L1-Covalent Bonding.pdf

Lesson 1 The Covalent Bond  Focus Question How do atoms bond in covalent molecules?  New Vocabulary covalent bond molecule Lewis structure sigma bond pi bond endothermic exothermic  Review Vocabulary chemical bond: the force that holds two atoms togeth...

Lesson 1 The Covalent Bond  Focus Question How do atoms bond in covalent molecules?  New Vocabulary covalent bond molecule Lewis structure sigma bond pi bond endothermic exothermic  Review Vocabulary chemical bond: the force that holds two atoms together  Why do atoms bond? Gaining Stability The stability of an atom, ion, or compound is related to its energy: lower energy states are more stable. Ionic bonds, metals, and nonmetals gain stability by transferring (gaining or losing) electrons to form ions with stable noble-gas electron configurations. Valence electron sharing allows atoms to acquire the stable electron configuration of noble gases.  What is a covalent bond? Shared Electrons Atoms in non-ionic compounds share electrons. A covalent bond is a chemical bond that results from electron sharing. A molecule is formed when two or more atoms bond covalently. The majority of covalent bonds form between atoms of nonmetallic elements.  What is a covalent bond? Covalent Bond Formation Diatomic molecules (H2, N2, F2, O2, I2, Cl2, Br2) exist because the two-atom molecules are more stable than the individual atoms.  What is a covalent bond? The most stable arrangement of atoms exists at the point of maximum net attraction, where the atoms bond covalently and form a molecule.  Single Covalent Bonds When only one pair of electrons is shared, the result is a single covalent bond. The figure shows two hydrogen atoms forming a hydrogen molecule with a single covalent bond, resulting in an electron configuration like helium. In a Lewis structure, dots or a line are used to symbolize a single covalent bond.  Single Covalent Bonds Group 17 and Single Bonds The halogens—the group 17 elements—have 7 valence electrons and form single covalent bonds with atoms of other non-metals. Group 16 and Single Bonds Atoms in group 16 can share two electrons and form two covalent bonds. Water is formed from one oxygen with two hydrogen atoms covalently bonded to it.  Single Covalent Bonds Group 15 and Single Bonds Atoms in group 15 form three single covalent bonds, such as in ammonia. Group 14 and Single Bonds Atoms of group 14 elements form four single covalent bonds, such as in methane. Lewis Structure of a Molecule Use with Example Problem 1. SOLVE FOR THE UNKNOWN Problem To draw a Lewis structure, first draw the Patterns on glass can be made by chemically electron-dot diagram for each of the atoms. etching its surface with hydrogen fluoride Then, rewrite the chemical symbols and draw (HF). Draw the Lewis structure for a a line between them to show the shared pair molecule of hydrogen fluoride. of electrons. Finally, add dots to show the unshared electron pairs. Response ANALYZE THE PROBLEM You are given the information that hydrogen and fluorine form the molecule hydrogen fluoride. An atom of hydrogen, a group 1 element, has only one valence electron. It can bond with any EVALUATE THE ANSWER nonmetal atom when they share one pair of Each atom in the new molecule now has a electrons. An atom of fluorine, a group 17 noble-gas configuration and is stable. element, needs one electron to complete its octet. Therefore, a single covalent bond forms when atoms of hydrogen and fluorine bond.  Single Covalent Bonds The Sigma Bond Single covalent bonds are called sigma bonds. They occur when the pair of shared electrons is in an area centered between the two atoms.  Multiple Covalent Bonds Double Bonds A double bond forms when two pairs of electrons are shared between two atoms. In a. of the figure on the next slide, two oxygen molecules form a double bond. Triple Bonds A triple bond forms when three pairs of electrons are shared between two atoms. In b. of the figure on the next slide, a triple bond forms between two nitrogen atoms. Multiple Covalent Bonds  Multiple Covalent Bonds The pi Bond A multiple covalent bond consists of one sigma bond and at least one pi bond. The figure shows the multiple bond between carbon atoms in C2H4. A pi bond forms when parallel orbitals overlap and share electrons.  The Strength of Covalent Bonds Bond Length The strength depends on the distance between the two nuclei, or bond length. As length increases, strength decreases. Bonds and Energy The amount of energy required to break a bond is called the bond dissociation energy. The shorter the bond length, the greater the energy required to break it.  The Strength of Covalent Bonds An endothermic reaction occurs 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 products. An exothermic reaction occurs when more energy is released during product bond formation than is required to break bonds in the reactants.  Quiz 1. Which of the following is true? A Metals gain stability by sharing electrons to form ions. B Atoms with higher energy states are more stable. C Atoms with lower energy states are more stable. CORRECT D Atoms in nonionic pairs gain or lose electrons.  Quiz 2. Which term refers to a chemical bond that results from sharing valence electrons? A ionic bond C covalent bond CORRECT B metallic bond D molecule  Quiz 3. How many electrons do group 17 elements need to form an octet? A 1 CORRECT B 2 C 3 D 8  Quiz 4. Which term refers to single covalent bonds? A pi C Lewis B ionic D sigma CORRECT  Quiz 5. Which of the following is true of a multiple covalent bond? A It consists of one sigma bond and at least one CORRECT pi bond. B It consists of more than one sigma bond. C It consists of two sigma bonds and more than two pi bonds. D None of the above is true.