Atom – the smallest unit of matter “indivisible”

Heliumatom

Chemical Chemical BondingBonding

Problems and questions —Problems and questions —

How is a molecule or How is a molecule or polyatomic ion held together?polyatomic ion held together?

Why are atoms distributed at Why are atoms distributed at strange angles?strange angles?

Why are molecules not flat?Why are molecules not flat?

Can we predict the structure?Can we predict the structure?

How is structure related to How is structure related to chemical and physical chemical and physical properties?properties?

Review of Chemical BondsReview of Chemical Bonds• There are 3 forms of bonding:There are 3 forms of bonding:• __Ionic____Ionic__—complete —complete transfer transfer of 1 of 1

or more electrons from one atom or more electrons from one atom to another (one loses, the other to another (one loses, the other gains) forming oppositely charged gains) forming oppositely charged ions that attract one anotherions that attract one another

• _Covalent__Covalent_——some valence some valence electrons electrons sharedshared between atoms between atoms

• Metallic__Metallic__ – holds atoms of a – holds atoms of a metal togethermetal together

Most bonds are Most bonds are somewhere in somewhere in between ionic between ionic and covalent.and covalent.

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons

C would like to N would like toO would like to

Gain 4 electronsGain 3 electronsGain 2 electrons

Why are electrons important?

1) Elements have different electron configurations different electron configurations mean

different levels of bonding

Electron Dot Structures

Symbols of atoms with dots to represent the valence-shell electrons

1 2 13 14 15 16 17 18

H He:

Li Be B C N O : F :Ne :

Na Mg Al Si P S :Cl :Ar :

The type of bond can The type of bond can usuallyusually be calculated by be calculated by finding the difference in electronegativity of finding the difference in electronegativity of

the two atoms that are going together.the two atoms that are going together.

Electronegativity Difference

• If the difference in electronegativities is between:– 1.7 to 4.0: Ionic– 0.3 to 1.7: Polar Covalent– 0.0 to 0.3: Non-Polar Covalent

Example: NaClNa = 0.8, Cl = 3.0Difference is 2.2, sothis is an ionic bond!

Ionic BondsIonic BondsIonic BondsIonic BondsAll those ionic compounds were made All those ionic compounds were made

from ionic bonds. Positive cations from ionic bonds. Positive cations and the negative anions are and the negative anions are attracted to one another (remember attracted to one another (remember the Paula Abdul Principle of the Paula Abdul Principle of Chemistry: Opposites Attract!)Chemistry: Opposites Attract!)

Therefore, ionic compounds Therefore, ionic compounds are usually between metals are usually between metals and nonmetals (opposite and nonmetals (opposite ends of the periodic table).ends of the periodic table).

Formation of Ions from Metals

Ionic compounds result when metals react with nonmetals

Metals lose electrons to match the number of valence

electrons of their nearest noble gas

Positive ions form when the number of electrons are

less than the number of protons

Group 1 metals ion 1+

Group 2 metals ion 2+

• Group 13 metals ion 3+

Formation of Sodium Ion

Sodium atom Sodium ion

Na – e Na +

2-8-1 2-8 ( = Ne)

11 p+ 11 p+

11 e- 10 e-

0 1+

Formation of Magnesium Ion

Magnesium atom Magnesium ion

Mg – 2e Mg2+

2-8-2 2-8 (=Ne)

12 p+ 12 p+

12 e- 10 e-

0 2+

Learning Check

A. Number of valence electrons in aluminum1) 1 e- 2) 2 e- 3) 3 e-

B. Change in electrons for octet1) lose 3e- 2) gain 3 e- 3) gain 5 e-

C. Ionic charge of aluminum 1) 3- 2) 5- 3) 3+

Solution

A. Number of valence electrons in aluminum3) 3 e-

B. Change in electrons for octet1) lose 3e-

C. Ionic charge of aluminum 3) 3+

Ions from Nonmetal Ions

In ionic compounds, nonmetals in 15, 16, and 17

gain electrons from metals

Nonmetal add electrons to achieve the octet

arrangement

Nonmetal ionic charge:

3-, 2-, or 1-

Fluoride Ion

unpaired electron octet

1 -

: F + e : F :

2-7 2-8 (= Ne)

9 p+ 9 p+

9 e- 10 e- 0 1 -

ionic charge

Ionic Bond

• Between atoms of metals and nonmetals with very different electronegativity

• Bond formed by transfer of electrons

• Produce charged ions all states. Conductors and have high melting point.

• Examples; NaCl, CaCl2, K2O

1). Ionic bond – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions.

