A Review of General Chemistry - Electrons, bonds, and Molecualr Properties
A Review of General Chemistry - Electrons, bonds, and Molecualr Properties Chem 249
Santa Ana College
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This 26 page Study Guide was uploaded by Study_Smarter_Now on Monday October 3, 2016. The Study Guide belongs to Chem 249 at Santa Ana College taught by Nguyen, W in Fall 2016. Since its upload, it has received 4 views. For similar materials see Organic Chemistry I in Chemistry at Santa Ana College.
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Date Created: 10/03/16
01 | A Review of General Chemistry - Electrons, bonds, and molecular properties 1.1 | INTRODUCTION TO ORGANIC CHEMISTRY Define organic chemistry. • Organic chemistry is literally defined as the study of compounds containing carbon atoms… but it is actually the study of electrons, not atom. ® Must recognize that reactions occur as a result of the motion of electrons. VITALISM: What is vitalism? The belief that compounds obtained from living sources possessed a special “vital force” that inorganic compounds lacked, stipulated that it should be impossible to convert inorganic compounds into organic compounds without the introduction of an outside vital force. How was vitalism disproved? • Vitalism was disproved: In 1828 when German chemist Fredrich Wohler demonstrated the conversion of ammonium cyanate (inorganic salt) into urea (organic compound) After vitalism was disproved, how was ® organic compounds became defined as organic chemistry redefined? those compounds containing carbon atoms, while inorganic compounds generally were defined as those compounds lacking carbon atoms. • Organic chemistry occupies a central role in the What are some central roles organic world around us: chemistry plays in the world around us? * The food we eat and the clothes that we wear are comprised of organic compounds. * Our ability to smell odor or see colors results from the behavior of organic compounds. 1 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties * Pharmaceuticals, pesticides, paints, adhesives, and plastics are all made from organic compounds. * Our bodies are constructed mostly from organic compounds (DNA, RNA, proteins, etc) whose behavior and function are determined by the guiding principles of organic chemistry. 1.2 | THE STRUCTURAL THEORY OF MATTER: CONSTITUTIONAL ISOMERS: Define constitutional isomers. Compounds that have the same molecular formula but differ in the way the atoms are connected. * have different physical properties and different names. Define valence. VALENCE: Describes the number of bonds usually formed by each element. Define tetravalence. • Tetravalent: Element that forms 4 bonds Define trivalence. • Trivalent: Element that forms 3 bonds Define divalent. • Divalent: Element that forms 2 bonds 2 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • Monovalent: Define monovalent. Element that forms 1 bond 1.3 | ELECTRONS, BONDS, AND LEWIS STRUCTURES WHAT ARE BONDS? • Bonds are the “glue” that holds atoms together. COVALENT BOND: What are covalent bonds? A bond resulting from two atoms sharing a pair of electrons ® When e-s are shared to form a bond, there is a decrease in energy, indicated by the negative value of DH. ® moving toward the left of the diagram, the hydrogen atom approaches each other, and there are several forces that must be taken When looking at energy diagram, when into account: moving toward the left of the diagram, what are the forces that must be taken into 1: The force of repulsion between the account as the hydrogen atoms approach two negatively charged e-s each other? 2: The force of repulsion between the two positively charged nuclei 3: The forces of attraction between the positively charged nuclei and the negatively charged electrons. * As hydrogen atoms get closer to each other, all of these forces get stronger. 3 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties DRAWING THE LEWIS STRUCTURE OF AN ATOM: Define Lewis structure. LEWIS STRUCTURE: A drawing structure of molecules in which the e-s take central state. • A few simple features of atomic structure: What is a nucleus composed of? * The nucleus of an atom is comprised of protons & neutrons. ® Proton = + ® Neutrons = neutral * For neutral atom, the number of protons is balanced by an equal number of electrons (-1 charge) ® 1 shell contains 2 e-s nd ® 2 shell contains up to 8 e-s * The number of valence e-s in an atom is identified by its group number in the periodic table. VALENCE ELECTRONS: Define Valence electrons? The electrons in the outermost shell of an atom. 4 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties DRAWING THE LEWIS STRUCTURE OF A SMALL MOLECULE: • Drawings are constructed based on the observation that atoms tend to bond in such a way so as to achieve the electron configuration of a noble gas. Define octet rule? OCTET RULE: The observation that second row elements (C,N,O,F) Will form the necessary number of bonds so as to achieve a full valence shell (8 electrons) What are lone pairs? LONE PAIRS: A pair of unshared, or no bonding electrons 1.4 | FORMAL CHARGES FORMAL CHARGE: What is a formal charge? Is associated with any atom that does not exhibit the appropriate number of valence electrons. • How to identify if a formal charge is required: What are the steps to require to identify a formal charge? 