Chap-2A

Chap-2A - Chapter 2 Structure and Properties of Organic...

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Unformatted text preview: Chapter 2 Structure and Properties of Organic Molecules Chapter 22 2-1 Wave Properties of Electrons in Orbitals •Electrons exhibit wave-particle duality. •There are 2 types of wave: • travelling (ripples on a pond) •standing waves (guitar string). •An electron in an atomic orbital can be described like a bound, stationary vibration – a standing wave. Chapter 23 •Consider a guitar string being plucked in the middle •We get a standing wave, which at one moment has all of the string up, and then the next moment, all of the string down. Chapter 24 •A 1s orbital is a 3D analog of this guitar string. •An orbital is described by its wave function, ψ , (psi), which is a mathematical description. •The electron density at any point is equal to ψ 2 . •A 1s orbital is spherically symmetrical, and is often represented as a circle (meaning a sphere). + •The + and – signs are not charges, just phases. The phase of the wave is either positive or negative. Chapter 25 •If we place our finger exactly half way along the string and pluck again, the string vibrates, we observe a standing wave but the midpoint does not move. •The amplitude at the midpoint is zero – a Node. •When one half of the string is up, the other is down, the two halves vibrate out of phase with one another. •This is the first Harmonic of the wave. •Extending to three dimensions – two out-of-phase lobes, separated by a nodal plane: a 2p orbital. Chapter 26 2-1A Linear Combinations of Atomic Orbitals (LCAO) • • Linear combination of atomic orbitals Linear combination of atomic orbitals Ø between different atoms is between different atoms is bond formation bond formation Ø on the same atom is on the same atom is hybridization hybridization . • • Conservation of orbitals Conservation of orbitals • • Waves that are in phase add together. Waves that are in phase add together. Amplitude increases. Amplitude increases. • • Waves that are out of phase cancel out. Waves that are out of phase cancel out. Chapter 27 2-2 Molecular Orbitals •Two atoms bond together to attain a lower energy. •The stability of a covalent bond comes from the large electron density in the space between the two nuclei (the bonding region). •The electrons shield the positive nuclei from each other, and allow them to get close. •There is an optimal distance for the nuclei to be separated: too close and the +charged nuclei will repel, too far and the electron sharing is weak. •This optimal distance is the Bond Length (Fig 2-5, p40). Chapter 28 2-2A The H 2 Molecule: Sigma Bonding •This is the simplest example of covalent bonding. •Consider bringing two Hydrogen atoms together: as they approach each other, their 1s orbitals will start to overlap. •The orbitals (waves) will interfere constructively and destructively. Chapter 29 • Constructive Interference •They interfere constructively when the orbitals are in phase (same sign). •The wave functions reinforce one another, electron density is increased in this region: it is a Bonding Molecular Orbital (bonding MO). Chapter 210...
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This note was uploaded on 04/08/2008 for the course CHE 201 taught by Professor Bong during the Spring '08 term at SUNY Buffalo.

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Chap-2A - Chapter 2 Structure and Properties of Organic...

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