COVALENT BONDbond formed by the sharing of electrons

Covalent Bond

• Between nonmetallic elements of similar electronegativity.

• Formed by sharing electron pairs

• Stable non-ionizing particles, they are not conductors at any state

• Examples; O2, CO2, C2H6, H2O, SiC

Bonds in all the polyatomic ions

and diatomics are all covalent bonds

when electrons are shared equally

NONPOLAR COVALENT BONDS

H2 or Cl2

2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons.

Oxygen AtomOxygen Atom Oxygen AtomOxygen Atom

Oxygen Molecule (O2)

when electrons are shared but shared

unequally

POLAR COVALENT BONDS

H2O

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

METALLIC BONDbond found in

metals; holds metal atoms together very strongly

Metallic Bond

• Formed between atoms of metallic elements

• Electron cloud around atoms

• Good conductors at all states, lustrous, very high melting points

• Examples; Na, Fe, Al, Au, Co

Metallic Bond, A Sea of Electrons

Metals Form Alloys

Metals do not combine with metals. They form Alloys which is a solution of a metal in a metal.Examples are steel, brass, bronze and pewter.

Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

• Remember from the electron chapter Remember from the electron chapter that valence electrons are the that valence electrons are the electrons in the OUTERMOST energy electrons in the OUTERMOST energy level… that’s why we did all those level… that’s why we did all those electron configurations!electron configurations!

• B is 1sB is 1s22 2s 2s22 2p 2p11; so the outer energy ; so the outer energy level is 2, and there are 2+1 = 3 level is 2, and there are 2+1 = 3 electrons in level 2. These are the electrons in level 2. These are the valence electrons!valence electrons!

• Br is Br is [Ar] 4s[Ar] 4s22 3d 3d1010 4p 4p55

How many valence electrons are present?How many valence electrons are present?

Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

Number of valence electrons of a main (A) Number of valence electrons of a main (A) group atom = Group numbergroup atom = Group number

Steps for Building a Dot Steps for Building a Dot StructureStructureSteps for Building a Dot Steps for Building a Dot StructureStructure

Ammonia, NHAmmonia, NH33

1. Decide on the central atom; never H. 1. Decide on the central atom; never H. Why?Why?

If there is a choice, the central atom is If there is a choice, the central atom is atom of lowest affinity for electrons. atom of lowest affinity for electrons. (Most of (Most of the time, this is the the time, this is the least electronegative atomleast electronegative atom…in advanced …in advanced chemistry we use a thing called formal charge to determine the chemistry we use a thing called formal charge to determine the central atom. But that’s another story!)central atom. But that’s another story!)

Therefore, N is central on this oneTherefore, N is central on this one2. Add up the number of valence electrons that 2. Add up the number of valence electrons that

can be used.can be used.

H = 1 and N = 5H = 1 and N = 5

Total = (3 x 1) + 5 Total = (3 x 1) + 5

= 8 electrons / 4 pairs= 8 electrons / 4 pairs

3.3. Form a single bond between Form a single bond between the central atom and each the central atom and each surrounding atom (each bond surrounding atom (each bond takes 2 electrons!)takes 2 electrons!)

H H

H

N

Building a Dot StructureBuilding a Dot Structure

H••

H

H

N4.4. Remaining electrons form Remaining electrons form LONE PAIRS to complete the octet LONE PAIRS to complete the octet as needed (or duet in the case of as needed (or duet in the case of H).H).

3 BOND PAIRS and 1 LONE 3 BOND PAIRS and 1 LONE PAIR.PAIR. Note that N has a share in 4 pairs (8 Note that N has a share in 4 pairs (8

electrons), while H shares 1 pair.electrons), while H shares 1 pair.

5.5. Check to make sure there are 8 Check to make sure there are 8 electrons around each atom electrons around each atom except H. H should only have 2 except H. H should only have 2 electrons. This includes SHARED electrons. This includes SHARED pairs. pairs.

Building a Dot StructureBuilding a Dot Structure

6. 6. Also, check the number of electrons in your Also, check the number of electrons in your drawing with the number of electrons from step drawing with the number of electrons from step 2. If you have more electrons in the drawing than 2. If you have more electrons in the drawing than in step 2, you must make double or triple bonds. in step 2, you must make double or triple bonds. If you have less electrons in the drawing than in If you have less electrons in the drawing than in step 2, you made a mistake!step 2, you made a mistake!

H••

H

H

N

Carbon Dioxide, COCarbon Dioxide, CO22Carbon Dioxide, COCarbon Dioxide, CO22

1. Central atom = 1. Central atom =

2. Valence electrons =2. Valence electrons =

3. Form bonds.3. Form bonds.

O OC4. Place lone pairs on outer atoms.4. Place lone pairs on outer atoms.

