Copyright Sautter 2003

ClH

CHEMICAL BONDS

BONDS HOLD ATOMSTOGETHER TO

FORM MOLECULES

TYPES OF CHEMICAL BONDS

•(1) IONIC BONDS - ELECTRONS AS TRANSFERRED FROM METALS TO NONMETALS (IONIC SALTS FOR EXAMPLE NaCl, CaCl2)•(2) COVALENT BONDS – ELECTRONS ARE SHARED BETWEEN NONMETALS (DIATOMIC MOLECULES FOR EXAMPLE THE HALOGENS, F2, Cl2, etc.)•(3) COORDINATE COVALENT BONDS – A BONDING PAIR OF ELECTRONS IS CONTRIBUTED BY ONLY ONE ATOM IN THE BOND (FOR EXAMPLE AMMONIUM ION, NH4

+, HYDRONIUM ION, H3O+)

WHY DO BONDS BETWEEN ATOMS FORM ?

• WHEN BONDS FORM THE STABILITY OF THE COMBINED ATOMS INCREASES AS COMPARED TO THAT OF THE INDIVIDUAL ATOMS. GENERALLY CHEMICAL STABILITY IS RELATED TO THE ABILITY OF ATOMS TO ATTAIN THE ELECTRON CONFIGURATION OF AN INERT GAS. FOR MANY ATOMS THIS MEANS ACQUIRING EIGHT ELECTRONS IN THE OUTER SHELL. THIS IS CALLED AN “OCTET” STRUCTURE.

• THE EXCEPTATIONS ARE SMALLER ATOMS (LIKE HYDROGEN, LITHIUM, BERYLLIUM, ETC.) THESE ATTEMPT TO ACQUIRE TWO ELECTRONS LIKE HELIUM.

• RECALL THE TERM “ISOELECTRONIC”. ATOMS ATTEMPT TO BECOME ISOELECTRONIC WITH THE NEAREST INERT GAS.

+9

+19 2e

1e

8e18e

7e

2e

8e

IONIC BOND FORMATION - ELECTRON TRANSFER BETWEEN METALS & NONMETALS

ISOELECTRONICWITH Ar

ISOELECTONICWITH Ne

REPRESENTING ATOMS AND MOLECULES USING ELECTRON DOT FORMULAE

(LEWIS DOT REPRESENTATIONS)

• LEWIS DOT FORMULAE USE THE ATOMIC SYMBOL TO REPRESENT THE “KERNEL” OF THE ATOM THAT IS THE NUCLEUS AND ALL INNER ENERGY LEVEL ELECTRONS. DOTS ARE THEN USED TO REPRESENT EACH OUTER ENERGY LEVEL ELECTRON (VALENCE ELECTRONS)

• THE PAIRING OF THESE ELECTRON “DOTS” APPROXIMATES THE PAIRINGS OF ELECTRONS IN ATOMIC AND MOLECULAR (BONDING) ORBITALS OF THE ATOM OR MOLECULE.

ELECTRON DOT REPRESENTATIONS FOR SOME COMMON ATOMS

(LEWIS DOT NOTATIONS)

COMMON PROPERTIES AND CHARACTERISTICS OF COVALENT BONDS

• COVALENT BONDS MAY BE SINGLE, DOUBLE OR TRIPLE BONDS

• (1) SINGLE BOND – ONE PAIR OF BONDING ELECTRONS JOINS TWO ATOMS TOGETHER

• (2) DOUBLE BOND – TWO PAIRS OF BONDING ELECTRONS JOIN TWO ATOMS TOGETHER (OCCURS PRIMARILY WITH CARBON, OXYGEN, NITROGEN AND SULFUR)

• (3) TRIPLE BOND – THREE PAIRS OF BONDING ELECTRONS JOIN TWO ATOMS TOGETHER (OCCURS PRIMARILY WITH CARBON AND NITROGEN)

COVALENT BONDING(ELECTRON SHARING – SINGLE BOND)

+9

+9 2e

7e

7e

2e

6e

6e

BOTH ISOELECTRONICWITH Ne

COVALENT BONDING(ELECTRON SHARING – DOUBLE BOND)

