usually the first in the formula) to the others with single bonds Step 3 complete valence shells of outer atoms Step 4 add any extra e- to central atom IF the central atom has 8 valence e- surrounding it . . YOU’RE DONE!
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Sometimes . . . You only have two atoms, so there is no central atom, but follow the same rules. Check & Share to make sure all the atoms are “happy”. Cl Br H O N HCl
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O O N N DOUBLE bond TRIPLE bond atoms that share two e- pairs (4 e-)
atoms that share three e- pairs (6 e-) N N
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Draw Lewis Dot Structures
You may represent valence electrons from different atoms with the following symbols x, , CO2 NH3
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Draw the Lewis Dot Diagram for polyatomic ions
Count all valence e- needed for covalent bonding Add or subtract other electrons based on the charge REMEMBER! A positive charge means it LOST electrons!!!!!
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Draw Polyatomics Ammonium Sulfate
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Types of Covalent Bonds
NON-Polar bonds Electrons shared evenly in the bond E-neg difference is zero Between identical atoms Diatomic molecules
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Types of Covalent Bonds
Polar bond Electrons unevenly shared E-neg difference greater than zero but less than 2.0 closer to more polar more “ionic character”
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Place these molecules in order of increasing bond polarity which is least and which is most?
HCl CH4 CO2 NH3 N2 HF a.k.a. “ionic character”
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Polar molecules (a.k.a. Dipoles)
Not equal on all sides Polar bond between 2 atoms makes a polar molecule asymmetrical shape of molecule
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non-polar MOLECULES Sometimes the bonds within a molecule are polar and yet the molecule is non-polar because its shape is symmetrical. H C Draw Lewis dot first and see if equal on all sides
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H Cl + -
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Space filling model “Electron-Cloud” model
H + -
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Water is asymmetrical + + H O -
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Water is a bent molecule
H H
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W - A - T - E - R as bent as it can be! Water’s polar MOLECULE!
The H is positive The O is not - not - not - not
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Making sense of the polar non-polar thing
BONDS Non-polar Polar Identical Different MOLECULES Non-polar Polar Symmetrical Asymmetrical
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IONIC bonds …. Ionic bonds are so polar that the electrons are not shared but transferred between atoms forming ions!!!!!!
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Intermolecular attractions
Attractions between molecules van der Waals forces Weak attractive forces between non-polar molecules Hydrogen “bonding” Strong attraction between special polar molecules
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van der Waals Non-polar molecules can exist in liquid and solid phases
because van der Waals forces keep the molecules attracted to each other Exist between CO2, CH4, CCl4, CF4, diatomics and monoatomics
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van der Waals periodicity
increase with molecular mass. Greater van der Waals force? F2 Cl2 Br2 I2 increase with closer distance between molecules Decreases when particles are farther away
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Hydrogen “Bonding” Strong polar attraction
Like magnets Occurs ONLY between H of one molecule and N, O, F of another H “bond”
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H is shared between 2 atoms of OXYGEN or 2 atoms of NITROGEN or 2 atoms of FLUORINE Of 2 different molecules
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Why does H “bonding” occur?
Nitrogen, Oxygen and Fluorine small atoms with strong nuclear charges powerful atoms very high electronegativities
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Intermolecular forces dictate chemical properties
Strong intermolecular forces cause high b.p., m.p. and slow evaporation (low vapor pressure) of a substance.
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Which substance has the highest boiling point?
HF NH3 H2O WHY? Fluorine has the highest e-neg, SO HF will experience the strongest H bonding and needs the most energy to weaken the i.m.f. and boil
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The Unusual Properties of Water
Unusually high boiling point Compared to other compounds in Group 16
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Density????
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H2O(s) is less dense than H2O(l)
The hydrogen bonding in water(l) molecules is random. The molecules are closely packed. The hydrogen bonding in water(s) molecules has a specific open lattice pattern. The molecules are farther apart.
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The End
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