Quantum Theory & Electron clouds

Niels Bohr (Danish) tried to explain the spectrum of hydrogen atoms.

•Energy is transferred in photon units (quanta), therefore specific amounts of energy are absorbed or emitted •Because the energy of an electron is quantized (discreet), there are only certain energy levels (orbits) for electrons•Therefore, this e-m radiation can behave as waves or particles = Wave-Particle Duality Theory•When an electron gains a certain amount of energy (absorbs a certain number of photons) it becomes excited and moves to a higher energy level

The Hydrogen Atom and Quantum Theory

If the electron radiates this energy, it jumps back toward ground (resting) state and emits colored light!!•The specific amounts of energy released when an electron jumps from an excited state to a lower state produce light of certain specific color

The de Broglie Hypothesis

Location __Probability__Friend's house 15%Shopping10%McDonald's 15%Home in bed 30%Cruising15%Work 15%Working in Chem Lab 0%

Basic Premise of Quantum Mechanics = small particles in small regions of space (e-) do NOT behave like large visible objects.•Therefore… we cannot tell exactly where an e- is at any given time or how it got there.•We CAN predict the position of the e-

Hypothetical example for comparison: If I want to find you on a Saturday afternoon…

•Probable places for finding electrons = ORBITALS

•Louis de Broglie (1924) theorized that if light can behave as particles, then particles (e-) could behave as waves!

•Why?? A wave fills the space of an atom better than a particle

•As an electron moves closer to the nucleus, its wavelength shortens, therefore frequency and energy increase - and the atom doesn't collapse

•Wave-Particle Duality Theory: greatly affects small particles whose behavior can be classed as either wave-like or particle-like

•To explain the position of an electron, both behaviors must be considered

•Heisenberg Uncertainty Principle: shows that we can only predict or estimate the position and momentum of the electron•Because we record position by measuring radiant energy from a particle, we can never know the exact position and the exact momentum of an electron!!!

Electrons are so small and so easily affected by energy (of any form) that "lighting up" an electron to see it causes it to change momentum (position and/or direction)

Wave-Mechanical View of the Hydrogen Atom

•When calculating the position of the electron, certain areas are more probable than others

•These areas tend to be close to the nucleus

•These areas are cloud-like with a fairly large space being filled by a few tiny electrons (e.g. fan blades)

•Therefore, the electron cloud takes up most of the space of the atom, while the small, dense nucleus fills the center.

QUANTUM THEORY: shows how the electron determines an atom's behavior and properties

•Schrodinger's Equation - supports the theory that quanta of energy are absorbed and emitted in whole-number units (in simple atoms!!)

•In larger atoms it is assumed that the electrons do not interfere with each other but they probably do (at least a little bit)

•4 Quantum Numbers must be used to describe the position of the electrons in an atom

•n, l, m, s are the "numbers"Each electron has a different set (just like each locker out in the hall has a different combination)

•Identifies the major energy levels that electrons can occupy

•Shows the distance from the nucleus

•Numbered 1,2,3,4,5,6,7,8 but sometimes referred to as K,L,M,N,O,P,Q,R shells

•Equation: 2n2 - shows how many electrons can be in each energy level (e.g. level 3: 2(3)2 = 18 e-'s)

•Principle Quantum Number - n

•Orbital Quantum Number - l•Identifies the shape of the sublevels of the main energy levels•s, p, d, f - used to identify the shape = "sphere, peanut, double peanut, flower"•1st energy level has 1 subshell (s)•2nd energy level has 2 subshells (s,p)•3rd energy level has 3 subshells (s,p,d)•4th energy level has 4 subshells (s,p,d,f)•At higher energies, these orbitals overlap•l can be from 0 to n-1•s=0•p=1•d=2•f=3

Shapes of the Charge Clouds

s for "Sphere": simplest shape, or shape of the simplest atoms like hydrogen and helium

•Electrons don't interfere with, or block, each other from the pull of the nucleus - ball shape

•Each energy level has an "s" orbital at the lowest energy within that level

•p for "Peanut": more complex shape that occurs at energy levels 2 and above

Shapes of the Charge Clouds

d for "Double Peanut": complex shape occurring at energy levels 3 and aboveThe arrangement of these orbitals allows for "s" and "p" orbitals to fit closer to the middle/nucleus

Shapes of the Charge Clouds

f for "Flower": 7 bizarre-shaped orbitals for electrons of very large atoms•electrons filling these orbitals are weakly attached to the atom because they are so far away from the pull of the nucleus

Shapes of the Charge Clouds

•Is indicated by the orientation of these orbitals in each dimension.

Magnetic Quantum Number m

•"p" orbitals line up on the x, y, z, axes in space

•"d" and "f" orbitals can line up on the axes as well as in between them

•Equation: there are n2 orbitals (of various shapes) per energy level

•e.g. level 3 has 32 = 9 orbitals (one s, 3 p's, 5 d's)

•m can be from –l to +l

•s orbital can have only one orientation (subshells)

•m can be 0 only, meaning 1 possible orientation in space

•p orbital can have 3 different orientations (subshells)

•m can be -1, 0, +1, meaning 3 possible orientations in space

•d orbital can have 5 different orientations (subshells)

•m can be -2, -1, 0, +1, +2 meaning 5 possible orientations in space

•f orbital can have 7 different orientations (subshells)

•m can be -3, -2, -1, 0, +1, +2, +3 meaning 7 possible orientations in space

•Spin Quantum Number (s): indicates the clockwise or counterclockwise spin of the electron

•Designated by -1/2 or +1/2

•Needed because NO 2 ELECTRONS CAN HAVE THE SAME SET OF QUANTUM NUMBERS!

•Sometimes called left-handed or right-handed spin

•No two electrons in an atom have the same set of 4 quantum numbers!

•Therefore, only 2 electrons can fit in any one orbital

•This works because spinning electrons act like tiny electromagnets and magnetically attract each other when they have opposite spin

Pauli Exclusion Principle

•Electrons fill the lowest energy levels first (always)•As principle quantum number increases, spacing between the shells decreases•Therefore, the third subshell and beyond start to overlap energy levels

Aufau Principle

Hund's Rule = electrons entering a subshell containing more than one orbital will spread out over the available orbitals with their spins in the same direction until all orbitals have one electron in them

•Three ways to describe the electron structure of atoms in the ground state: orbital notation, electron configuration notation, electron dot notation

Order of Fill for sublevels