Components and Structure of the Atom

1. Lecture #5 Components and Structure of the Atom Chemistry 142 A James B. Callis, Instructor Winter Quarter, 2006

2. NeonSignsLow 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: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/rutherford/

4. Deflection-> mass/charge

6. Millikan’s Experiment1. 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. <- 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

13. 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 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 ( )

18. For each of the following ions, indicate the total number of protons and electrons and neutrons in the ion. • 127I- (c)27Al+3