1. ORGANIC CHEMISTRY ATOMIC ORBITALS andMOLECULAR ORBITALS by: J. B. Andres-Catipon

2. OBJECTIVE Be able to know about atomic orbitals and molecular orbitals andunderstand them throughbond formation. 3. TERMS AND DEFINITIONS Orbitals - They represent the probability of finding an electron in any oneplace. They correspond to different energies. So an electron in an orbitalhas definite energy. Orbitals are best described with quantum mechanics. Atomic Orbitals the region in space just outside the nucleus of the atomwhere the probability of finding the electrons is at the highest (95%). Molecular Orbitals formed as a result from the overlap of two atomicorbitals, wherein a pair of electrons occupying. Electron Density a measure of the probability of finding an electron in anorbital. Wave Function mathematical description of the volume of spaceoccupied by an electron having a certain amount of energy. A node in an orbital is the place where a crest and a trough meet. Quantum Mechanics is based on the wave properties of matter.Quantization of energy is the consequence of these properties. 4. FOREWORDAs it has been studied previously, electrons in atoms treatedas waves effectively than as compact particles in circular orelliptical orbits. Such particles like electrons, atoms or moleculesdo not obey Isaac Newtons Law but rather obeys a different kindof mechanics called quantum mechanics.One of the underlying principles of quantum mechanics isthat we cannot determine precisely the paths that electrons followas they move about atomic nuclei (HEISENBERG UNCERTAINTYPRINCIPLE). Because of this, scientists resort to statistical approachand speak of the probability of finding an electron within specifiedregion in space ( ATOMIC ORBITAL). So, quantum numbers are usedto designate the electronic arrangements in all atoms (ELECTRONICCONFIGURATIONS) and play important roles in describing theenergy levels and the shapes of orbitals that describe thedistributions of electrons in space. 5. Each electron is said to occupy an atomic orbital definedby the set of quantum numbers . The main shell of the atomicorbital is indicated by the principal quantum number. These shellsare referred to as the electron energy levels. Each shell has ssubshell. Beginning with the 2nd energy level, it has a p subshell.Each shell has also p subshell. Each of these subshells contains aset of three p atomic orbitals. Each set of atomic p orbitalsresembles three mutually perpendicular equal arm dumbbells. Thenucleus defines the origin of a set of cartesian coordinates withthe usual x, y and z axes which indicate the axis along which eachof the orbitals is directed. Beginning with the 3rd shell, each shellalso contain a third subshell d, composed of five atomic orbitals. Ineach of the fourth shell and larger shells, there is also a fourthsubshell f, which composed of seven atomic orbitals. 6. We also learned that wave function represents atomic orbital.The overall sign on the wave function that describes an atomic orbitalis not important but when we combine two orbitals ( covalent bond-sharing of electron), their signs become very significant. When wavesare combined, they may interact constructively and destructively.Constructive interaction of waves occurs in the region between thenuclei while destructive interaction of waves reduces the probability offinding electrons between the nuclei. These interactions occur in thebonding of MOLECULAR ORBITAL. 7. ATOMIC ORBITALS The energy levels about the nucleus contain group of these atomicorbitals. Each orbital ( designated as s, p, d, and f) has a unique energyassociated with it, can contain a maximum of two electrons and variesin shape and spatial orientation. We are mainly concerned with the s and p orbitals since most of theelements found in organic molecules have their electrons in the1s, 2s, and 2p orbitals. For the shapes of f orbitals, are quite complicated. Higher d and f orbitals are utilized by elements further down in theperiodic table . These are further discussed by inorganic chemists. The s orbital is spherical, like a fuzzy hollow ball with its center at thenucleus of the atom. The are three p orbitals of equal energy, designated px, py, and pz. Each p orbital is dumbbell shaped. Each consists of two lobes withatomic nucleus lying between them and each has a nodal plane at thenucleus, where the probability of the electrons location is zero. 8. Shape of the s orbitalIt contains no nodesbecause it is theclosest to the nucleus.It has the lowestenergy of all theatomic orbitals1s 9. Shape of the s orbital The 2s atomic orbital has a small region of . electrondensity surrounding the nucleus, but most of the eletron density is farther from the nucleus, beyond a node. 2s 10. Shape of the s orbital 3s 11. For any atom there is only one 1s orbital. The "1" represents the factthat the orbital is in the energy level closest to the nucleus. The "s"refers to the shape of the orbital. S orbitals are spherically symmetricalaround the nucleus. For any atom there is only one 2s orbital. This is similar to a 1s orbitalexcept that the region where there is the greatest chance of findingthe electron is further from the nucleus - this is an orbital at thesecond energy level. There is a also a region of slightly higher electrondensity nearer the nucleus called a spherical node. For any atom there is only one 3s orbital. The intensity of colourationindicates the positions where the electron is likely to be found on anyplane cutting through the nucleus. There are two spherical nodes inthe 3s orbital. 