1: Determine the appropriate number of valence electrons from an atom 2: Determine whether the atom exhibits the appropriate number of electrons * Group number indicates the appropriate number of valence electrons for each atom. * After ID the appropriate number of electrons for each atom in a Lewis structure, the next task is to determine if any of the atoms in the Lewis structure exhibit an unexpected number of electrons. 5 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties 1.5 | INDUCTION AND POLAR COVALENT BONDS What are the 3 categories chemist classify • Chemist classify bonds into 3 categories: bonds? 1: Covalent Electrons are considered to be equally shared 2: Polar Covalent Electrons are not shared equally between the atoms 3: Ionic Electrons are not shared at all, the attraction between oppositely charged ions. Define electronegativity. ELECTRONEGATIVITY: A measure of the ability of an atom to attract electrons. • Rough guidelines to classify a bond: When using electrochemistry, what are the rough guidelines to classify a bond? * If the difference in electronegativity is less than 0.5 Results in a covalent bond. 6 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties * If the difference in electronegativity is between 0.5 and 1.7 Results in a polar covalent bond * If the difference in electronegativity is greater than 1.7 Results in ionic bond The result of the force of attraction between the two oppositely charged ions. INDUCTION: Define induction. The withdrawal of electron towards one atom over the other, causes the formation of partial positive and partial negative charges. PRACTICALLY SPEAKING – ELECTROSTATIC POTENTIAL MAPS What are electrostatic potential maps? ELECTROSTATIC POTENTIAL MAPS: 3D, rainbow-like images showing partial charges of molecule. • red represents a region that is d- • blue represents a region that is d+ 7 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties 1.6 | ATOMIC ORBITALS QUANTUM MECHANICS • (1924): French physicist Louis de Broglie suggested In 1924 what did French physicist Louis de that electrons are particles, but also exhibited Broglie suggest, which introduced a major wavelike properties. concept that developed the field of quantum mechanics? • An equation is constructed to describe the total energy of a hydrogen atom (one proton + one Based on quantum mechanics, what is a electron). wave equation? ® The equation called the wave equation take into account the wavelike behavior of an electron that is in the electric field of a proton. What is a wavefunction? • The wave equation is then solved to give a series of solutions called wavefunctions. ® y (psi) is used to denote each wave function (y 1y ,y2). 3 * Each wavefuntion corresponds to an allowed energy level for the electron. * Suggests that an electron, when contained in an atom, can only exist at discrete energy levels ® The energy of the electron is quantized. • Each wavefunction is a function of spatial location. ® It provides information that allows us to assign a numerical value for each location in 3D space relative to the nucleus. 2 ® The square value y for any particular What does the square value of the location indicates the probability of finding the 2 electron in that location. wavefunction (y ) indicate? \ 2 a 3D plot of y will generate an image of an atomic orbital 8 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties ELECTRON DENSITY AND ATOMIC ORBITALS What is an orbital? ORBITAL: A region of space that can be occupied by an electron. • Electron clouds only come in a small number of shapes and sizes (as defined by the orbitals) • We must think of an electron cloud as a single entity, even through it can be thicker in some places and thinner in other places. • An electron cloud does not have defined edges. The shape of an orbital refers to the region • The shape of an orbital refers to a region of space of space that contains what % of the that contains 90-95% of the electron density. electron density? ELECTRON DENSITY: Define electron density? The density associated with the probability of finding an electron in a particular region of space. ATOMIC ORBITAL (AO): What is an atomic orbital? The region of space which is defined with respect to the nucleus of a single atom containing electron density Examples: s, p, d, f PHASES OF ATOMIC ORBITALS • The wavefunction (y) mathematically describes the wave, and the value of the wavefunction is dependent on location. ® Locations above the average level of the lake have a positive value for y (red) ® Locations below the average level of the lake have a negative value for y (blue) 9 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties Define node. NODE: Locations where the value of y = 0 • A positive value for y does not imply a positive charge. ® The value of y (+ or -) is a mathematical convention that refers to the phase of the wave. What is the electron density at a node? ® @ a node, where y=0, the electron density 2 (y ) will also be zero. * MEANS: there is no electron density located at a node. FILLING ATOMIC ORBITALS WITH ELECTRONS • Electrons are lowest in energy when they occupy a 1s orbital, b/c the 1s orbital is closest to the nucleus and it has no nodes (the more nodes that an orbital has, the greater its energy) 10 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • The 2s orbital has one node and is farther away from the nucleus \ higher in energy than the 1s orbital • After the 2s orbital, there are three 2p orbitals that are all equivalent in energy to one another. What are degenerate orbitals? DEGENERATE ORBITALS: Orbitals with the same energy level • The order in which orbitals are filled by electrons is What are the 3 simple principles that determined by 3 simple principles: determine the order in which orbitals are filled by electrons? 1: AUFBAU PRINCIPLE: the lowest energy orbital is filled first 2: PAULI EXCLUSION PRINCIPLE: Each orbital can accommodate a maximum of two electrons that have opposite spin. ® In order for the orbital to accommodate 2 e-s, the e-s must have opposite spin states. 3: HUND’S RULE: When dealing with degenerate orbitals, such as p orbitals, one electron is placed in each degenerate orbital first before electrons are paired up. 11 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties 1.7 | VALENCE BOND THEORY • A covalent bond is formed from the overlap of When is a covalent bond formed? atomic orbitals. • Two commonly used theories for describing the What are two commonly theories used for nature of atomic orbital overlap are: describing the nature of atomic orbital 1: Valence bond theory overlap? 2: Molecular orbital (MO) Theory Define constructive interference. CONSTRUCTIVE INTERFERENCE: Produces a wave with larger amplitude Define destructive interference. DESTRUCTIVE INTERFERENCE: Results in waves canceling each other out, which produces a node What is the valence bond theory? VALENCE BOND THEORY: A bond is simply the sharing of electron density between two atoms as a result of the constructive interference of their atomic orbitals. SIGMA (s) BOND: Define a sigma bond. The electron density of this bond is primarily located on the bond axis. Characterized by circular symmetry WRT the bond axis. N 12 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • All single bonds are s bonds. 1.8 | MOLECULAR ORBITAL THEORY MOLECULAR ORBITAL (MO) THEORY: What is the molecular orbital theory? Describes a bond in terms of the constrictive interference between two overlapping atomic orbitals. • uses mathematics as a tool to explore the consequences of atomic orbital overlap. • When 2 atomic orbitals overlap they ceases to exist & are replaced by 2 molecular orbitals, each of which is associated with the entire molecule. Define Linear combination of atomic LINEAR COMBINATON OF ATOMIC ORBITALS (LCAO): orbitals (LCAO). The mathematical method used as a tool to explore the consequences of atomic orbital overlap to describe molecular orbital theory. MOLECULAR ORBITALS: What are molecular orbitals? The combination of atomic orbitals. • Are considered to be a single entity held together by many electron clouds (some expanding the entire length of the molecule) • filled with electrons in a particular order 13 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • e-s first occupy the lowest energy orbitals, with a maximum of two e-s per orbitals. • An atomic orbital is a region of space associated with Compare and contrast an atomic orbital vs. an individual atom, while a molecular orbital is A molecular orbital. associated with an entire molecule. BONDING MO: What is a bonding molecular orbital? The lowest energy molecular orbital, resulting from constructive interference of the original two atomic orbitals. What is an antibonding molecular orbital? ANTIBONDING MO: The higher energy molecular orbital, resulting from destructive interference. • The antibonding MO has one node, which explains why it is higher in energy. ® Both electrons occupy the bonding MO in order to achieve a lower energy state. ® This lowering in energy is the essence of the bond. • For every molecule, two of its molecular orbitals will For every molecule, what are two of its be of particular interest: molecular orbitals that are of particular 1: the highest energy orbital from among the interest? occupied orbitals (HIGHEST OCCUPIED MOLECULAR ORBITAL – HOMO) 14 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties 2: The lowest energy orbital from among the unoccupied orbitals (LOWEST UNOCCUPIED MOLECULAR ORBITAL – LUMO) 1.9 | HYBRIDED ATOMIC ORBITALS 3 METHANE AND sp HYBRIDIZATION • Linus Pauling introduced the concept of What scientist introduced the concept of hybridization hybridization? • The hybridization process does not represent real What is hybridization? physical process that the orbitals undergo. ® it is a mathematical procedure that is used to arrive at a satisfactory description of the observed bonding. 