Chemical Bonding Part Two: Metallic and Covalent Bonds Metallic Bonds & Properties of Metals v The bonding in metals is explained by the Electron Sea Model v Atoms in a metallic solid contribute their valence electrons to form a “sea” of electrons that surrounds metallic cations v These delocalized electrons are not held by any specific atom and can move easily throughout the solid v Metallic bond is the attraction between these electrons and a metallic cation

Electron Sea Model Properties of Metals v Properties of metals are determined by the number of electrons they can contribute to the metallic bond v Group IA metals have only one electron to contribute to the bond and are therefore soft v Group IIA with two electrons are a little harder v The hardest metals are found in groups VIB through VIIIB where electrons from partially illed “d” sublevels take part in the metallic bond Metal Alloys v A mixture of elements that has metallic properties is called an alloy v Two basic types of Alloys: v Substitutional alloy: Atoms of the original metal are replaced by other metal atoms of similar size

v Interstitial alloy: Small holes in a metallic crystal are filled by other smaller atoms

Substitutional Alloys

v Examples are sterling silver, pewter and 10 carat gold

Interstitial Alloys

v An example is carbon steel- Carbon fills in the holes in the iron lattice which makes the iron harder stronger and less ductile

Check for Understanding 1. What is meant by the electron sea model? 2. What is the metallic bond? 3. Give three properties of metals. 4. Which would be the harder metal, Ca or Cr? Sc or Fe? 5. Brass is an alloy of Copper and Zinc. What type of alloy is it, substitutional or interstitial? Why?

Check for Understanding-Answers 1. Cations are ixed points in a mobile “sea” of electrons 2. The attraction between cations and sea of electrons 3. Shiny, Malleable, Ductile, High Melting Pt 4. Cr, Fe 5. Substitutional because they are about the same size

Covalent Bonds vCharacterized by a sharing of electrons vThe attraction of two atoms for a shared pair of electrons is called a covalent bond v In a covalent bond, atoms share electrons and neither atom has an ionic charge vA compound whose atoms are held together by covalent bonds is a covalent compound

Properties of Covalent Bonds vThe particle that results from a covalent bond is a molecule vA molecule is an uncharged group of two or more atoms held together by covalent bonds vMost covalent compounds are formed between atoms of nonmetals

vExamples are H2, H2O, NH3 ,and CH4

Formation of Water Through Electron Sharing v By sharing an electron pair with the oxygen, each hydrogen gets two electrons in its outer level like Helium v The oxygen, by sharing two electrons with two hydrogens, gets a stable octet in its outer level v By this method, each atom achieves a stable noble gas coniguration Single and Multiple Covalent Bonds

v The covalent compounds considered so far (water, methane, ammonia, and hydrogen) have single covalent bonds formed by a single shared pair of electron Single and Multiple Covalent Bonds

v They result from atomic orbitals overlapping end to end and electrons being concentrated in a bonding orbital between the two atoms vThis is a region where bonding electrons will most likely be found

Single and Multiple Covalent Bonds

v When more than one pair of electrons are shared between atoms multiple covalent bonds form vThese can be double or triple bonds depending on the number of pairs of electrons shared Single and Multiple Covalent Bonds Characteristics of Covalent Bonds v Low melting points

v Do not conduct electricity in any state

v Solids are often soft or brittle

v Many are gases at room temperature

v Most are less soluble in water than ionic compounds and are not electrolytes Formulas for Covalent Bonds Number Preix Number Preix

1 mon(o)- 6 hexa- 2 di- 7 hepta- 3 tri- 8 octa- 4 tetra- 9 nona- 5 penta- 10 deca- Examples:

carbon monoxide CO sulfur dioxide SO2

phosphorous trichloride PCl3 diarsenic trisulide Ar2S3

dinitrogen tetroxide N2O4

Formulas for Covalent Bonds 1. Write the symbol of the irst element named -If a preix accompanies the name, place a subscript equal to the preix after the symbol Example: Dinitrogen  N2 2. Write the symbol of the second element - Place a subscript after its symbol if the preix means two or higher

- Example: Dinitrogen trioxide  N2O3

Naming Covalent Compounds 1. Write the name of the irst element 2. If the symbol is followed by a subscript of 2 or more, use the appropriate preix 3. Write the root of the name of the 2nd element with an “–ide” ending 4. Use appropriate preix to denote the number of the 2nd element (1 or more) Examples: CO carbon monoxide

IF5 iodine pentaluoride

N2O5 dinitrogen pentoxide