Lecture #5
Components and Structure
of the Atom
Chemistry 142 A
James B. Callis, Instructor
Winter Quarter, 2006
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.
Movies to Illustrate the Nature
of Atoms
• Primitive Cathode Ray Tube:
http://micro.magnet.fsu.edu/electromag/java/crookestube/
• Millikan Oil Droplet Experiment:
http://chemistry.umeche.maine.edu/~fgamar/Millikan.html
• Rutherford Scattering Experiment:
http://www.howstuffworks.com/framed.htm?parent=atom.
htm&url=http://micro.magnet.fsu.edu/electromag/java/ruth
erford/
Deflection -> mass/charge
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-
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.
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! ”
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.
Notes: mass of e- tiny relative to p+, n.
p+, n have same mass (almost).
e-, p+ have same charge, opposite sign.
<- 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 = 10-13 cm
Atomic Definitions I: Symbols, Isotopes,Numbers
A
X
Z
The Nuclear Symbol of the Atom, or Isotope
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
Neutral ATOMS
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( )
For each of the following ions, indicate the total
number of protons and electrons and neutrons in
the ion.
(a) 127I-
(c) 27Al+3