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CHEMICAL BONDING
∑ Chemical bonds are strong attractive force that exists between the atoms of a substance
Chemical Bonds are commonly classified into 3 types:
- IONIC BONDING ÿ Ionic bonds usually form between metals and nonmetals ÿ Bonds form by transfer of electrons ÿ Ex: NaCl, MgBr 2 , AlF 3 , etc
- COVALENT BONDING ÿ Covalent bonds usually form between atoms of nonmetals ÿ Nonmetallic atoms may be the same or different ÿ Bonds form by sharing of electrons ÿ Ex: H 2 , Cl 2 , N 2 , HCl, H 2 O, NH 3 , CO 2 , CCl 4
- METALLIC BONDING ÿ Metallic bonds form between metallic ions ÿ Bonds are formed through “non localized” sharing of electrons commonly referred to as “Sea of electrons” ÿ Ex: Cu, Al, Na
LEWIS ELECTRON – DOT SYMBOLS
∑ When atoms bond, the bonding is usually achieved by the interaction of valence electrons ∑ Recall: Valence electrons are the “s” and “p” electrons of the outermost shell ∑ In Lewis Electron – Dot Symbols the dots represent valence electrons
Examples:
Na Ga
1s
2
2s
2
2p
6
3s
1
1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
1
1s
2
2s
2
2p
6
3s
1
1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
1
[Ne] 3s^1 [Ar] 3d^10 4s^2 4p^1
Lewis
Electron Dot Na Ga
Symbol
Note: Na Ga
stands for stands for
[Ne] [Ar] 3d
10
Noble Gas Core Pseudo Noble Gas Core
1s
2
2s
2
2p
6
1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
[Ne] [Ar] 3d
10
Conclusion: In Lewis Electron – Dot Symbols: ÿ dots represent valence electrons ÿ symbols of elements represent Noble Gas Cores or Pseudo – Noble Gas Cores
IONIC BOND
∑ Ionic bonds are commonly found in “SALTS”
∑ Recall SALTS are made up of : CATIONS and ANIONS any positive ion any negative ion other than H+^ other than OH-
∑ Examples of Salts: NaCl KNO 3 (NH 4 ) 2 SO 4 NH 4 C 2 H 3 O 2
Properties of Salts: ÿ crystalline solids at room temperature (RT) ÿ have high Melting Points ÿ are Strong Electrolytes in molten form and in aqueous solution
FORMATION OF AN IONIC BOND
Na + Cl Na
Cl
electron transfer
[Ne]3s
[Ne] 3s
3p
[Ne] [Ne] 3s
3p
[Ar]
Oppositely charged
ions attract
∑ The Ionic Bond is the electrostatic attraction between the oppositely charged ions.
OTHER EXAMPLES OF FORMATION OF IONIC COMPOUNDS
Mg + Cl Mg
2 Cl MgCl 2
[Ne]3s
[Ne] 3s
3p
[Ne] [Ar]
Ionic Formula Empirical
Cl (Simplest)
Formula
[Ne] 3s
3p
2 Na + S 2 Na
S
Na 2 S
[Ne]3s
[Ne] 3s
3p
[Ne] [Ar]
2 e
Ionic Empirical
Formula (Simplest)
Formula
ENERGY INVOLVED IN IONIC BONDING
Na(s) + ½ Cl 2 Na+Cl-(s) DH = 411 kJ/mol NaCl
(exothermic)
∑ Where is this energy coming from? The reaction can be considered to take place in 5 steps:
Step 1: Sublimation of Sodium (solid Na Æ gaseous Na)
Na (s) Na (g) DH 1 = + 108 kJ/mol Na endothermic
Step 2: Dissociation of Chlorine molecules (Cl 2 molecules Æ Cl atoms)
½ Cl 2 Cl (g) DH 2 = + 120 kJ/mol Cl endothermic
Step 3: Ionization of Sodium (Na atoms Æ Na+^ ions)
Na (g) Na+^ (g) + e -^ DH 3 = + 496 kJ/mol Na endothermic
(First Ionization Energy of Na)
Step 4: Formation of Cl-^ (Cl atoms Æ Cl-^ ions)
Cl (g) + e -^ Cl-^ (g) DH 4 = –349 kJ/mol Na exothermic
(Electron Affinity of Cl
Step 5: Formation of NaCl(s) from ions
Na
(g) + Cl
(g) Na
Cl
(s) DH 5 = –786 kJ/mol Na exothermic
(Lattice Energy of NaCl = U) energy given off when ions form a crystalline structure
Net Energy Released = + 108 kJ + 120 kJ + 496 kJ – 349 kJ – 786 kJ = – 411 kJ/mol
