Chapter 9 Chemical Bonding Section 9 1 Bonding

Chapter 9 Chemical Bonding Section 9. 1 – Bonding of Atoms

Ionic and Covalent Bonds Ionic: Conduct electricity, high MP, high BP, dissolve in water Covalent: Do not conduct electricity, low MP, low BP, do not dissolve in water You can make predictions about the properties, not always right because compounds can be partly covalent and partly ionic

Pure ionic: Sharing is so unequal that it is best described as a complete transfer of e. Pure covalent: e- are shared equally Most compounds fall between these two extremes (they have ionic and covalent characteristics) Partly covalent and partly ionic – electrons may be shared equally, only slightly (almost not at all)

ELECTRONEGATIVITY (EN) Measure of the ability of an atom in a bond to attract e(tug-of-war for shared e-) – Table on p. 302

Atoms w/ large EN values (Fig. 9. 5 p. 304) attract valence e- stronger than atoms w/ small electronegativities.

In General: EN values increase as you move from left to right on the periodic table. EN values decrease as you go down a group on the periodic table Reasoning: Increase left to right b/c of the # of protons increases and the attraction of the nucleus for valence e- increases.

�Decreases down because the number of energy levels increase and the valence e- are farther from the nucleus and the shielding effect (e- in the inner levels tend to block the attraction of the nucleus for valence e-)

EN values The farther the bonding atoms are from each other on the periodic table, the greater the EN difference. The greater the EN difference (ΔEN), the more unequally the e- are shared. ΔEN = larger EN – smaller EN

Ionic Bonding When ΔEN is 2. 0 or greater, sharing of e- is so unequal (e- transfer is occurring) ΔEN ≥ 2. 0 Ionic The greater the ΔEN = the more ionic the bond F-Cs ΔEN = 4. 0 (F) – 0. 7 (Cs) = 3. 3

Covalent Bonding ΔEN = 0 <0. 5 Covalent No difference or small ΔEN: Diatomic molecules: Pure covalent ΔEN of C-H C=2. 5 H= 2. 1 2. 5 – 2. 1 = 0. 4

Polar Covalent Bonding Polar Covalent (partial transfer of e- to the more electronegative atom) ΔEN = 0. 5 - 2. 0

Polar Covalent Bonds Polar covalent bonds are called polar because unequal e - sharing creates two poles across the bond (-) pole = more electronegative atom (+) pole = less electronegative atom Charges are not 1+ or 1 -, instead δ+ and δ-

Water has polar bonds: ΔEN for O-H (3. 5 -2. 1) bond is 1. 4 O 3. 5 - H 2. 1 Compounds with polar covalent bonds have different properties from compounds with pure covalent bonds

H 2 O boiling point: 100˚C due to its polar bonds e- on O repel each other to be as far apart from each other as possible Geometric shape: Bent- Water angles 105 ˚C Water molecule:

Metallic Bonds Name some properties of metals: malleable - shaped ductile – drawn into wires conductivity – ease of electron flow

Why? Metallic bonds can be described as: a “sea of valence electrons” shared pool of electrons

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