Reactivity of ElementsHow many valence electrons do the noble gases have?
8 electrons = octet
Compounds form when two or more elements come together, ideally so each can have a full octet
Ionic compound:
electron(s) transferred from one atom to another
Covalent compound:
electron(s) shared between two atoms
Electron interactions generate chemical bonds, which hold together atoms in a compound
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• Metals– often lose e-
– CATION
• Nonmetals– often gain e-
– ANION
– “-ide”
Ions
Practice
Predict the ion each of the following elements becomes:
• O• I• S• Mg• Cs• Al• N
Ionic Compounds
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• Composed of ions• Transfer of electrons• Neutral (no net charge)
• Charges must balance
• Subscripts may be used
Binary ionic compounds– Two elements
– Usually metal + nonmetal
– Solids at room temp
Ionic Compound: Sodium Cloride
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Naming Binary Ionic Compounds
Cation + Anion
name of metal + root of nonmetal-ide
Example: Strontium and Iodine
strontium iodide
Example: Nitrogen and Cesium
cesium nitride
• How would you name the following compound?– Calcium and Fluorine
Calcium Fluoride
– Lithium and Bromine
Lithium Bromide
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Naming Binary Ionic Compounds
Ionic Compounds
• Net ionic charge = 0• Examples:
– Sodium chloride
– Calcium bromide
– Magnesium sulfide
– Lithium oxide
Metals that form more than one ion
• Usually in B groups (transition metals)• Ex. Iron
Fe2+ Fe3+
FeCl2 FeCl3Iron (II) chloride Iron (III) chloride
chromium
copper gold iron lead tin
Cr2+ Cu+ Au+ Fe2+ Pb2+ Sn2+
Cr3+ Cu2+ Au3+ Fe3+ Pb4+ Sn4+
Polyatomic Ions• Polyatomic ions:
• ions made up of more than one atom
Polyatomic Ions• Nitrate: NO3 charge = -1
• Sulfate: SO4 charge = -2
Examples:
Potassium nitrate
Calcium nitrate
Potassium sulfate
Calcium sulfate
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Covalent CompoundsAtoms share electrons to form molecules
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H H+ H H H H H2
F F+ FF FF F2
Lone pair electrons
Covalent bonds usually between nonmetal + nonmetal
Bonding electrons
C H+ C H C H
+H
C H
H
C H
H
C HH
H
H
Carbon atom:• 4 valence electrons• Wants to form 4 bonds
C + 4 H
N H+
+H
Nitrogen atom:• 5 valence electrons• 3 unpaired electrons• Wants to form 3 bonds
+ 3 HN N HH
H
N H N H
N H
H
N H
H
H+
+H
Oxygen atom:• 6 valence electrons• 2 unpaired electrons• Wants to form 2 bonds
+ 2 H
O O H O H
O H
H
O H
H
OO H
H
Multiple Bonds
Unpaired electrons left!
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N N+ N N N N
C ++ OO CO O
CO O CO O
Resonance Structures
When two or more electron-dot formulas can be drawn, these are called resonance structures
Example: O3
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O O O
18 electrons total
O O O O O O
O O O
Each bond is a“one and a half”
bond
Covalent Compounds• Combination of two different elements
(usually nonmetal + nonmetal)
Rules for Naming
-Element with lower group number is first word;
higher group number element is second word.
-Same group? element with higher period number first
-Second element: “root”-ide
-Add Greek numerical prefixes
Naming Covalent Compounds
Example: CS2
1.Which element is in lower group?
2. (Not in same group so doesn’t apply)
3.Second element is root-ide
4.Add prefix 2 sulfurs
Carbon Disulfide
Naming Covalent Compounds
Example: N2O
1.Which element is in lower group? 2.(Not in same group so doesn’t apply)3.Second element is root-ide 4.Add prefix 2 nitrogens
Dinitrogen oxide
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Electronegativity Electronegativity: ability of an atom to attract electrons
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(EN)
Types of Bonding / Variations
• Ionic bond: large difference in electronegativity; polar
• Covalent bond: small difference in electronegativity
• Some difference in electronegativity: polar covalent
• No difference in electronegativity: nonpolar
• Polar bonds creates a dipole
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Look up electronegativities (Fig. 4.5)
Find the Difference between the two
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Shapes of Molecules
VSEPR: Valence Shell Electron Pair Repulsion
Minimize electron repulsion
Electron groups pushed as far apart as possible
Electron groups:• Bonding electrons (bonds)
– Note: single, double, triple bonds all count as ONE electron group
• Lone pairs of electrons27
Basic Molecular Shapes
2 electron groups linear
O=C=O
3 electron groups
trigonal planar
bent
SOO
B F
F
F
Basic Molecular Shapes: 4 electron groups
tetrahedral
bent
OHH
pyramidal
CHH
H
H
N HH
H
Polarity of MoleculesPolar bonds: differences in negativities of bonded atoms
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H-H F-F Non-polar (atoms equal EN)
Polar (F more EN than H; lone pairs around
F)
H-F
OHH
Add polar bond together to see if molecule is polar overall (head to tail)
Polar O partially negativeH partially positive
Polarity of Molecules
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Non-polar
Add polar bonds (head to tail)
C HH
H
H
Molecules with polar bonds may not be polar overall!
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