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CHEMICAL BONDSCHEMICAL BONDS
CHAP 9CHAP 9
Three Types of Chemical BondsThree Types of Chemical Bonds(1) Ionic bonds
• Electrons are transferred from one atom to another
• Occur between a metal atom and a nonmetal atom
(2) Covalent bonds
• Electrons are shared between two atoms• Occur between two nonmetal atoms
(3) Metallic bonds
• Metal atoms share many electrons in a “sea” that is free to move throughout the metal
Na 11 protons11 electrons Na+ 11 protons
10 electrons
Cl 17 protons17 electrons
Cl-17 protons18 electrons
Formation of an ionic bond
+ e−
Net reaction: Na + Cl → Na+Cl− + energy
This energy is called the heat of formation
Fig. 9.6 Model of the sodium chloride crystal
−+
No molecules in an ionic compound!No molecules in an ionic compound!
Na+ ionsCl− ions
NaCl is a“formula unit”
Fig. 9.7 The cubic crystal structure of sodium chloride
• Ionic compounds consist of a combination of positive ions and negative ions
• Ions are formed as metal atoms lose and nonmetal atoms gain electrons to achieve stable noble gas structure
• The sum of the charges on the positive ions and negative ions in each formula unit must equal zero
• How do we determine the formula for an ionic compound?
• Key: All ionic formulas must be neutral!
5 6 7
8
3 4
1
2
Fig 8.19 The Periodic Table
8
Formula of Ionic Compounds
Al2O3
2 x +3 = +6 3 x -2 = -6
Al3+ O2-
CaBr2
1 x +2 = +2 2 x -1 = -2
Ca2+ Br-
Al and O
Ca and Br
9
Covalent bond - chemical bond in which two or more electrons are shared by two atoms.
Why should two atoms share electrons?
F F+
7e- 7e-
F F
8e- 8e-
F F
F F
electronic structure of F2
lone pairslone pairs
lone pairslone pairs
single covalent bond
single covalent bond
Fig. 9.8 Overlap of two atomic orbitals to form amolecular orbital
11
8e-
H HO+ + OH H O HHor
2e- 2e-
Electronic structure of water
Double bond – two atoms share two pairs of electrons
single covalent bonds
O C O or O C O
8e- 8e-8e-double bonds double bonds
Triple bond – two atoms share three pairs of electrons
N N8e-8e-
N N
triple bondtriple bond
or
How do we know if a bond between two atoms will be ionic or covalent?
Electronegativity - ability of an atom in a chemical bond to attract the electrons to itself
In general:
• metal and nonmetal → ionic bond• nonmetal and nonmetal → covalent bond
Specifically:
• we use:
Fig. 9.10 Electronegativities of the elements
Very activemetals
Very activenonmetals
14
Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
Classification of bonds by difference in electronegativity
Difference Bond Type
0.5 ≥ Covalent
≥ 1.7 Ionic
0.5 - 1.7 Polar Covalent
Table 9.5
Fig. 9.11 Electron distribution and kinds of bonding
Polar covalent bond or polar bond - a covalent bond with greater electron density around one of the two atoms
H F
electron richregion
electron poorregion
Box Fig 9.1 How a microwave oven cooks food
18
Metallic Bonds
• Held together by metallic bonds• Malleable and ductile• Good conductors of heat and electricity
Cross Section of a Metallic Crystal
nucleus &inner shell e-
mobile “sea”of e-
19
Composition of Ionic Compounds
Common names:
“salt”→ sodium chloride
“lye” → sodium hydroxide
“rust” → iron oxide
“baking soda” → sodium bicarbonate
“chalk” → calcium carbonate
Fig. 9.12 Three substances containing sodium and some form of the carbonate ion