1. NOTES: 5.1 – Revising the Atomic Model

2. Atomic Models ~ REVIEW!
● ATOM: the smallest particle that has the properties of an element.
● From the early Greek concept of the atom to the modern atomic theory, scientists have built on and modified existing models of the atom.

3. ATOM BASICS
● Atoms are composed of a positively charged nucleus surrounded by an electron cloud.
-NUCLEUS: (99% of atom’s mass) uncharged neutrons and positively charged protons.
-ELECTRON CLOUD: negatively charged electrons in constant motion creating a “cloud” like a fan.

4. Electron arrangements…
● the chemical properties of atoms, ions, and molecules are related to the arrangement of the electrons within them
● models of the atom will be reviewed with an emphasis on the electrons

5. JOHN DALTON: Dalton’s Atomic Theory:
In 1808, this English schoolteacher proposed his own atomic theory that became widely accepted.
Dalton’s Atomic Theory:
-All elements are made of tiny atoms.
-Atoms cannot be subdivided.
-Atoms of the same element are exactly alike.
-Atoms of different elements can join to form molecules.

6. Millikan Thomson Ernest Rutherford
● As it turns out, the atom can be divided into subatomic particles.
● Thomson and Millikan are given credit for the first discoveries relating to electrons.
● Rutherford discovered the positively charged nucleus
Millikan
Thomson
Ernest Rutherford

7. 3 types of particles that make up atoms:
Particles: Mass (amu): Charge:
PROTON (pos.)
NEUTRON (neutral)
ELECTRON (neg.)

8. J.J. Thomson (1856-1940) ● proposed the “plum-pudding” model:
-neg. charged electrons are dispersed throughout a lump of pos. charged material (like raisins stuck in dough)

9. How is the atom structured?
● each atom has a tiny, positively charged, massive (very dense) center called an ATOMIC NUCLEUS.
● The area around the nucleus is mostly empty space.
● How do we know?

10. Rutherford’s “Scattering” Experiment:
-positively charged alpha particles (helium nuclei) were shot through a thin gold foil.
-most alpha particles passed through the foil, or were deflected through moderate angles.
-a few were reflected at extreme angles, or even came shooting right back to the source!

11. Rutherford’s Scattering Experiment

12. Rutherford’s Scattering Experiment

13. Rutherford’s Scattering Experiment

14. NIELS BOHR
● In 1913, this Danish scientist suggested that electrons “orbit” the nucleus.
● In Bohr’s model, electrons are placed in different energy levels based on their distance from the nucleus. (lowest energy level is closest to nucleus)

15. Summary of atomic structure:

16. Bohr’s Atomic Model
● What prevents an electron from falling into the nucleus?
● The electrons in a particular path have a fixed energy level
● The energy levels are like rungs of a ladder:
-lowest rung of ladder = lowest energy level
-one can climb up or down a ladder by going from rung to rung = an electron can “jump” from one energy level to another (by gaining or losing energy)

18. ● The Bohr Atom was a “solar system” or “planetary” model.

19. Bohr’s Atomic Model
● In the process of changing energy levels the electron gains or loses energy by absorbing or emitting a quantum of electromagnetic energy.
● the higher an electron is on the energy “ladder”, the farther it is from the nucleus
● in the atom, the steps become closer together as an electron climbs higher
● similarly, the higher energy level occupied by an electron, the easier the electron escapes from the atom…WHY?

20. An electron jumping down gives off energy
Only certain orbits are allowed:
An electron jumping down gives off energy

21. An electron jumping up absorbs energy
Only certain orbits are allowed:
An electron jumping up absorbs energy

23. Orbits – Energy Levels
n = 5
n = 4
n = 3
n = 2
n = 1

24. Orbits – Energy Levels
n = 5
n = 4
n = 3
n = 2
n = 1

26. Bohr’s Atomic Model Defects: 1) Contradicted known physics.
2) Couldn’t explain various intensities of the line spectra.
3) Couldn’t explain WHY only certain orbits were allowed.
4) Only worked for hydrogen.

27. In review… History of the Atomic Model VIDEO
Bohr Model of the Atom VIDEO

28. Physicists were mystified, but intrigued by Bohr’s theory of the atom.
Why are the energies of the hydrogen electron quantized?
Why is the electron restricted to orbiting at certain fixed distances?

29. This is very difficult stuff!
Erwin Schrodinger ( )
This is very difficult stuff!
In 1926 Schrodinger proposed an equation describing the location and energy of an electron in a hydrogen atom.

30. Quantum Mechanical Model:
Solving the Schrodinger Equation gives the probability of finding the electron at a given place around the nucleus.
The solution also gives rise to energy level sub levels.

31. Quantum Mechanical Model:
● The quantum mechanical model of the atom predicts energy levels for electrons; it is concerned with the probability, or likelihood of finding an electron in a certain location.
● In quantum theory, each electron is assigned a set of 4 quantum numbers
**analogy: like the mailing address of an electron!

32. Quantum Mechanical Model:
● The first number is called the principal quantum number (n). It describes the main energy level the electron is on.
● The second quantum number (l) describes the sub level, or orbital type, the electron is in (s, p, d, f)

33. Playing Chopin with Boxing Gloves
“Trying to capture the physicists’ precise mathematical description of the quantum world with our crude words and mental images is like playing Chopin with a boxing glove on one hand and a catcher’s mitt on the other.”
(1996). Johnson, George. On skinning Schrodinger’s Cat. New York Times.

34. Electron Configurations
● Regions where electrons are likely to be found are called orbitals. EACH ORBITAL CAN HOLD UP TO 2 ELECTRONS!

35. S Orbitals

39. Energy level Sublevel Orbitals
n=1 1s 1 (1s)
n=2 2s 1 (2s)
2p 3 (2p)
n=3 3s 1 (3s)
3p 3 (3p)
3d 5 (3d)

40. Sublevel # Orbitals Max # elec.p d f

41. Quantum Mechanical Model!
VIDEO

44. Electron Configuration!
VIDEO