12. Shapes of the p orbitals 13. The three p orbitals in the second shell of electrons are totallydifferent from the 1s and 2s orbitals. Each p orbital consists of adumbbell or teardrop shape on either side of the nodal planethat runs through the center of the nucleus. Their orientation is 90 from each other in the three spatialdirection and have identical energies and shapes. Chemists call them as degenerate orbitals. Because electrons in the three 2p orbitals are farther from thenucleus than those in the 2s orbital, they are at a higher energylevel. 14. Shapes of d orbitals 15. Shapes of the f orbitals4fy3 - 3x2y 4fxyz4f5yz2 - yr2 16. Shapes of the f orbitals 4fz3 - 3zr2 4fzx2 - zy24f5xz2 - xr2 4fx3 - 3xy2 17. The 4f y3 - 3x2y orbital corresponds to n=4, =3, and m=-3. Six lobes pointto the corners of a regular hexagon in the xy plane, with one pair oflobes along the x-axis. Three nodal planes pass between the lobes andintersect at the z axis. The 4fxyz orbital corresponds to n=4, =3, and m=-2. Eight lobes point tothe corners of a cube, with four lobes above and four lobes below the xyplane. The x and y axes pass through the centers of four of the cubesfaces (between the lobes). The three nodal planes are defined by thex, y, and z axes. The 4f5yz2 - yr2 orbital corresponds to n=4, =3, and m=-1. Six lobes pointto the corners of a regular hexagon in the yz plane, with one pair oflobes along the x-axis. The three nodal planes pass between the lobesand intersect at the y axis. The 4fz3 - 3zr2 orbital corresponds to n=4, =3, and m=0. Two lobes pointalong the z-axis, with two bowl-shaped rings above and below the xyplane. The nodal surfaces are the xy plane and a conical surface passingthrough the nucleus and between the rings and the lobes. 18. The 4f5xz2 - xr2 corresponds to n=4, =3, and m=+1. Six lobes point to thecorners of a regular hexagon in the xz plane, with one pair of lobes alongthe y-axis. The three nodal planes pass between the lobes and intersect atthe x axis. The 4fzx2 - zy2 orbital corresponds to n=4, =3, and m=+2. It has the sameshape as the 4fxyz orbital, but the corners of the cube are in the planesdefined by the x, y, and z axes and the three nodal planes cut between thelobes and intersect along the z axis. The 4fx3 - 3xy2 orbital corresponds to n=4, =3, and m=+3. It is identical to theorbital with m_=-3 except that a lobe lies along the y axis instead of alongthe x axis. 19. MOLECULAR ORBITALS Bonding between atoms occurs when they come into close enough proximity fortheir orbitals to overlap. Thus, when two atoms are brought close enough togetherto permit overlap of their orbitals, their electron pair and go into a single orbitalencompassing both nuclei. As two atoms form a bond, they interact very much like waves on a body of water.When two waves are traveling in the same directions and one overtakes the otherthe amplitude of the new wave is greater than the amplitude of either of the twothat created it. In contrast, when two waves travel in opposite directions, and theymeet, their amplitudes cancel each other. A pair of electrons encompassing two or more nuclei is said to occupy aMOLECULAR ORBITAL. As with atomic orbitals, a molecular orbital may not contain more than twoelectrons. The molecular orbital represents a lower energy state for the system than do twoseparate atomic orbitals at the characteristic internuclear distance. Energy is liberated during the overlap, and a stable covalent bond is formed. 20. MOLECULAR ORBITALS During bonding,atoms overlap either in-phase or out-of-phase. In-phaseoverlap if there is constructive bonding (added, wave functions with the samesigns), while out-of-phase overlap if there is destructive bonding(subtracted, wave functions of opposite signs). In the in-phase overlap, the wave functions reinforce one another. Thisreinforcement increases the probability of finding the electrons in the regionbetween the nuclei. This is so-called the BONDING MOLECULAR ORBITAL. An out-of-phase overlap forms an ANTIBONDING MOLECULAR ORBITAL. Anode develops between the two nuclei. It is also formed for each bondingmolecular orbital that forms. BONDING MOLECULAR ORBITAL always has lower energy than the energies ofcombining atomic orbitals and the more stable the molecule or ion becomes. ANTIBONDING MOLECULAR ORBITAL always has a higher energy than theenergies of the two separate atomic orbitals, leading to the repulsion betweentwo atoms, and the less stable the molecule or ion becomes. In a bonding MO, the electron density is high between two atoms where itstabilizes the arrangement by attracting both nuclei. 21. Types of Molecular Orbital 22. TYPES OF MOLECULAR ORBITAL1.) Sigma () molecular orbital orbital that is symmetrical about the molecularaxis. The two electrons in it are called the bonds. A sigma molecular orbital may be formed by the direct or head-on overlapthe following orbitals.a.) Two 1s atomic orbitals + 1s1s1s-1s bond s-s MOb.) Two px atomic orbitals + px pxpx-px bondpx-px MO 23. c.)1s and px atomic orbitals1s pxs-px bonds-px MO 24. 2.) Pi () molecular orbital In a molecular orbital, the electrondensity is concentrated above and below the line joining the twonuclei of the bonding atoms. The electrons in it are called electrons and the bond is referred to as bond. A double bond isone bond and one bond, a triple bond consists of one bondand two bonds.A molecular orbital may be formed by the sideways overlapof the following orbitals.a.) Two pz atomic orbitals+ pz pzpz pz MO 25. b.) Two py atomic orbitals+pypy py py MO+ 26. c.) py or pz and dxz atomic orbitals+ dxzpz dxz pz MO+