Chemical Bonding

CHEMICAL BONDS

Cl

H

BONDS HOLD ATOMS TOGETHER 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.

IONIC BOND FORMATION - ELECTRON TRANSFER BETWEEN METALS & NONMETALS +9 2e 7e 8e 1e 18e 8e +19

2e

ISOELECTRONIC WITH Ar

ISOELECTONIC WITH 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 2e 6e 7e

6e 7e +9

2e

BOTH ISOELECTRONIC WITH Ne

COVALENT BONDING (ELECTRON SHARING – DOUBLE BOND) +8 2e 4e 6e

6e 4e +8

2e

BOTH ISOELECTRONIC WITH Ne

COVALENT BONDING (ELECTRON SHARING – TRIPLE BOND) +7 2e 5e 2e

5e 2e +7

2e

BOTH ISOELECTRONIC WITH Ne

ELECTRON DOT REPRESENTATIONS OF POLYATOMIC MOLECULES AND IONS WRITING THE DOT FORMULA FOR METHANE - CH4 STEP I – DETERMINE THE TOTAL NUMBER OF VALENCE ELECTRONS FOR ALL ATOMS. CARBON (C) HAS 4 VALENCE ELECTRONS HYDROGEN (H) ATOMS HAVE 1 VALENCE ELECTRON FOR 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 SYMMETRICALLY AROUND IT.

WRITING THE DOT FORMULA FOR METHANE CH4 OCTET (8) FOR CARBON HYDROGEN REQUIRES ONLY 2 ELECTRONS 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)

WRITING THE ELECTRON STRUCTURE FOR MONOCHLOROETHENE - C2H3Cl 2C=2x4=8e 3H=3x1=3e 1 Cl = 1 x 7 = 7 e TOTAL VALENCE ELECTRONS =18 OCTETS COMPLETE 8 ELECTRONS REMAIN COMPLETE OCTET FOR CHLORINE DOUBLE BOND

2 ELECTRONS STILL REMAIN PLACE THEM TO COMPLETE THE OCTETS FOR BOTH CARBONS

WRITING ELECTRON DOT REPRESENTATIONS FOR POLYATOMIC 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 ELECTRONS REMAIN COMPLETE THE OCTETS ALL ELECTRONS ARE USED BUT AN OCTET FOR CARBON MUST BE CREATED

RESONANCE BOND

-2

RESONANCE STRUCTURES OCCUR WHEN A DOUBLE OR TRIPLE BOND MAY EQUALLY WELL BE PLACED AT MORE THAN ONE BONDING SITE FOR EXAMPLE IN THE CARBONATE ION PREVIOUSLY SHOWN, THE DOUBLE BOND COULD HAVE BEEN PLACE BETWEEN ANY OF THE THREE CARBON – OXYGEN BOND SITES. IT COULD HAVE BEEN PLACED HERE OR HERE

-2

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

+δ

-δ

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.

LITHIUM WITH NO HYBRIDIZATION UNHYBRIDIZED LITHIUM (BEFORE REACTING) (CAPABLE OF FORMING ONE BOND) WHEN BONDSPROMOTION FORM WITH NO ELECTRON OR 1 HYDROGEN (H 1s ) HYBRIDIZATION OCCURS COMPOUND LiH RESULTS BONDING ORBITAL

3P 2S 1S

BERYLLIUM IN HYBRIDIZED STATE UNHYBRIDIZED BERYLLIUM (BEFORE REACTING) (CAPABLE OF FORMING TWO BONDS) WHEN BONDS FORM WITH ELECTRON PROMOTION AND 1 HYDROGEN (H 1s ) HYBRIDIZATION OCCURS COMPOUND BeH2 RESULTS

BONDING ORBITALS

2S 1S

3P SP HYBRID ORBITALS

BORON IN HYBRIDIZED STATE UNHYBRIDIZED BORON (BEFORE REACTING) (CAPABLE OF FORMING THREE BONDS)

WHEN BONDS FORM WITH ELECTRON PROMOTION AND HYDROGEN (H 1s1) HYBRIDIZATION OCCURS

BONDING ORBITALS

2S 1S

3P

SP2 HYBRID ORBITALS

COMPOUND BH3 RESULTS

UNHYBRIDIZED (BEFORE REACTING) CARBON CARBON IN HYBRIDIZED STATE (CAPABLE OF FORMING FOUR BONDS) WHEN BONDS FORM WITH ELECTRON PROMOTION AND HYDROGEN (HOCCURS 1s1) HYBRIDIZATION

BONDING ORBITALS

2S 1S

COMPOUND CH4 RESULTS

3P

SP3 HYBRID ORBITALS

NITROGEN IN HYBRIDIZED STATE UNHYBRIDIZED NITROGEN REACTING) (CAPABLE OF FORMING(BEFORE THREE BONDS WITH ONE LONE ELECTRON PAIR) ELECTRON WHEN BONDS PROMOTION FORM WITH AND HYBRIDIZATION HYDROGEN (H OCCURS 1s1) LONE e-PAIR

BONDING ORBITALS

3P SP3 HYBRID ORBITALS

2S 1S

COMPOUND NH3 RESULTS

OXYGEN IN HYBRIDIZED STATE UNHYBRIDIZED OXYGEN (BEFORE (CAPABLE OF FORMING TWO BONDSREACTING) WITH TWO LONE ELECTRON PAIRS) WHEN BONDS FORM WITH ELECTRON PROMOTION AND 1 HYDROGEN (H 1s ) HYBRIDIZATION OCCURS TWO LONE ePAIRS

BONDING ORBITALS

2S 1S

3P

SP3 HYBRID ORBITALS

COMPOUND H2O RESULTS

FLOURINE IN HYBRIDIZED STATE UNHYBRIDIZED CARBON (BEFORE REACTING) (CAPABLE OF FORMING ONE BOND WITH THREE LONE ELECTRON PAIRS) ELECTRON PROMOTION AND WHEN BONDS FORM WITH HYBRIDIZATION HYDROGEN OCCURS (H 1s1) THREE LONE ePAIRS BONDING ORBITALS

3PORBITALS SP3 HYBRID 2S 1S

COMPOUND HF RESULTS

COORDINATE COVALENT BOND FORMATION

LONE e- PAIR

HYDROGEN ION WITH NO ELECTRONS

AMMONIUM AMMONIA ION NH3 IS FORMED NH4+