1. Components and Structure of the Atom
Lecture #5
Components and Structure of the Atom
Chemistry 142 B
James B. Callis, Instructor
Autumn Quarter, 2004

2. Neon Signs Low pressure neon gas in evacuated tube, between electrodes
Neon Signs Low pressure neon gas in evacuated tube, between electrodes. High voltage separates +, - charges in Ne atoms: + particle goes to - electrode. - particle goes to + electrode. Measure current between electrodes: Shows atoms made of +, - charges.

3. Movies to Illustrate the Nature of Atoms
Primitive Cathode Ray Tube:
Millikan Oil Droplet Experiment:
Rutherford Scattering Experiment:

4. Deflection -> mass/charge

6. Millikan’s Experiment 1
Millikan’s Experiment 1. Measured rate of droplet’s fall without voltage: gave its mass. 2. Voltage across plates influenced speed, due to charge of droplet. 3. Quantitative effect of voltage w/ laws of physics -> amt. of charge on droplet. 4. RESULT: Different droplets had different charge, but always a multiple of same number -> elementary charge on electron: e = 1.602x10-19 coulombs (negative). 5. (Mass/charge) x e- = mass of e-

8. Rutherford Experiment
Alpha (i.e., subatomic) particles bombarding the atom.
Rationale - to study the internal structure of the atom, and to know more about the mass distribution in the atom.
Bombarded a thin Gold foil with alpha particles from radium.

9. Ernest Rutherford (1871-1937) Won the Nobel Prize in Chemistry in 1908
“It was quite the most incredible event..... It was almost as if a gunner were to fire a shell at a piece of tissue and the shell bounced right back! ”

10. The Neutron
Protons cannot be the only particle in the nucleus because the mass of the protons in is less than the mass of the nucleus.
Therefore, a third, neutrally charged particle must exist!
James Chadwick discovered the missing subatomic particle, the neutron. He bombarded beryllium foil with alpha particles and noticed a neutral stream of particles coming out. The particles have about the same mass as a proton.

11. Notes: mass of e- tiny relative to p+, n.
p+, n have same mass (almost).
e-, p+ have same charge, opposite sign.

12. Moving electron cloud surrounding nucleus. Almost all the mass in
<- D = 2 x 10-8 cm ->
Moving electron cloud
surrounding nucleus.
Almost all the mass in
the nucleus.
Radius of Atom
= ~10-8 cm
Diameter of Nucleus
= ~10-13 cm
__________________
Diameter = cm

13. X = Atomic symbol of the element, or element symbol
Atomic Definitions I: Symbols, Isotopes,Numbers A X
The Nuclear Symbol of the Atom, or Isotope Z
X = Atomic symbol of the element, or element symbol
A = The Mass number; A = Z + N
Z = The Atomic Number, the Number of Protons in the Nucleus
(All atoms of the same element have the same no. of protons.)
N = The Number of Neutrons in the Nucleus
Isotopes = atoms of an element with the same number of protons,
but different numbers of Neutrons in the Nucleus

15. Neutral ATOMS Numbers of each particle:
If neutral, then number of electrons = number of protons.
Numbers of each particle:
51 Cr = p+ ( ), e- ( ), n ( )
239 Pu = p+( ), e-( ), n ( )
15 N = p+( ), e-( ), n( )
56 Fe = p+( ), e-( ), n ( )
235 U =p+( ), e-( ), n ( )