Quantum Mechanics

Electromagnetic Spectrum

Visible Spectrum

Line Spectrum Each element has its own unique line spectrum Each line is caused by an electron dropping from one energy level to another The color (frequency, wavelength, energy) of each line in the spectrum is determined by the difference in energy level

Visible Spectrum

Excitation ElectronOrbits.html ElectronOrbits.html

Bohr Model Electrons move around the nucleus in only certain allowed circular orbits Each orbit has a certain quantum number (energy level) associated with it.

Energy Levels Bohr’s calculate values for the energy levels of Hydrogen with experiments He found that when the electron absorbed energy and released energy, colour was emitted.

Excitation ElectronOrbits.html ElectronOrbits.html

Line Spectrum

However... Bohr’s explanation only worked for the Hydrogen atom Mercury has more electrons than Hydrogen and when electrons move in the Mercury atom there are more colours emitted than the ones observed from the Hydrogen experiment

So... Scientists, knew that there must be more to the atomic structure then what Bohr predicted Schrödinger developed a new theory where the electrons are not in orbits but in probability regions known as orbital

Orbital An orbital is a mathematical function describing the standing wave that gives the probability of the electron manifesting itself at any given location in space. What?!?! Basically an orbital is the region of space occupied by an electron

So what’s the difference? What is the difference between an “Orbit” and an “Orbital”? Well, things in an orbit follow an absolute path. To plot a path for something you need to know exactly where the object is and be able to work out exactly where it's going to be an instant later. You can't do this for electrons.

Heisenberg Uncertainty Principle The Heisenberg Uncertainty Principle says - loosely - that you can't know with certainty both where an electron is and where it's going next. (What it actually says is that it is impossible to define with absolute precision, at the same time, both the position and the momentum of an electron.)

Quantum Mechanical Model of the Atom

Homework Read pages Bohr’s Model of the Atom – The Quantum Mechanical Model of the Atom Answer questions # 4-7 on page 130 based on class and readings

Review Last class we learned that when electrons move from different energy levels they gain or release energy. This release of energy is emitted as colour. Different colours are emitted depending on how much energy is emitted.

More Review We also talked about how Bohr’s Model of the Atom was not entirely true. We talked about the Uncertainty Principle which caused scientists to look for other answers. We found that electrons are NOT in an orbit around the nucleus, but rather that they are in an “orbital”

Quantum Mechanical Model of the Atom

Orbitals... Each kind of orbital is characterized by a set of quantum numbers n, l, and m. These relate, respectively, to the average distance of the electron from the nucleus, to the shape of the orbital, and to its orientation in space.

Principal Quantum Number (n) n represents the energy shell (energy level) i.e. whether it is the first, second, third etc. shell away from the nucleus Possible values for n are positive integers starting at 1. (1,2,3,4,5,6....) Think back to our Bohr model

Orbital-Shape Quantum Number ( l ) l (L) represents the type of orbital within a particular shell i.e. s,p,d and f orbitals. We know how there are different levels of orbitals, but how are there different types of orbitals? Well, there are different shapes!

Orbital-Shape Quantum Number Positive integer ranging from 0 onward. (0,1,2,3,4,5,...) Corresponds to the principal quantum level (n). Has a maximum value of (n-1) ex: if n=4 there are 4 possible sublevels l =(4-1)=3 (all possible values range from 0) l =0,1,2,3 (total of four)

s, p, d, f,...what? Remember we said “Each kind of orbital is characterized by a set of quantum numbers n, l, and m.”? Well where did this s, p, d, and f come from?! s, p, d and f represent different shapes of sublevels.

Sublevel Shapes We now know that an energy level can have sublevels. We also know that these sublevels are different shapes and have the letters s, p, d and f. But how so we know which is what? l =0=s l =1=p l =2=d l =3=f

Example To find the name of the sublevel we combine the value of n and l. If n=3 then l could equal 0,1,2 If we are looking to name n=3, l =0 we would have the sublevel 3s If we are looking for n=2, l =1 we would have 2p.

Magnetic Quantum Number (m l ) m l represents the third quantum number. m l is an integer ranging from – l to + l. This number describes the orientation of the sublevel around the nucleus. For the sublevel of s ( l =0) there is only a m l value of 0.

Sample Problems If n=3 what are the possible values for l and m l n=3 so l = 0 to (n-1) = 0 to (3-1) = 0 to 2 Therefore l = 0, 1, 2 m l = -l to +l so when: – l = 0, m l = 0 – l =1, m l = -1, 0, +1 – l = 2, m l = -2, -1, 0, +1, +2

We now know what the quantum numbers represent so let’s discuss the shapes of the orbitals. We know that the shapes are represented by s, p, d and f. But what do they look like?

s Orbitals s orbitals are spherically symmetric around the nucleus - in each case, like a hollow ball made of rather chunky material with the nucleus at its centre. Looks like a ball around the nucleus.

s Orbitals

p Orbitals Unlike an s orbital, a p orbital points in a particular direction- for example up and down. A p orbital is rather like 2 identical balloons tied together at the nucleus

p Orbitals

d orbitals Similar to 2 p orbitals combined.

d Orbitals

Let’s try to draw the orbitals

Homework take up Take up homework from last class Home work for tomorrow: – Study for quiz – Questions on Atomic mass, number – Element symbols and the periodic table (1-39) – Bohr model, Lewis structures – Covalent and ionic bonds

Tomorrow Quiz Read pages Questions: Next week we will finish chapter 3 and then have a quiz on the Quantum Mechanical Theory before moving on to our next unit: Aqueous Solutions.