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!