Chapter 9 Covalent Bonding Section 9 1 n

Chapter 9 Covalent Bonding

Section 9. 1 n n n Atoms bond together because they want a stable electron arrangement consisting of a full outer energy level. Two ways that atoms can bond together are ionically & covalently. A covalent bond is a chemical bond that results from the sharing of the valence electrons. Covalent bonds are usually formed between elements close to each other on the periodic table and nonmetallic elements.

Nonmetals bonded with nonmetals

Section 9. 1 n n A molecule is formed when 2 or more atoms bond covalently. Example HI n F 2 n Practice problems 1. HBr 2. O 2 n 3. H 2 O

` n n Diatomic molecules are molecules of like atoms. Diatomic molecules include hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, & iodine. Write the formulas & structures for the diatomic molecules.

n The number of covalent bonds that these groups form are: Halogens single covalent bond n Group 6 A double/two covalent bond n Group 5 A triple/three covalent bond n Group 4 A four covalent bonds n

n Practice n HF n OCl 2 n H 2 S n N 2 n PH 3 n Si. H 4

Section 9. 1 n n Single covalent bonds are also called sigma bonds, symbolized by the Greek letter s A sigma bond results when orbitals overlap. s orbitals can overlap s orbitals n s orbitals can overlap p orbitals n p orbitals can overlap p orbitals n

Section 9. 1 n n Multiple covalent bonds occur when more than one pair of electrons are shared. A pi bond, p, is formed when parallel orbitals overlap to share electrons. A double bond is made of 1 sigma bond & 1 pi bond n A triple bond is made of 1 sigma bond & 2 pi bonds n

Practice Draw the lewis dot structure for each molecule. n 1. PH 3 n 2. H 2 S n 3. HCl n 4. CCl 4 n 5. Si. H 4 n 6. CO 2

n n If H 2 O is water, what is H 2 O 4? Drinking, bathing, washing, swimming…

Section 9. 1 n n n The strength of covalent bonds depend on the length of the bond, the distance between the bonded nuclei. Bond length is determined by the size of the atoms & the number of shared electron pairs. As the number of shared pairs increases, the bond length decreases.

n n n The shorter the bond, the stronger the bond. Energy is released when a bond forms & energy is added to break a bond. Which has a shorter bond? H 2 or S 2 The amount of energy needed to break a bond is the “Bond Dissociation Energy” Among F 2, O 2, & N 2, which would have the greatest bond dissociation energy? Least?

Section 9. 1 n n Endothermic reactions occur when a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in the product molecules Exothermic reactions occur when more energy is released forming new bonds than is required to break bonds in the initial reactants

Measuring heat n Exothermic Rxn n Endothermic Rxn

Section 9. 2 Naming Binary Molecular Compounds 1 st element—use entire element name 2 nd element—use root word & “ide” use prefixes to indicate # of atoms for each element n n n prefixes n n n n n mono— 1 di— 2 tri— 3 tetra— 4 penta— 5 hexa— 6 hepta— 7 octa— 8 nona— 9 deca--10

Section 9. 2 **NEVER USE mono WITH 1 ST ELEMENT** Example n n N 2 Cl 3 CO 2 Practice n n 1. P 2 O 5 2. NO 2 3. CO Common names of some molecular compounds— p. 249 ** Worksheet “Naming molecular compounds” n

n n n What is the name for H 2 O? What is the name for CH 2 O? Sea water

n n Naming acids Binary acids—hydrogen plus one other element n Hydro + root of 2 nd element + ic acid 1. 2. 3. 4. HCl hydrochloric acid HBr hydrobromic acid H 2 S hydrosulfuric acid Hydroiodic acid

n Oxyacids—contain hydrogen & oxygen and one other element n Root of other element + ic acid H 2 CO 3 n H 2 SO 4 n HC 2 H 3 O 2 n Phosphoric acid n Nitric acid n n ** Worksheet “Naming acids”

Section 9. 3 n n n Structural formulas use letter symbols & bonds to show relative positions of atoms. Hydrogen is always a terminal atom because it can bond with only 1 other atom. Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.

Resonance n Example HCl n SO 2 n n Practice 1. SO 3 n 2. O 3 n

n A coordinate covalent bond forms when one atom donates both electrons to be shared.

9. 4 Molecular Shapes n n n n Molecular shape is determined by the overlap of orbitals that share electrons. The VSEPR model is used to determine the molecular shape of the molecule. VSEPR stands for Valence Shell Electron Pair Repulsion model.

n n n F 2 has linear shape CO 2 has linear shape BH 3 has trigonal planar shape CF 4 has tetrahedral shape NH 3 has pyramidal shape H 2 O has bent shape

n n Hydridization is a process where atomic orbitals are mixed to form new, identical hybrid orbitals. Carbon is the most common element that undergoes hybridization. The hybrid orbital of carbon is called sp 3. It takes shape to look like a tetrahedral.

9. 5 Electronegativity & Polarity n n Remember that electronegativity is defined as the tendency of an atom to attract electrons. Fluorine has the highest EN at 3. 98 and Francium has the lowest EN at 0. 7.

Polar/nonpolar n n Polar covalent bond is when electrons are unequally shared—large difference in EN. Electrons spend more time around the element that is more EN. The more EN element has a partial negative charge. A polar covalent bond is also called a dipole. Electronegativity occurs in molecules. For example H 2 O & HCl n Page 263

Polar/nonpolar n n Nonpolar covalent bond is when electrons are shared equally—have small or no EN differences. Symmetrical molecules with balanced charges are nonpolar. (CCl 4) These are pure covalent bonds. Ionic bonds generally form when EN differences are 1. 7 or greater.

Electronegativity difference 0. 00 - 0. 4 Bond Type 0. 5 – 1. 6 Polar covalent 1. 7 – 3. 2 ionic Nonpolar covalent

Polar, Nonpolar, or Ionic? n Example: n n HCl Practice: 1. 2. 3. 4. SCl 2 H 2 S CF 4 CS 2

Properties of covalent compoundsp. 266 -267 “like dissolves like” n Many different types of forces between molecules. **Worksheet “Shape/polarity of molecules” n

n Mainly gases (some liquids), melt/vaporize easily, hardness varies. n Oxygen gas, acids, parafinn, graphite, diamonds