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UMass Amherst CHEM 261 - Exam 1 Study Guide
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Exam 1 Study Guide Lectures: 1 - 12Outline:I. Lectures 1-12Lecture 1:-Atomic number (Z) : number of protons in the nucleus of an atom**nucleus=protons and neutrons (positively charged and small) , electrons orbit nucleus, are in a cloud around it, with a diameter ~2 x10-10 m, or 200 picometers (Most of organic chemistry is, “where are the electrons?” or “to where are they moving?”)-Mass number (A): number of protons and neutrons in the nucleus of an atom-All atoms of an element have the same atomic number! What can change is the mass numberIsotope: atoms of the same element that have a different number of neutrons, which causes them to have a different massAtomic Mass: (atomic weight) of an element is the weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes Orbitals:Energy Levels:Lecture 2: CHEM 261 1st Edition1. Lewis dot Structures:-“Electron dot structures”-Valence shell electrons of an atom are represented as dots2. Kekule (line) structures-2 electron covalent bonds represented as a line, rather than as dots, drawn between atoms**both concepts are also applicable to both double and triple bonds**The “octet rule”: elements tend to combine in such a way as to fill the outer most valence shellsso that they contain 8 electrons, or a noble gas electron configuration (very stable). This can be done by forming…-ionic bonds: electrostatic interaction in which electrons are completely transferred between atoms-covalent bonds: shared electron bond, forms a molecule. Can be polar (unequal sharing) or nonpolar (equal sharing)Valence Bond Theory: Bonds form when 2 atoms get close to each other AND one singly occupied orbital on one atom overlaps with a single occupied orbital on another. The electrons are now paired in overlapping orbitals and are attracted to the nuclei of both atoms, which bonds the atoms together.** as bond strength increases, bond length decreasesHybridization:Sp3 orbital: Formed by combining an s orbital with 3 p orbitals. The overlapping orbitals allow for stronger bonds than un-hybridized s and p orbitals. Sp3 are tetrahedral in shape** the “3” in sp3 refers to how many of each type of orbital combine-NOT how many electrons occupy itSp2 orbital: formed by combining an s orbital with 2 p orbitals. Can create a bent shapeSp: 2 sp orbitals combine, create a linear shape. Triple bonds!!Sigma bonds: bonds formed by the head-on overlap of 2 orbitals along a line drawn between the nucleiLecture 3: CHEM 261 1st EditionCovalent bond: 2 electrons share equally. Similar electronegativity between atoms. Polar covalent = unequal sharing, partial +, partial -Ionic Bond: electron transfer, held by electrostatic interactions. BIG difference in electronegativity between atoms. Periodic trend: Electronegativity increases left to right, top to bottom + electronegative: wants electrons, doesn’t give them up“-“ electronegative: give away electronsInductive effect- bond polarity contributes to unequal sharing of electronsDipole moment- The net dipole of a molecule. Electron pairs contribute to the partial negative character of a molecule. Lecture 4:Formal Charge: (# of valence electrons) - # of bonds formed by atom - # of nonbonding electrons on atomResonance: A tool to help draw structures in 2 dimensions. In reality, resonance structures exist somewhere in between the states depicted1. Resonance bonds only change for π (pi bonds) or non-bonding electrons2. Octet rule still applies3. Remember electronegativity! (negative charges go on electronegative atoms)Brønsted-Lowry Acids and Bases: Acid: Donates an H+ , Base: Accepts an H+ CHEM 261 1st EditionXH + SX- + HS (XH= acid, S=base, X-=conjugate base, HS= conjugate acid)Acid strength and pKa:pKa = -log(Ka), where Ka is the acid dissociation constant**big Ka value = strong acid**big pKa value = weak acid** -pKa = very strong acid!!Lecture 5: -No new material covered, was a recap of everything learnedLecture 6:** protons will move from a lower to a higher pKa Lewis Acids and Bases: Lewis acid- electron ACCEPTOR, Lewis base- electron DONORLewis acids: no scale!Non-covalent interactions:1. Hydrogen bonds (strongest)A hydrogen bond is the attractive force between the hydrogen (partial positive charge) attached to an electronegative atom (partial negative charge) of one molecule and the electronegative atom of another nearby molecule. The best example of hydrogen bonding is WATER!2. Dipole-Dipole InteractionsDipole-dipole interactions occur between the positive end of one polar molecule and the negative end of another, different polar molecule. 3. Van der Waals Forces/Dispersion forces (weakest)Electron distribution can change, and these temporary changes causes for brief attractions and repulsions between moleculesAlkanes: CHEM 261 1st Edition-Molecular formulae are CxHx….only for NON-RING structures, all alkanes are sp3 bonded compounds made up of carbon and hydrogen (hydrocarbons!), all sigma bonded/single bonds. All carbons are saturated. **Alkane names correspond to the # of carbonsA functional group is a group of atoms within a molecule that has a characteristic chemical behavior. An “alkyl” group is any alkane as a functional group CHEM 261 1st EditionLecture 7:Moiety: functional group or components of a functional group** can focus on moiety of functional group or moiety of a component of functional group (one atom’s moiety)Carbon is classified by its connections CHEM 261 1st EditionIsomers:Constitutional Isomers: Isomers with the same molecular formula but have different connectivities. You must make or break a bond to get them to have the same connectivity. **When the number of carbons is greater or equal to 4 in a molecule, there is a greater chance that that molecule will have isomers** if connectivity is equal, then it is the same molecule. If breaking connectivity, they are not equal (isomers)Naming Branched Alkanes: Step 1: Find the parent hydrocarbon (longest continuous chain of carbon atoms. If two chains exist that are of equal length, the parent chain is the one with the larger number of branch points. You may have to “turn corners” to find the parent chain; it may not be apparent)Step 2: Number the atoms in the longest chain (Beginning at the end nearest to the first branch point, number each carbon atom in the parent chain. If there is branching an equal distance away from both ends of the parent chain, begin numbering


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