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.
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)
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