Chemistry Notes The Quantum Theory The Bohr Model

Chemistry Notes The Quantum Theory

The Bohr Model l Recall: Bohr’s model describes the atom as having definite energy levels l l e- will absorb a “quantized” amount of energy, which excites the e-, when e- returns to its ground state it always emits light at specific wavelengths These wavelengths correspond to definite changes in the electrons energy l l Bohr used Max Plank’s idea: E = hc/λ Bohr’s model refers to quanta as packets of energy (or particles)

From Bohr to the Quantum Model l Bohr’s model had issues considers electrons to have both a known radius and orbit l It makes poor predictions regarding the spectra of larger atoms. l It does not predict the relative intensities of spectral lines. l The Bohr Model does not explain fine structure in spectral lines. l

From Bohr to the Quantum Model l Heisenberg Uncertainty Principle: it is impossible to know the exact position and the exact momentum of an electron at the same time. (This challenged Bohr’s Model) In order to know where an electron is we have to “see” it. To see something it must be hit by a photon. Photons and electrons are roughly the same size, when they collide the electron will no longer be in the same position. l

From Bohr to the Quantum Model l Photoelectric Effect showed that r frequency high light can eject electrons. l i. e. – Light can act as a particle See Physics notes l De Broglie – can quanta (particles) have wave properties? Can particles act like waves? l Wave-particle duality

From Bohr to the Quantum Model l Erwin Schrodinger investigated the wave nature of the electron l Developed a mathematical equation that related the electron's amplitude to any point in space around the nucleus

Schrodinger's Equation • Ψ is the wave function or probable position of the electron. • m is the mass of the electron. • E is the total energy of the system. • V is the potential energy and is a function of x, y and z. The probability of finding the electron decreases as you move away from the center of the nucleus.

Schrodinger's Equation l The solutions of the Schrodinger equation can tell us things about the electron. l The size of electron cloud l The energy of the electron cloud l The shape of the cloud

The Quantum Model l Summary: Quantum model seeks to describe the behavior of subatomic particles in terms of waves l Electrons have properties of both waves and particles

The Quantum Model Bohr’s model showed definite positioning of electrons (they were in orbits). l The Quantum model is primarily a mathematical one that shows the probable location of an electron l Notice: the circles denote the probability of finding an electron in that region of space.

The Quantum Model l A fuzzy cloud represents the probability of finding an electron within a certain volume of space. This Fuzzy Cloud is called an orbital Denser cloud = higher probability of finding an electron Less dense cloud = lower probability of finding an electron l l l

The Quantum Model Orbits (Bohr’s Model) vs. Orbitals (Quantum Model) (PATHS e- travel in (Regions of space where e- may be)

The Quantum Model l Letters that denote atomic orbitals in sublevels: s – spherical shaped l p – dumbbell shaped (3 orbitals) l d – four of the five kinds of d orbitals are clover shaped. (5 orbitals) l f – very oddly shaped (7 orbitals) l l The maximum number of electrons in each orbital is 2.

AGAIN!!! The Quantum Theory is not trying to show what the atom looks like. l The QT is a mathematical model l l The QT is trying to show us the probable location of electrons in an atom l The orbitals represent the probability of finding an electron

Describing the Distribution of Electrons Quantum Numbers: like a zip code for electrons n (principle quantum number)– Indicates principle energy level l l Describes relative size of the electron cloud Corresponds to the energy level 1, 2, 3, …n

Describing the Distribution of Electrons l (second quantum number) – Indicates sublevel l Describes shape of cloud l s: l = 0; p: l = 1; d: l = 2; f: l = 3

Describing the Distribution of Electrons m (third quantum number) – Indicates orientation in space of the cloud l For the p orbitals the values range from – 1 to 1 l l For the d orbitals the values range from – 2 to 2 l l px (-1) / py (O) / pz (+1) dx 2 y 2 (+2) / dz 2 (+1) / dxy (O) / dxz (-1) / dyz (-2) For the f orbitals the values range from – 3 to 3 l Don’t worry about it.

Describing the Distribution of Electrons s (fourth quantum number) - distinguish between electrons in same orbital. l +1/2 denotes a clockwise spin l -1/2 denotes a counter clockwise spin. l The Pauli exclusion principal says: electrons in the same orbital have opposite spins. (See Fig. 5. 12 on p. 123 in book)

Describing the Distribution of Electrons What do the following numbers tell us: 4, 1, -1, +1/2 Electron is in the 4 th PEL l Electron is in the p sublevel l Electron is oriented in the orbital along xaxis l Electron has a clockwise spin l

Describing the Distribution of Electrons l Quantum numbers are like zip codes l. N United States / PEL l L Michigan / sublevel l M Bridgman / orbital l S facing north / spin See table 5. 1 in book l Please read pages 109 -125 l

The Quantum Theory Summary l l Explain how the quantum model is part of the on going development of the atomic model. State the Heisenburg Uncertainty Principle and explain how it influenced the modern view of the atom. Explain how the Quantum Model is different than other views of the atom. Use quantum numbers to locate the approximate location of electrons.