Na
(g) + Cl(g)
E. A. of Cl
I. E. of Na (–349 kJ/mol)
(+ 496 kJ/mol)
Na
(g) + Cl
(g)
Na(g) + Cl(g)
Dissociation of Cl 2
(+ 120 kJ/mol)
Na(g) + ½ Cl 2 (g)
Sublimation of Na Lattice Energy
(+ 108 kJ/mol) (–786 kJ/mol)
Na(s) + ½ Cl 2 (g)
DHf (NaCl)
411 kJ/mol
IONS OF MAIN GROUP ELEMENTS
IA IIA IIIA IVA VA VIA VIIA
Period 1 H–
Period 2 Li+^ covalent covalent covalent N3–^ O2–^ F–
Period 3 Na+^ Mg2+^ Al3+^ covalent P3–^ S2–^ Cl–
Period 4 K+^ Ca2+^ Ga3+^ covalent As3–^ Se2–^ Br–
Period 5 Rb+^ Sr2+^ In3+^ Sn2+^ Sn4+^ Sb3+^ Te2–^ I–
Period 6 Ca+^ Ba2+^ Tl3+^ Tl+^ Pb2+^ Pb4+^ Bi3+^ covalent radioactive
NOTE:
- Group IA through Group IVA ÿ Charge of Cation = Group Number ÿ Atoms lose all their valence electrons (“s” and “p” electrons) ÿ Electronic Configuration of cations = Preceding Noble Gas electronic Configuration
- Group IIIA through Group VA ÿ Charge of Cation = Group Number – 2 Atoms lose only their “p” valence electrons ( “s” electrons are kept)
Electronic Configuration of cations = Preceding Pseudo Noble Configuration + ns^2
Sn – 2e Sn2+^ Similarly: Pb2+ [Kr] 5s^2 4d^10 5p^2 [Kr] 5s^2 4d^10
Sb – 3e Sb3+^ Similarly: Bi3+ [Kr] 5s^2 4d^10 5p^3 [Kr] 5s^2 4d^10
- Group VA through Group VIIA ÿ Charge of Anion = Group Number 8
Atoms gain electrons to achieve stable octet (or doublet in the case of H) Electronic Configuration of anions = Noble Gas electronic Configuration following the element.
CATIONS OF TRANSITION METALS
∑ These ions are very often colored
Loss of electrons Ionic Charge Examples
The “ns” electrons are lost first (most common) 2+ Zn2+^ Cd2+
In some cases in addition to the “s” electrons, some “d” electrons from an inner shell (n 1) may be lost.
and/or larger than 2+
Fe2+^ Fe3+
Unusual charges are due to irregular electronic configurations associated with the stability of half filled or filled subshells
and/or 1+
Cu2+^ Cu+ Ag+
Fe2+^ [Ar] ≠Ø ≠ ≠ ≠ ≠.
4s 3d
Fe
0
[Ar] ≠Ø ≠Ø ≠ ≠ ≠ ≠ Fe3+^ [Ar] ≠ ≠ ≠ ≠ ≠.
4s 3d 4s 3d
Cu
2+
Cu
Ag
[Ar] 4s
0
3d
9
[Ar] 4s
0
3d
10
[Kr] 5s
0
4d
10
RADII OF CATIONS
Radii of atoms > Radii of Cations
Na^0 Na+
1s^2 2s^2 2p^6 3s^1 1s^2 2s^2 2p^6
3 shells 2 shells isoelectronic with Ne
∑ Isoelectronic = species having the same number of electrons and electron configuration.
∑ Arrange the following species in order of increasing size:
Fe 0 Fe 3+^ Fe 2+
26 p, 26 e 26 p, 23 e 26 p, 24 e
[Ar] 4s
2
3d
6
[Ar] 3d
5
[Ar] 3d
6
4 shells 3 shells 3 shells
weaker stronger interelectron interelectron repulsions repulsions smaller ion larger ion
Fe
3+
< Fe
2+
< Fe
0
smallest largest
Conclusion: ∑ For cations of the same element (same number of protons):
The larger the positive ionic charge, the smaller the cation
∑ Reason: Larger positive ionic charge means fewer electrons
Size of Cations in The Periodic Table (Main Group Elements)
IA IIA IIIA
2 Li
(isoelectronic with He)
3 Na
Mg
Al
(isoelectronic with Ne) 11 protons 12 protons 13 protons
4 K
Ca
Ga
(isoelectronic with Ar) 19 protons 20 protons
5 Rb
Sr
In
(isoelectronic with Kr) 37 protons 38 protons
6 Cs
Ba
Tl
(isoelectronic with Xe) 55 protons 56 protons
Size of Anions in the Periodic Table
VA VIA VIIA Noble Gases
2 N
O
F
(isoelectronic with Ne)
7 protons 8 protons 9 protons
3 P
S
Cl
(isoelectronic with Ar) 15 protons 16 protons 17 protons
4 As
Se
Br
(isoelectronic with Kr) 33 protons 34 protons 35 protons
Te
I
5 52 protons 53 protons (isoelectronic with Xe)