This leaves 12 electrons (6 pair).This leaves 12 electrons (6 pair).

5. Check to see that all atoms have 8 electrons around it 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.except for H, which can have 2.

C 4 e-C 4 e-O 6 e- X 2 O’s = 12 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electronsTotal: 16 valence electrons

Carbon Dioxide, COCarbon Dioxide, CO22Carbon Dioxide, COCarbon Dioxide, CO22

••O OC

•• ••

••••••

••O OC

•• ••

••••••

••O OC

•• ••

••

••O OC

•• ••

••

6. There are too many electrons in our drawing. We 6. There are too many electrons in our drawing. We must form DOUBLE BONDS between C and O. must form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and pairs. So one pair is taken away from each atom and replaced with another bond.replaced with another bond.

C 4 e-C 4 e-O 6 e- X 2 O’s = 12 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electronsTotal: 16 valence electrons

How many are in the drawing?How many are in the drawing?

Double and Double and even triple even triple bonds are bonds are commonly commonly observed for C, observed for C, N, P, O, and SN, P, O, and S

••O OC

•• ••

••

••O OC

•• ••

••

HH22COCO

SOSO33

CC22FF44

Violations of the Octet Violations of the Octet RuleRule

(Honors only)(Honors only)

Violations of the Octet Violations of the Octet RuleRule

(Honors only)(Honors only)Usually occurs with B and elements of Usually occurs with B and elements of higher periods. Common exceptions higher periods. Common exceptions are: Be, B, P, S, and Xe. are: Be, B, P, S, and Xe.

BF3BF3

SF4SF4

Be: 4Be: 4

B: 6B: 6

P: 8 OR 10P: 8 OR 10

S: 8, 10, OR 12S: 8, 10, OR 12

Xe: 8, 10, OR 12Xe: 8, 10, OR 12

VSEPRVSEPR • VValence alence SShell hell EElectron lectron

PPair air RRepulsion theory.epulsion theory.• Most important factor in Most important factor in

determining geometry is determining geometry is relative relative repulsion between repulsion between electron pairs.electron pairs.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

MOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRY

Some Common Some Common GeometriesGeometries

LinearLinear

Trigonal PlanarTrigonal Planar TetrahedralTetrahedral

VSEPR chartsVSEPR charts

• Use the Lewis structure to determine the geometry of the molecule

• Electron arrangement establishes the bond angles• Molecule takes the shape of that portion of the

electron arrangement• Charts look at the CENTRAL atom for all data!• Think REGIONS OF ELECTRON DENSITY

rather than bonds (for instance, a double bond would only be 1 region)

Structure Determination by Structure Determination by VSEPRVSEPR

Structure Determination by Structure Determination by VSEPRVSEPR

Water, HWater, H22OOThe electron pair geometry is TETRAHEDRALTETRAHEDRAL

The molecular geometry is BENTBENT..

The molecular geometry is BENTBENT..

H O H••

••

H O H••

••

2 bond 2 bond pairspairs

2 lone 2 lone pairspairs

Structure Determination Structure Determination by VSEPRby VSEPR

Structure Determination Structure Determination by VSEPRby VSEPR

Ammonia, NHAmmonia, NH33

The electron pair geometry is The electron pair geometry is tetrahedral.tetrahedral.

H

H

H

lone pair of electronsin tetrahedral position

N

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

Bond PolarityBond PolarityBond PolarityBond PolarityHCl is HCl is POLARPOLAR because it because it

has a positive end and a has a positive end and a negative end. (difference negative end. (difference in electronegativity)in electronegativity)

Cl has a greater share in Cl has a greater share in bonding electrons than does bonding electrons than does H.H.

Cl has a greater share in Cl has a greater share in bonding electrons than does bonding electrons than does H.H.

Cl has slight negative charge Cl has slight negative charge (-(-)) and H has and H has slight positive charge slight positive charge (+ (+ ))

H Cl••

••

+ -••H Cl

••

••

+ -••

Bond PolarityBond PolarityBond PolarityBond Polarity

• ““Like Dissolves Like”Like Dissolves Like”

– Polar dissolves PolarPolar dissolves Polar

– Nonpolar dissolves Nonpolar dissolves NonpolarNonpolar

Diatomic ElementsDiatomic Elements

• These elements do not exist as a single atom; they always appear as pairs

• When atoms turn into ions, this NO LONGER HAPPENS!– Hydrogen– Nitrogen– Oxygen– Fluorine– Chlorine– Bromine– Iodine

Remember: Remember: BrINClHOFBrINClHOF