Atomic Mass is a Weighted Average Review

Final Exam Review From Exam 1: ● ● ● ● ● ● ● ● ● ● ● ● ● atomic mass is a weighted average of all the naturally occurring isotopes of that element ○ atomic mass = ∑ (mass number of element)(% abundance) kinetic energy is energy that results from motion ○ thermal potential energy is energy possessed by an object by virtue of its position ○ chemical ○ electrostatic the electromagnetic spectrum is a continuum of radiation The Double Slit Experiment ○ the light sources recombine after passing through slits, they do so constructively where the 2 waves are in phase (light lines) and de-constructively when out of phase (dark lines) The Quantum Theory ○ black body radiation is electromagnetic radiation emitted when a solid is heated ○ a quantum is the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation ○ E = hv where E is the energy for a single quantum (photon) The Photoelectric Effect ○ electrons are ejected from the surface of a metal exposed to light ○ the light must be of a certain minimum frequency (threshold frequency) ○ the number of electrons ejected is proportional the the light intensity but the energies of the ejected electrons aren’t ○ photons are “particles” of light ○ if the frequency of light is… below threshold the photon will bounce off and not eject any electrons equal to threshold it will dislodge the most loosely held electrons above threshold it will dislodge electrons, imparting kinetic energy to them The Atomic Line Spectra ○ a line spectrum shows the emission of light only at specific wavelengths ○ every element has a unique emission spectrum ○ proves that the energy of electrons are quantized ○ energy is emitted when an electron moves from a higher energy excited state to a lower energy excited state (or ground state) Wave Properties of Matter ○ nodes in a wave don’t move and have an amplitude of 0 ○ electrons in an atom behave like a standing wave ○ only certain wavelengths are possible Quantum Mechanics ○ the uncertainty principal states that it is impossible to know simultaneously both the momentum and position of a particle with certainty ○ the Schrödinger equation incorporates both particle behavior (mass) and wave behavior (wave function Ψ) the probability of finding an electron in a certain region in space is proportional to Ψ2 ○ an atomic orbital is the wave function of an electron in an atom/region in space surrounding the nucleus where there is a high probability of finding an electron ● different from an orbit, which is a fixed path for which you can know both location and velocity of an electron ○ the quantum mechanical model shows that electrons have a wave-like nature ● Atomic Orbitals ○ S orbitals ○ there is an S sub shell in every shell ○ each sub shell contains just one orbital ● P orbitals ○ there is a p sub shell in every shell for which n≥2 ○ each p sub shell contains 3 p orbitals (px, py, pz) ● orbitals in the same shell have the same energy regardless of their sub shell ● sub shells refer to shape (s, p, d) ● The Pauli Exclusion Principle ○ no 2 electrons in an atom can have the same 4 quantum numbers ○ if 2 electrons in an atom have the same n, l, and ml, they must have a different ms (spin) ● Hund’s Rule states that the most stable arrangement of electrons in orbitals of equal energy is when the number of electrons with the same spin is maximized ● The Aufbau Principle states that electrons will fill the lowest energy orbitals first ● isoelectronic- species that have the same electron configurations ● degenerate- having equal energy ● diamagnetic- all electrons are paired (repelled by a magnetic field) ● paramagnetic- one or more unpaired electrons (attracted by a magnetic field) From Exam 2: ● Forces, Potential Energy, and Coulomb’s Law ○ the potential energy of a gravitational system decreases as the 2 objects come close together ○ as the distance between 2 oppositely charged particles increases, the attractive forces between them decreases ○ as the distance between 2 negatively charged particles decrease, the repulsive forces between them increases ○ the potential energy of the system increases as 2 negatively charged particles approach each other ○ looking at a single atom, the potential energy of the system is higher if the electron is farther away from the nucleus ● Effective Nuclear Charge and Shielding ○ an orbital that penetrates into the region occupies by core electrons is less shielded from nuclear charge than an orbital that doesn’t penetrate and therefore has lower energy ○ ○ ○ ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● nuclear charge (Z) is the number of protons in the nucleus effective nuclear charge (Zeff) is the magnitude of positive charge experienced by an electron in the atom ○ the value of Zeff increases from left to right across the periodic table because the number of core electrons remains the same Ionization Energy ○ the energy required to remove the most loosely held electron of an atom ○ decreases down a group because as you add more electrons you increase the number of