Components and Structure of the Atom

Chemistry 142 A

James B. Callis, Instructor

Winter Quarter, 2006

Lecture #5

Neon SignsLow 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/rutherford/

Deflection -> mass/charge

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-

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.

Moving electron cloudsurrounding nucleus.Almost all the mass in the nucleus.Radius of Atom = ~10-8 cm

Diameter of Nucleus = ~10-13 cm

__________________

<- D = 2 x 10-8 cm ->

Diameter = 10-13 cm

Atomic Definitions I: Symbols, Isotopes,Numbers

XA

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

The Nuclear Symbol of the Atom, or Isotope

Neutral ATOMS

• 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 (

)

If neutral, then number of electrons = number of protons.

Numbers of each particle:

For each of the following ions, indicate the total number of protons and electrons and neutrons in the ion.

(a) 127I-

(c) 27Al+3