1 Mr. ShieldsRegents Chemistry U06 L03

2 Bohr Model e - transitions from a higher energy levels to lower energy levels release energy in the form of photons. Bohr’s model correctly predicted hydrogen’s 4 visible Lines in the emission spectra but incorrectly predicted the emission spectra for all other atoms.

3 Bohr Model Niels Bohr Obviously the Bohr model didn’t Accurately describe the atom. In the 1920’s a new model of the Atom began to emerge. It was known as the Quantum Mechanical model. Let’s look at some of the key concepts That lead to this model

4 The electron as a Wave Louis Victor de Broglie – As a graduate student de Broglie began considering Bohr’s quantized atom. If light could behave as a particle Then why, de Broglie wondered, couldn’t particles (like electrons) behave as waves?

5 The wave nature of an electron De Broglie developed his idea and established a Mathematical relationship that related wavelength To mass & velocity of a moving object. h = x j sec In other words, Anything that moves behaves as a wave!

6 Heisenberg In 1913 Bohr showed how electrons were quantized In 1924 De Broglie showed how an electron could behave as a wave. In 1925 Werner Heisenberg established what was known as matrix mechanics to explain atomic behavior Werner Heisenberg

7 Schrodinger In 1926 Erwin Schrodinger proposed a Wave Mechanics model of the atom. The matrix and wave quantum model of the atom divided scientists into two camps. Erwin Schrodinger

8 The debate In 1926 Werner Heisenberg began his job as Assistant to Niels Bohr in Copenhagen. Later that year Schrodinger came To debate the two alternative theories with Bohr. Neither model was satisfactory but Schrodinger showed the equivalence of the matrix and wave versions of Quantum Mechanics. After Bohr presented a statistical interpretation of the wave function, these theories formed the basis of what is now regarded as quantum mechanics.

9 Uncertainty Principle Heisenberg proposed what he is most well Uncertainty Principle Known for … The Uncertainty Principle “It is impossible to simultaneously know both the Position and Velocity (a vector) of a particle at the Same time.” A photon has about the same energy as an electron. A photon striking an electron causes the electron To change both position and velocity. So an electrons Position can not be specified with precision.

10 Uncertainty principle The consequence is that we can only give a Probability of finding an electron in a given location. Is the electron * here Ans: yes to all but each has a different probability * Or here? * Some location probabilities are higher than others.

11 The Quantum Mechanical model Electrons behave as a wave or a particle with specific allowable energy values – they are quantized. Electrons behave as a wave or a particle with specific allowable energy values – they are quantized. Electrons are located in “probability regions” of space Electrons are located in “probability regions” of space These are known as atomic orbitals. They are not the same as Bohr’s circular orbits! These are known as atomic orbitals. They are not the same as Bohr’s circular orbits! An Orbital is defined as a region in space in which there is a 90% probability of finding the electron An Orbital is defined as a region in space in which there is a 90% probability of finding the electron

12 Hydrogen’s 1 st orbital; electron probability plot

13 The Quantum Mechanical model Just like the Bohr Model there are Principal quantum numbers (n) Just like the Bohr Model there are Principal quantum numbers (n) n indicates distance from the nucleus n indicates distance from the nucleus As n increase distance from the nucleus increases As n increase distance from the nucleus increases n Specifes major energy levels called the Principal Energy Levels n Specifes major energy levels called the Principal Energy Levels There are currently 7 principal energy levels There are currently 7 principal energy levels Principal energy levels contain Energy Sublevels Principal energy levels contain Energy Sublevels

14 Energy Sublevels Each Principal Energy level contains 1 more sublevels than the last principal energy level, beginning with 1 sublevel in the 1 st Principal Energy Level Sublevels are labeled s, p, d, and f (there are no others) So…number of nsublevelsSublevel 111s 222s 2p 333s 3p 3d 444s 4p 4d 4f 545s 5p 5d 5f 64 6s 6p 6d 6f 7 4 7s 7p 7d 7f

15 Energy Sublevels Furthermore, within each energy sublevel there is anywhere from 1 to 7 orbitals Sublevel# of Orbitals s1 s orbital p3 p orbitals d5 d orbitals f7 f orbitals

16 Energy Sublevels Electrons occupy the energy sublevels. These energy sublevels refer to the 3 dimensional regions in space called atomic Orbitals. Remember: orbitals are defined as the region In space in which there is a 90% probability of finding the electron. So what do these orbitals look like?

17 The “s” orbital 90% probability Inside circle All “s” orbitals are spherical

18 The s orbitals of the 1 st three Energy Levels n=1n=2 n=3 2s Note the growth in size as n increases

19 The 3 “p” orbitals of the 2 nd Principal Energy Level The “Dumbbell” shaped p Orbitals

20 So what do these 1s 2s and 2p atomic orbitals look like as they surround the nucleus ? The number of orbitals in a given principle energy level (n) is equal to n 2 Remember “2n 2 “ for calculating the # of e - in each Bohr orbit ? Well 2 is the max # of e - allowed in each orbital hence 2n 2 = max # of e - per principle quantum number Calculating # of Orbitals

21 Neon with fully occupied 1s, 2s, 2p orbitals

22 All the orbitals of the s, p, d, and f sublevels Only need to know these

23 Orbital Energy As the principal quantum number (n) increases, in Other words as we move further from the nucleus, The energy of the electrons in those principal Energy levels increase; Energy: 1 < 2 < 3 < 4 < 5 < 6 < 7 Also, within any principal energy level the energy of The sublevels increase from s to f; Energy: s < p < d < f

24 Electron Orbital filling This increasing energy sequence defines into which Orbitals Electrons go as they are added to the atom. For example lets look at Rubidium …

25 3d > 4s 4d > 5s 4f > 5s,5p,6s Sublevel Energy increases From s to f. Some sublevels With a lower n may Actually be at a Higher energy Than some e - in a higher n ! We’ll look at this “SWITCHING” Again later