shells, so the outer electron experiences more shielding and is further from the nucleus ○ increases as you go across the periodic table because you add the same number of protons and electrons, but the electrons are in the same shell thus are poor shielders electron-electron repulsion makes removing an electron easier Solid Types ○ crystalline- has a unit cell and is usually something that is ionic ○ amorphous- has no specific repeating pattern Unit Cell ○ piece that repeats ○ lattice point is in the middle of the atom ○ simple cubic atoms per unit cell: 1 atom packing efficiency: 52% coordination number: 6 edge length: 2r 1/8 of an atom at each corner body centered cubic atoms per unit cell: 2 atoms packing efficiency: 68% coordination number: 8 edge length: 4r/√3 face centered cubic ○ atoms per unit cell: 4 atoms ○ packing efficiency: 74% ○ coordination number: 12 ○ edge length: 2√8 the coordination number describes how many atoms are touching the center Lattice Energy ○ the amount of energy required to convert a mole of ionic solid to its constituent ions in the gas phase ○ the greater the lattice energy the more stable the compound ○ depends on magnitude of charges and distance between them (Coulomb’s Law) Periodic Trends ○ Atomic Radius ○ decreases from left to right ○ increases from top to bottom ● Ionization Energy ○ increases from left to right with a few exceptions: IE1 of 3A is smaller than 2A, IE1 of 6A is smaller than 5A ● Electron Affinity ○ the energy released when an atom in the gas phase accepts an electron ○ increases from left to right (due to the increase in Zeff from left to right) with a few exceptions: EA of 2A is lower than 1A, EA of 5A is lower than 4A ● Ionic Radius ○ same trends as atomic radius ○ when an atom loses an electron and becomes a cation its radius decreases due in part to a reduction in electron-electron repulsions (therefore reduction in shielding) in the valence shell ● an acid is a substance that produces H+ ions when dissolved in water ● ionic crystals are composed of charged spheres held together by Coulombic attraction ex. NaCl From Exam 3 and after: ● in general, a longer bond is a weaker bond ● always write the formal charge on atoms that bear them ● always refer to Coulomb’s Law when comparing the strength of 2 bonds ● second row elements can never violate the octet rule ● group 3 elements can be stable with 6 valence electrons ● Hybridization ○ the mixing of atomic orbitals ○ 1s + 1p = 2sp orbitals ○ 1s + 2p = 3sp2 orbitals ○ sp3 ● orbitals attached: sp3 sp3 sp3 sp3 ● all 4 hybrid orbitals are of equal energy ● sp2 ● orbitals attached: sp2 sp2 sp2 p ● sp ● orbitals attached: sp sp p p ○ second row elements will hybridize ○ sigma bonds are perpendicular to the p orbitals that make pi bonds ○ it’s the singly occupies p orbitals not involved in hybridization that give rise to multiple bonds ○ a sigma bond is a bond in which the shared electron density is concentrated directly along the internuclear axis ○ a pi bond is a bond that forms from the interaction of parallel p orbitals ○ if a bond angle is close to 90° then it is using non-hybridized p orbitals to make the bond ● Resonance ○ 1. 2. 3. ● ● ● ● ● ● ● ● resonance structures- 2 or more Lewis structures for a single molecule that differ only in the positions of electrons ○ in most cases, the resonance structures of a molecule aren’t equivalent and don’t contribute equally to the hybrid ○ resonance lowers potential energy ○ resonance delocalization happens in π systems (or p orbitals) ○ how to determine which resonance structure contribute more: As long as the octet rule is not exceeded for second row elements, the contributing structure with the greater number of covalent bonds contributes more to the resonance hybrid. This rule is more important than rules 2 and 3. When 2 or more structures satisfy the octet rule, the major contributor is the one with the smallest separation of oppositely charged atoms. Among structural formulas that satisfy the octet rule and in which one or more atoms bear a formal charge, the major contributor is the one in which the negative charge resides on the most electronegative atom and the positive charge on the least electronegative atom. lone pairs repel more strongly than bonds lone pairs take up more space than bonding pairs a lone pair on a central atom is attracted only to the nucleus of that atom, whereas a bonding pair of electrons is simultaneously attracted by the nuclei of both the bonding atoms multiple bonds rebel more strongly than single bonds VSEPR Model ○ electrons pairs in the valence shell of an atom repel one another ○ electron domain- a lone pair or bond ○ electron domain geometry is the arrangement of electron domains around a central atom ○ molecular geometry is the arrangement of bonded atoms Valence Bond Theory ○ atoms share electrons when an atomic orbital on one atom overlaps with an atomic orbital on the other ○ each of the overlapping atomic orbitals must contain a single unpaired electron ○ formation of a bond results in a lower potential energy for the system Isomers ○ different compounds that have the same molecular formula ○ constitutional- isomers that differ in the order that the atoms are connected (also called structural) ○ stereoisomers- isomers with the same constitution but differ in the arrangement of their atoms in space IUPAC Rules ○ name branched alkanes as substituted derivatives of the unbranched parent alkanes ○ How to: 1. Pick out the longest continuous carbon chain and find the IUPAC name (methane, ● ● ● ● ● ● ● ● ethane, etc). 