+8

+8 2e

6e

6e

2e

4e

4e

BOTH ISOELECTRONICWITH Ne

COVALENT BONDING(ELECTRON SHARING – TRIPLE BOND)

+7

+7 2e

5e

5e

2e

2e

2e

BOTH ISOELECTRONICWITH Ne

ELECTRON DOT REPRESENTATIONS OF POLYATOMIC MOLECULES AND IONS

WRITING THE DOT FORMULA FOR METHANE - CH4

STEP I – DETERMINE THE TOTAL NUMBER OFVALENCE ELECTRONS FOR ALL ATOMS.CARBON (C) HAS 4 VALENCE ELECTRONS

HYDROGEN (H) ATOMS HAVE 1 VALENCE ELECTRONFOR EACH OF THE FOUR ATOMS (4 x 1 = 4)

THE TOTAL NUMBER OF VALENCE ELECTRONS = 8

STEP II-SELECT THE CENTRAL ATOM (THE ONE THAT CAN FORM THE GREATEST NUMBER OF BONDS)

AND PLACE THE OTHER ATOMS SYMMETRICALLYAROUND IT.

WRITING THE DOT FORMULA FOR METHANE CH4

STEP III -PLACE TWO DOTS (ELECTRONS) BETWEEN EACH BONDED PAIR OF ATOMS.

STEP IV -FILL IN OCTET STRUCTURES FOR ANY REMAINING ATOMS UNTIL ALL THE ELECTRONS

ARE USED.*(HYDROGEN, BERYLLIUM & BORON ARE COMMON EXCEPTATIONS TO THE OCTET RULE)

OCTET (8) FOR CARBON

HYDROGENREQUIRES

ONLY 2 ELECTRONS

WRITING THE ELECTRON STRUCTURE FOR MONOCHLOROETHENE - C2H3Cl

2 C = 2 x 4 = 8 e3 H = 3 x 1 = 3 e1 Cl = 1 x 7 = 7 e

TOTAL VALENCE ELECTRONS =18

8 ELECTRONSREMAIN

COMPLETE OCTET FORCHLORINE

2 ELECTRONSSTILL REMAINPLACE THEM

TO COMPLETE THE OCTETS

FOR BOTHCARBONS

OCTETSCOMPLETE

DOUBLE BOND

WRITING ELECTRON DOT REPRESENTATIONS FORPOLYATOMIC IONS CONTAINING RESONANCE

STRUCTURES (CARBONATE ION - CO3-2 )

1 C = 1 x4 = 4 e 3 O = 3 x 6 = 18 e-2 CHARGE = 2 e

TOTAL VALENCE ELECTRONS = 24

18 ELECTRONSREMAIN

COMPLETETHE OCTETS

ALL ELECTRONS ARE USED BUT AN OCTET

FOR CARBON MUSTBE CREATED

-2

RESONANCE BOND

RESONANCE STRUCTURES OCCUR WHEN A DOUBLEOR TRIPLE BOND MAY EQUALLY WELL BE PLACED

AT MORE THAN ONE BONDING SITE

FOR EXAMPLE IN THE CARBONATE IONPREVIOUSLY SHOWN, THE DOUBLE BOND COULD

HAVE BEEN PLACE BETWEEN ANY OF THETHREE CARBON – OXYGEN BOND SITES.

-2IT COULDHAVE BEEN

PLACED HEREOR HERE

WHEN RESONANCE OCCURS THE DOUBLE BOND IS SAID TO BE DELOCALIZED WHICH MEANS THAT IT MOVES CONTINUALLY FROM ONE

BONDING SITE TO THE NEXT.

-2

-2

-2

COMMON PROPERTIES AND CHARACTERISTICS OF COVALENT BONDS

(CONT’D)• COVALENT BONDS MAY BE POLAR OR NONPOLAR• (1) POLAR BONDS OCCUR WHEN SHARED ELECTRON

PAIRS ARE SHIFTED AWAY FROM ONE OF THE BONDED ATOMS AND TOWARDS THE OTHER. ONE END OF THE BOND THEREFORE IS MADE MORE NEGATIVE AND THE OTHER END OF THE BOND IS LEFT MORE POSITIVE. THE GREATER THE ELECTRON SHIFT, THE MORE POLAR THE BOND BECOMES.

