From the Bohr Model to the Quantum Mechanical Model

1. From the Bohr Model to the Quantum Mechanical Model Advanced Chemistry Ms. Grobsky

2. Shortcomings of the Bohr Model • Bohr’s model was too simple • Worked well with only hydrogen because H only has one electron • Could only approximate spectra of other elements with more than one electron • Electrons do not move in circular orbits • So there is more to the atomic puzzle…

3. de Broglie and the Dual Nature of Light • As a result of Planck’s and Einstein’s work, light was found to have certain characteristics of particulate matter • No longer purely wavelike • Waveicle • But is the opposite also true? • A physicist named de Broglie asked the question: Does matter exhibit wave properties? • The answer is Yes! • As shown through X-ray diffraction patterns

4. Quantum Mechanical Model of the Atom • Things that are very small behave differently from things big enough to see • Come to find out, electrons bound to the nucleus are similar to standing waves • Standing waves do not propagate through space • Standing waves are fixed at both ends • Think of a guitar or violin • A string is attached to both ends and vibrates to produce a musical tone • Waves are “standing” because they are stationary – the wave does not travel along the length of the string Standing Wave Video

5. The Quantum Mechanical Model • Remember, energy is quantized (i.e. it comes in chunks) • Since the energy of an atom is never “in between”, there must be a quantum leap in energy • Also, a physicist named Heisenberg stated that “[t]here is a fundamental limitation on how precisely we can know both the position and momentum of a particle at a given time” • It is impossible to know both the velocity and location of an electron at the same time

6. The Quantum Mechanical Model • Erwin Schrodinger derived a mathematical equation that described the energy and position of electrons in an atom that became known as the “Quantum Mechanical Model” • An overview of the model: • Electrons are found in energy levels and ORBITALS

7. Atomic Orbitals • Within each energy level, the complex math of Schrodinger’s equation describes several shapes • These are called atomic orbitals • Orbitals are NOT circular orbits for electrons • Orbitals ARE areas of probability for locating electrons • Electron density maps (probability distribution) indicates the most probable distance from the nucleus • These DO NOT describe: • How an electron arrived at its location • Where the electron will go next • When the electron will be in a particular location • Orbitals of the same shape grow larger as number of energy levels increases • # of nodes (areas in which there is zero electron probability) increase as well

8. Electron Density Maps??? WHAT????? • With a partner, complete page 204 - “Locating an ‘s’ Electron in an Atom by Analogy” • Be sure the marbles are caught after their first bounce • Be sure marbles are dropped from a consistent height • Change in procedure and data analysis • No carbon paper – you must mark each landing spot with an “X” using a pencil! • Y-axis in graph is (# of dots/cm2)*10 • Before you begin, do you expect each group to get the same pattern? • Do you expect a marble to land exactly in the middle?

9. Sample Marble Drop Probability

10. So What do These Probability Distributions (Atomic Orbitals) Look Like?

11. Atomic Orbitals Simulation • Follow along on page 205! Atomic Orbitals Java Applet

12. The s-Orbital • Spherical shape • Single orbital • Seen in all energy levels • Can hold up to 2 electrons

13. The p-Orbital p (x) y-axis z-axis p (y) x-axis p (z) • Dumbbell-shaped • Seen in 2nd energy level and above • Can hold up to 2 electrons PER SUBORBITAL (6 electrons total)

14. The d-Orbital • Five clover-shaped orbitals • Can hold up to 2 electrons per suborbital (10 electrons total) • Seen in 3rdenergy level and above

15. The f-Orbital • Seven equal energy orbitals • Each suborbital can hold up to 2 electrons (14 electrons total) • Shape is not well-defined • Seen in 4th energy level and above

16. Summary of Atomic Orbital Shapes

17. Arrangement of Atomic Orbitals • The orbitals of an atom are LAYERED!