Chemical Bonds, Orbital Shapes, and Orbital Hybridization

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Elinor Franklin
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1 Chemical Bonds, Orbital Shapes, and Orbital Hybridization PRELAB ASSIGNMENT Read the entire laboratory write up. Write an objective and answer the following questions in your laboratory notebook before coming to the lab. Complete the prelab assignment (the objective, hazards, and answers to the prelab questions) so that it may be checked as you come to lab. Read the entire laboratory write up before answering the following questions. 1. List at least three principles of valence bond theory. 2. What determines the type of hybridization around the central atom of an ion or molecule? 3. What type(s) of orbitals overlap to form pi bonds? 4. How many sigma bonds can form between any two atoms? 5. Why can alkali and alkali earth metals not form pi bonds? BACKGROUND INFORMATION The properties of chemical compounds are directly related to the ways in which atoms are bonded together to make the molecules of the compound. Valence electrons, or an atom s outermost electrons, participate in bond formation. The Group number of a main Group (Group A) element can tell you the number of valence electrons each element has. Valence electrons are distributed among s, p and sometimes d orbitals. Valence Bond (VB) Theory says that bonds are formed by the overlap of atomic orbitals to form new hybridized orbitals. Two electrons of opposite spin can be accommodated in the hybridized orbitals formed. Sigma ( ) bonds are formed from the hybridization or s, p, and sometimes d orbitals in the plane of the nuclei of bonding atoms. The electron density of a sigma bond is greatest along the axis of the bond. The orbital shape around the central atom of a molecule determines the type of hybridization around that atom. The names of the types of hybridization are based on the numbers and types of atomic orbitals that are hybridized. For example one s and one p atomic orbital hybridize to form two sp hybrid orbitals while one s and two p atomic orbitals hybridize to form three sp 2 hybrid orbitals. Bonds & Orbital Hybrid Unhybridized Example e Pairs (e Pair) Type p Orbitals Around the Shape Central Atom 1 or 2 linear sp 2 p s HCN 3 trigonal sp 2 1 p BF 3 planar 4 tetrahedral sp 3 none CH 4 5 trigonal sp 3 d none SF 4 bipyramidal 6 octahedral sp 3 d 2 none SF 6

2 Pi ( ) bonds are formed by overlapping unhybridized p orbitals above and below the plane of the nuclei of the bonding atoms. Note that alkali and alkali earth metals cannot form pi bonds because they do not have valence electrons in p orbitals. The electron density in a pi bond is above and below the plane of the nuclei of the bonding atoms. Pi bonds can only form if bonding atoms have unhybridized p orbitals and these orbitals are perpendicular to the plane of the bonding nuclei and parallel to each other. Only one sigma bond can form between two atoms. The second bond in a double bond is a pi bond. Triple bonds contain one sigma bond and two pi bonds. Please note that a double bond contains one sigma and one pi bond. Therefore, it is not correct to say that a pi bond is a double bond. PROCEDURE Copy the following table into your lab notebook and, for each molecule or ion complete the number of valence electrons, Lewis structure, number of bonds or lone pairs around the central atom, orbital shape, hybridization, and number of unhybridized p orbitals around the central atom. Molecule/Ion Total Valence Electrons Bonds & E Pairs Around The Central Atom Lewis Structure Orbital (or e Pair) Shape Hybridization Unhybridized p Orbitals Silicate ion SiO Tetrahedral sp 3 none Oxygen, O 2 Hydrogen Cyanide, HCN Nitrate, NO 3

3 Phosgene COCl 2 Sulfur dioxide, SO 2 Carbon monoxide CO Carbon dioxide CO 2 Boron fluoride BF 3 (this is an ionic compound and violates the octet rule) Sulfur trioxide SO 3 Oxygen diflouride OF 2

4 Sulfur Pentafluoride Ion SF 5 (this is an expanded octet) Boron Tetrahydride Ion BH 4 Chlorine Trifluoride ClF 3 (this is an expanded octet) Acetone CH 3 COCH 3 Ethane CH 3 CH 3 Ethylene CH 2 CH 2

5 Ethyne (Acetylene) CHCH QUESTIONS Answer all questions in your laboratory notebook. 1. Develop and write down a set of instructions for how to determine the hybridization around the central atom in a molecule or ion? 2. What trends do you see regarding which molecules or ions form double and/or triple bonds? 3. In what type of bonds (ionic, polar covalent, or covalent) is multiple bonding most likely to occur? 4. What type of molecules (organic or inorganic) violate the octect rule.

Hybridization of Orbitals

Hybridization of Orbitals Structure & Properties of Matter 1 Atomic Orbitals and Bonding Previously: Electron configurations Lewis structures Bonding Shapes of molecules Now: How do atoms form covalent