CHEMICAL BONDS CHAP 9

Three 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 protons 11 electrons Na + 11 protons 10 electrons Cl 17 protons 17 electrons Cl - 17 protons 18 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! Na + ions Cl − 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!

Fig 8.19 The Periodic Table

8 Formula of Ionic Compounds Al 2 O 3 2 x +3 = +63 x -2 = -6 Al 3+ O 2- CaBr 2 1 x +2 = +22 x -1 = -2 Ca 2+ 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 - FF 8e - F F F F electronic structure of F 2 lone pairs single covalent bond

Fig. 9.8 Overlap of two atomic orbitals to form a molecular orbital

11 8e - H H O ++ O HH O HHor 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 - double bonds Triple bond – two atoms share three pairs of electrons N N 8e - N N triple 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 Electronegativities of the elements Very active metals Very active nonmetals

14 Covalent share e - Polar Covalent partial transfer of e - Ionic transfer e - Increasing difference in electronegativity Classification of bonds by difference in electronegativity DifferenceBond Type 0.5 ≥Covalent ≥ 1.7 Ionic Polar Covalent Table 9.5

Fig 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 rich region electron poor region

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 Three substances containing sodium and some form of the carbonate ion