3 3 sp HYBRIDIZED ORBITALS: Define sp hybridized orbitals? 4 orbitals produced by averaging one s orbital & 3 p orbitals. 3 ( 4 sp orbitals) 15 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties DOUBLE BONDS AND sp HYBRIDIZATION: 2 2 2 Define sp hybridized orbitals. sp HYBRIDIZED ORBITALS: 3 orbitals produced by averaging one s orbital & two p orbitals. (3 sp orbitals & 1 p orbital) 16 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties Pi (p) BOND: Define Pi bond. The interaction of two p orbitals overlapping with each other. How is a double bond produced? • The overlapping of a s bond along with the overlapping of a p bond causes a double bond. TRIPLE BONDS AND sp HYBRIDIZATION sp HYBRIDIZATION: Define sp hybridization. The interaction of one s orbital averaged with only one p orbital. (1 sp orbital & 3 p orbitals) 17 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties BOND STRENGTH AND BOND LENGTH: • Single bond: has only one bonding interaction How are single bonds formed? ( one s bond) • double bond: has two bonding interactions (one s bond & one p bond) 18 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties How are triple bonds formed? • Triple bond: has three bonding interactions (one s bond & two p bonds) • Bond strength: How is the bond strength of a single, single < double < triple double, and triple bond ranked? - < = < º 1.10 | VSEPR THEORY: PREDICITING GEOMETRY • Valance Shell Electron Pair Repulsion (VSEPR) How is the geometry of a small compound • The geometry of a small compound is predicted by predicted? focusing on the center atom and counting the number of s bonds and lone pairs. STERIC NUMBER: The total (s bonds plus lone pairs) Define steric number. ® Indicates the # of electron pairs (bonding and nonbonding) that are repelling each other. 19 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties GEOMETRIES RESULTING FROM sp HYBRIDIZATION 3 • Tetrahedral: 4 sp bonds forms what shaped molecule? 4 sp bonds • Trigonal pyramidal 3 sp and 1 lone pair forms what shaped 3 sp bonds & 1 lone pair molecule? 3 • Bent 3 2 sp bonds and 2 lone pairs forms what 2 sp bonds & 2 lone pairs shaped molecule? GEOMETRIES RESULTING FROM sp HYBRIDIZATION 2 • Trigonal planar 3 sp bonds 3 sp bonds forms what shaped molecule? • Bent 2 sp bonds and 1 lone pair forms what 2 sp bonds & 1 lone pair shaped molecule? 20 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties GEOMETRIES RESULTING FROM sp HYBRIDIZATION • Linear 2 sp bonds form what shaped molecule? 2 sp bonds SUMMARY: 1.11 | DIPOLE MOMENTS AND MOLECULAR POLARITY DIPLE MOMENT (µ): What is a dipole moment? An indicator of polarity µ = d x d d = the amount of partial charge on either end of the dipole d = the distance of separation Define debye. DEBYE (D): The unit used to report diploe 18ment 1 deby = 10 esu ∙ cm 21 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • Measuring the dipole moment of a particular bond How is the percent ionic characteristic of a allows us to calculate the percent ionic characteristic bond determined? of that bond. Example: CCl bond This bond has a bond length of 1.772 x 10 cm, -8 -10 and an electron has a charge of 4.80 x 10 esu. If the bond were 100% ionic, then the dipole moment would be: µ = e x d -10 -8 µ = (4.80 x 10 esu-18 (1.772 x 10 cm) µ = 8.51 x 10 esu ∙ cm or 8.51 D. In reality the bond is not 100% ionic & the experimentally observed dipole moment is measured at 1.87 D \ the percent ionic characteristic of a CCl bond: MOLECULAR DIPOLE MOMENT: The vector sum of all the individual dipole Define molecular dipole moment. moments. Takes into account both the magnitude and the direction of each individual dipole moment. 22 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties • The presence of a lone pair has a significant effect on the molecular dipole moment. 1.12 | INTERMOLECULAR FORCES & PHYSICAL PROPERTIES INTERMOLECULAR FORCES: What are intermolecular forces. The attractive forces between the individual molecules - Are electrostatic ELECTROSTATIC FORCES: What are electrostatic forces. Forces that occur as a result of the attraction between opposite charges. What are the 3 types of intermolecular • The 3 types of intermolecular forces: forces? 1: Dipole-dipole interactions 2: hydrogen bonding interactions 3: London dispersion interactions DIPOLE-DIPOLE INTERACTIONS • The resulting net attraction between the molecules results in an elevated melting point and boiling point. 23 01 | A Review of General Chemistry - Electrons, bonds, and molecular properties Define dipole-dipole interaction. DIPOLE-DIPOLE INTERACTION: The resulting net attraction between two dipoles. HYDROGEN BONDING HYDROGEN BONDS: What are hydrogen bonds? • A specific type of dipole-dipole interaction
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