1. Atomic Structure (History & Background

2. ATOMS MAKE UP ELEMENTS!! ATOMS ARE MADE OF PROTONS, NEUTRONS, & ELECTRONS!! SO…WHERE DID THE IDEA OF ATOMS ORIGINATE????

3. Early Greeks
Democritus, Aristotle first to devise idea of “atoms”

4. Dalton’s Model Noticed compounds always have a fixed composition
Ex: H2O
John Dalton

5. Dalton’s Atomic Theory
Elements are made of atoms that cannot be divided
Atoms of same element have same mass, atoms of different elements have different masses
Compounds contain atoms of more than one element
In compounds, atoms of different elements always combine in the same way

6. Dalton’s Atomic Model
- Solid Sphere cannot be divided
(No protons, neutrons, electrons)

7. JJ Thomson’s Experiments
Cathode ray tube (sealed glass with gas inside, electric current passed through)
Current caused beam to glow

8. Thomson’s Experiments
Placed 2 charged plates (+ and -) on either side of tube
Beam Bent towards (+) plate +
Beam -
Electric current

9. Thomson’s Conclusion
Beam was negative charged particles from inside atoms
First evidence of subatomic particles
Discovered electron

10. Thomson’s “Plum Pudding” Model
Electrons evenly mixed in a sphere of (+) charge

11. Mass of the Electron
The oil drop apparatus
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

12. Conclusions from the Study of the Electron:
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

13. Conclusions from the Study of the Electron:
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

14. Rutherford’s Gold Foil Experiment
Passed (+) charged beam of alpha particles at gold foil
Expected only slight deflection

15. Results showed more deflection than expected

17. Rutherford’s Model
Atoms have a small, dense, positive center: (Nucleus) surrounded by mostly empty space containing electrons
Small dense nucleus
Empty space ( electrons)

18. Bohr Model
Bohr was correct in saying electrons were located in energy levels
He was wrong about electrons moving in predictable orbits around nucleus

19. Electron Cloud Model
Heisenberg uncertainty principle: the exact speed and position of an electron cannot be determined
Can only determine the probability of finding electrons at possible locations outside nucleus

20. Electron Cloud Model Propeller in motion Propeller at rest
(cannot see exact location-
Only blur)
Propeller at rest
(can see exact location)

21. Highest probability of
finding electron (near
the nucleus)

22. Electron cloud
Cloud is denser where probability of finding electron is high
Cloud is less dense where probability of finding an electron is low

23. Orbital: 3-D Region of space around nucleus that indicates probable location of electron.
Each orbital can have maximum of 2 electrons

24. Subatomic Particles Particle Charge Mass (g) Location -1 9.11 x 10-28
Electron
(e-) -1
9.11 x 10-28
Electron cloud
Proton (p+) +1
1.67 x 10-24
Nucleus
Neutron
(no)
Most of the mass of an atom is in the nucleus;
Relative mass (p=1, n=1, e=1/2000)
Most of the volume of an atom is in the e- cloud;