2. Identify the substituent groups attached to the parent. 3. Number the longest continuous chain in the direction that gives the lowest number to the substituent at the first point of branching. when 2 or more different substituents are present they are listed in alphabetical order in the name nomenclature of alkyl halides ○ functional class nomenclature- the alkyl group and the halide (fluoride, chloride, bromide, etc) are designated as separate words ex. methyl fluoride, 1-ethylbutyl bromide ○ substitutive nomenclature- treats the halogen as a halo (fluoro-, chloro-, bromo-, iodo-, etc) substituent on an alkane chain ex. 1-fluoropentane ○ substitutive nomenclature is most often preferred ○ when the carbon chain bears both a halogen and an alkyl subsistent, the 2 are considered equal rank nomenclature of alcohols ○ replace the -e ending of the corresponding alkane with -ol ○ the hydroxyl (-OH) groups take precedence over alkyl groups and halogens in determining the direction in which a carbon chain is numbered Newman Projections ○ staggered confirmation is more stable ○ eclipsed conformation is less stable and has higher potential energy ○ 60° is gauche ○ 180° is anti staggered anti is most stable and has least potential energy ○ torsional strain is decreased stability of a molecule associated with eclipsed bonds ○ steric strain is the destabilization of a molecule as a result of van der Waals repulsion, distorted bond distances, bond angles, or torsion angles ○ van der Waals strain is destabilization that results when 2 atoms or groups approach each other too closely the most stable conformations of higher alkanes such as pentane and hexane have their carbon backbones arranged in a zigzag Enantiomers ○ stereoisomers that are related as an object and its non superimposable mirror image ○ constitution is the order in which the atoms are connected ○ configuration is the spatial arrangement of atoms or groups at a chirality center or double bond ○ conformation is the rotation about a single bond ○ physical properties (density, melting point, boiling pint, etc) are identical for both enantiomers of a chiral compound diastereomers are stereoisomers that are not enantiomers (not mirror images of one another) ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 2. 3. Chirality ○ a molecule is chiral if its 2 mirror image forms are not superimposable in 3D ○ a molecule that has a single asymmetric carbon will be chiral ○ a molecule is achiral if it’s superimposable on its mirror image ○ a molecule that has an internal mirror plane will be achiral ○ chirality center (asymmetric carbon)- an atom that has 4 nonequivalent atoms or groups attached to it ○ chiral compounds rotate plane polarized light (physical property) ○ meso forms are achiral molecules that have chirality centers a conformational change (rotational) doesn’t change configuration Intermolecular Forces ○ dipole-dipole interactions attractive forces between polar molecules the partial positive charge on one molecule is attracted to the partial negative charge on a neighboring molecule the magnitude of the attractive forces depends on the magnitude of the dipole hydrogen bonding special type of dipole-dipole interaction hydrogen bond donor- any polar species that contains a hydrogen covalently bonded to an N, O, or F hydrogen bond acceptor- any polar species with a long electron pair on N, O, or F dispersion forces attractive force between molecules, whether they are polar or not, due to instantaneous or induced dipoles arise from the movement of electrons in non polar molecules the magnitude depends on how mobile the electrons in the molecule are ion-dipole interactions Coulombic attractions between ions and polar molecules magnitude depends on charge and size of the ion, and dipole moment and size of the polar molecule R S Notation ○ The Cahn-Ingold-Prelog Sequence Rules: 1. Higher atomic number takes precedence over lower. 2. When 2 atoms directly attached to the chirality center are the same, compare the atoms attached to them on the basis of their atomic numbers, precedence is determined at the first point of difference. 3. Work outward from the point to attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. 4. When working outward from the point of attachment, always evaluate substituent atoms one by one, never as a group. 5. An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom. R is clockwise S is counterclockwise ● a molecule with a single asymmetric center with an R configuration always has an enantiomer with the S configuration