• (2) NONPOLAR BONDS OCCUR WHEN ELECTRON PAIRS ARE SHARED EQUALLY AND THE BOND PAIR IS CENTRALLY LOCATED BETWEEN THE ATOMS. THIS BOND HAS NO POSITIVE OR NEGATIVE END.

COMMON PROPERTIES AND CHARACTERISTICS OF COVALENT BONDS

(CONT’D)

• THE DEGREE OF BOND POLARITY DEPENDS ON HOW WELL ONE ATOM ATTRACTS ELECTRONS AS COMPARED TO THE OTHER BONDED ATOM.

• MORE NONMETALLIC ATOMS (HIGH ELECTRONEGATIVITIES) ATTRACT ELECTRONS THE BEST WHILE METALLIC ATOMS (LOW ELECTRONEGATIVITIES) ATTRACT ELECTRONS MOST POORLY.

• WHEN BOTH BONDED ATOMS ARE NONMETALLIC THEIR ELECTRONEGATIVITY VALUES ARE COMPARED TO DETERMINE THE DEGREE OF BOND POLARITY.

COMMON PROPERTIES AND CHARACTERISTICS OF COVALENT BONDS

(CONT’D)

• THE LARGEST POSSIBLE DIFFERENCE IN ELECTRONEGATIVITIES BETWEEN BONDED ATOMS IS THAT FOR A CESIUM FLOURINE BOND ( EN =3.3) THIS REPRESENTS THE MOST IONIC POSSIBLE BOND.

• THE SMALLEST POSSIBLE DIFFERENCE IN ELECTRONEGATIVITIES BETWEEN BONDED ATOMS OCCURS IN ALL DIATOMIC MOLECULES. SINCE EACH AOM HAS THE SAME ELECTRONEGATIVITY, THE DIFFERENCE IS ZERO ( EN = 0). THIS REPRESENTS A COMPLETELY NONPOLAR BOND.

• ELECTRONEGATIVITY DIFFERENCES BETWEEN THESE EXTREMES (3.3 AND 0) INDICATE THE RELATIVE POLARITY OF A BOND

- +

KINDS OF CHEMICAL BONDS

IONIC BONDING

EN = (4.0 – 0.70) = 3.3

POLAR COVALENT

BONDING

EN = (3.0 – 2.5) = 0.5

NON POLAR COVALENT BONDING

EN = (3.0 –3.0) = 0

BOND FORMATION AND HYBRIDIZATION

• ATOMS GENERALLY FORM AS MANY BONDS AS IS POSSIBLE. OFTEN THE BONDING CAPACITY OF AN ATOM CAN BE INCREASED BY A PROCESS KNOWN AS HYBRIDIZATION.

• IN THIS PROCESS, ORBITALS THAT ARE CLOSE TO EACHOTHER (IN TERMS OF ENERGY) MERGE TOGETHER FORMING NEW ORBITALS CALLED HYBRIDS. THE FORMATION OF THESE NEW HYBRIDS ALLOW PREVIOUSLY PAIRED ELECTRONS TO SEPARATE AN MOVE INTO NEW ORBITALS THEREBY ALLOWING FOR MORE BONDS TO BE FORMED.

• ORBITALS COMMONLY ENGAGING IN THIS PROCESS ARE S AND P ORBITALS AT THE SAME ENERGY LEVEL. EVEN S, P AND D ORBITALS CAN HYBRIDIZE.

BOND FORMATION AND HYBRIDIZATION(CONT’D)

• RULES GOVERNING ORBITAL HYBRIDIZATION• (1) THE NUMBER OF HYBRID ORBITALS THAT ARE

FORMED EQUAL THE NUMBER OF ATOMIC ORBITALS USED IN THE HYBRIDIZATION.

• (2) ALL THE NEWLY FORMED HYBRID ORBITALS ARE OF EQUAL ENERGY.

