The Structure of Atoms

1
CHAPTER 5

• The Structure of Atoms

2
Fundamental Particles

• Three fundamental particles make up atoms

3
The Discovery of Electrons

• Late 1800s early 1900s
• Cathode ray tube experiments showed that very
  small negatively charged particles are emitted by
  the cathode material.

• 1897 J. J. Thomson
• Modified the cathode ray tube and measured the
  charge to mass ratio of these particles. He
  called them electrons.
• (Nobel prize in physics, 1906)

4
The Discovery of Electrons

• 1909 Robert A. Millikan
• Determined the charge and the mass of the
  electron from the oil drop experiment.
• (The second American to win Nobel prize in
  physics in 1923)

• 1910 Ernest Rutherford
• Gave the first basically correct picture of the
  atoms structure.
• (Nobel prize in chemistry in 1908)

5
Rutherfords Atom

• The atom is mostly empty space
• It contains a very small, dense center called the
  nucleus
• Nearly all of the atoms mass is in the nucleus
• The nuclear diameter is 1/10,000 to 1/100,000
  times less than atoms radius

6
The Discovery of Protons

• 1913 H.G.J. Moseley
• Realized that the atomic number defines
  the element
• Each element differs from the preceding element
  by having one more positive charge in its nucleus
• Along with a number of observations made by
  Rutherford and some other physicists, this led to
  the discovery of the proton
• The elements differ from each other by the number
  of protons in the nucleus

7
The Discovery of Neutrons

• 1932 James Chadwick
• recognized existence of massive neutral
  particles which he called neutrons
• (Nobel prize in physics in 1935)
• The atomic mass of an element is mainly
  determined by the total number of protons and
  neutrons in the nucleus
• The atomic number of an element is determined by
  the total number of protons in the nucleus

8
Mass Number and Atomic Number

• Mass number A
• Atomic number Z
• Z protons
• A protons neutrons
• protons electrons

• The way we typically write this

full nuclide symbol
short nuclide symbol
9
Isotopes

• Atoms of the same element but with different
  masses
• The same element means that the number of protons
  is the same,
• then different masses mean that the number of
  neutrons differs

protium (or hydrogen)
deuterium
tritium
10
Isotopes Example
11
Experimental Detection of Isotopes

• 1919-1920 Francis Aston
• Designed the first mass-spectrometer
• (Nobel prize in chemistry in 1922)

• Factors which determine a particles path in the
  mass spectrometer
• accelerating voltage, V
• magnetic field strength, H
• mass of the particle, m
• charge on the particle, q

12
Mass Spectrometry Isotopes

• Mass spectrum of Ne ions
• This is how scientists determine the masses and
  abundances of the isotopes of an element

13
Mass Spectrometry Isotopes

• Lets calculate the atomic mass of Ne using the
  mass-spectrometry data

14
Atomic Weight Scale

• A unit of atomic mass (atomic mass unit) was
  defined as exactly 1/12 of the mass of a 12C atom
• Two important consequences of such scale choice
• The atomic mass of 12C equals 12 a.m.u.
• 1 a.m.u. is approximately the mass of one atom of
  1H, the lightest isotope of the element with the
  lowest mass.
• The atomic weight of an element is the weighted
  average of the masses of its isotopes

15
Isotopes and Atomic Weight

• Naturally occurring chromium consists of four
  isotopes. It is
• 4.31 50Cr, mass 49.946 amu
• 83.76 52Cr, mass 51.941 amu
• 9.55 53Cr, mass 52.941 amu
• 2.38 54Cr, mass 53.939 amu
• Calculate the atomic weight of chromium

16
Isotopes and Atomic Weight

• Naturally occurring Cu consists of 2 isotopes.
  It is 69.1 63Cu with a mass of 62.9 amu, and
  30.9 65Cu, which has a mass of 64.9 amu.
  Calculate the atomic weight of Cu to one decimal
  place.
• A.W.(Cu) (62.9 amu ? 0.691) ( 64.9 amu ?
  0.309)
• 63.5 amu

17
Electromagnetic Radiation

• Any wave is characterized by 2 parameters
• Wavelength (?) is the distance between two
  identical points of adjacent waves, for example
  between their crests
• It is measured in units of distance (m, cm, Å)
• Frequency (?) is the number of wave crests
  passing a given point per unit time (for example,
  per second)
• It is measured in units of 1/time, usually s-1
• 1 s-1 1 Hz (Hertz)

18
Electromagnetic Radiation

• The speed at which the wave propagates
• c ? ? ?
• The speed of electromagnetic waves in vacuum has
  a constant value
• c 3.00?108 m/s
• This is the speed of light
• Given the frequency of the electromagnetic
  radiation, we can calculate its wavelength, and
  vice versa

19
Electromagnetic Radiation

• Max Planck
• (Nobel prize in physics in 1918)

• Electromagnetic radiation can also be described
  in terms of particles called photons
• Each photon is a particular amount of energy
  carried by the wave
• Plancks equation relates the energy of the
  photon to the frequency of radiation
• E h ? ?
• (h is a Plancks constant, 6.62610-34 Js)

20
Electromagnetic Radiation

• What is the energy of green light of wavelength
  5200 Å?