• (3) LIKE ATOMIC ORBITALS, A MAXIMUM OF TWO ELECTRONS CAN BE PRESENT.

• (4) AN ELECTRON PAIR PRESENT IN AN ORBITAL MAY BE A BONDING ELECTRON PAIR OR A LONE ELECTRON PAIR WHICH DOES NOT FORM A BOND.

• (5) ALL BONDS DO NOT REQUIRE HYBRIDIZATION IN ORDER TO FORM.

HYBRIDIZATION AND PERIODIC TRENDS

• TRENDS IN BONDING BETWEEN ATOMS OCCUR IN SIMILAR FASHION AMONG MEMBERS OF THE SAME CHEMICAL FAMILY (COLUMNS ON THE PERIODIC TABLE).

• THE BONDING CHARACTERISTICS OF EACH FAMILY ON THE PERIODIC TABLE ARE DEMONSTRATED IN THE FOLLOWING FRAMES.

• ALTHOUGH ELEMENTS IN A FAMILY GENERALLY DO ACT SIMILARLY, NOT ALL THE ELEMENTS IN A PARTICULAR FAMILY ACT IN EXACTLY THE SAME WAY.

UNHYBRIDIZED LITHIUM (BEFORE REACTING)

3P 2S

1S

LITHIUM WITH NO HYBRIDIZATION (CAPABLE OF FORMING ONE BOND)

NO ELECTRON PROMOTION ORHYBRIDIZATION OCCURS

BONDINGORBITAL

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

COMPOUNDLiH

RESULTS

UNHYBRIDIZED BERYLLIUM (BEFORE REACTING)

3P 2S

1S

SP HYBRID ORBITALS

BERYLLIUM IN HYBRIDIZED STATE (CAPABLE OF FORMING TWO BONDS)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURS

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

BONDINGORBITALS

COMPOUNDBeH2

RESULTS

UNHYBRIDIZED BORON (BEFORE REACTING)

3P 2S

1S

SP2 HYBRID ORBITALS

BORON IN HYBRIDIZED STATE (CAPABLE OF FORMING THREE BONDS)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURS

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

BONDINGORBITALS

COMPOUNDBH3

RESULTS

UNHYBRIDIZED CARBON (BEFORE REACTING)

3P 2S

1S

SP3 HYBRID ORBITALS

CARBON IN HYBRIDIZED STATE (CAPABLE OF FORMING FOUR BONDS)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURS

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

COMPOUNDCH4

RESULTSBONDINGORBITALS

UNHYBRIDIZED NITROGEN (BEFORE REACTING)

3P

2S

1S

SP3 HYBRID ORBITALS

NITROGEN IN HYBRIDIZED STATE (CAPABLE OF FORMING THREE BONDS

WITH ONE LONE ELECTRON PAIR)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURS

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

COMPOUNDNH3

RESULTS

BONDINGORBITALS

LONE e-PAIR

UNHYBRIDIZED OXYGEN (BEFORE REACTING)

3P 2S

1S

SP3 HYBRID ORBITALS

OXYGEN IN HYBRIDIZED STATE (CAPABLE OF FORMING TWO BONDS WITH TWO

LONE ELECTRON PAIRS)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURS

WHEN BONDS FORM WITHHYDROGEN (H 1s1)

COMPOUNDH2O

RESULTS

BONDINGORBITALS

TWO LONE e-

PAIRS

UNHYBRIDIZED CARBON (BEFORE REACTING)

3P 2S

1S

SP3 HYBRID ORBITALS

FLOURINE IN HYBRIDIZED STATE (CAPABLE OF FORMING ONE BOND

WITH THREE LONE ELECTRON PAIRS)

ELECTRON PROMOTION ANDHYBRIDIZATION OCCURSWHEN BONDS FORM WITH

HYDROGEN (H 1s1)

COMPOUNDHF

RESULTS

THREE LONE e-

PAIRS

BONDINGORBITALS

AMMONIANH3

HYDROGENION WITH

NO ELECTRONS

COORDINATE COVALENT BOND FORMATION

AMMONIUMION IS

FORMED NH4+

LONE e- PAIR