Chapter 5 The Structure of Atoms

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Fundamental Particles

  • Three fundamental particles make up atoms:

The Discovery of Electrons

  • Late 1800’s & early 1900’s

  • Cathode ray tube experiments showed that very small negatively charged particles are emitted by the cathode material.

The Discovery of Electrons

Rutherford’s Atom

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

The Discovery of Protons

The Discovery of Neutrons

Mass Number and Atomic Number

  • Mass number – A

  • Atomic number – Z

    • Z = # protons
    • A = # protons + # neutrons
    • # protons = # electrons


  • Atoms of the same element but with different masses

Isotopes: Example

Experimental Detection of Isotopes

  • Factors which determine a particle’s path in the mass spectrometer:

    • accelerating voltage, V
    • magnetic field strength, H
    • mass of the particle, m
    • charge on the particle, q

Mass Spectrometry & Isotopes

  • Mass spectrum of Ne+ ions

    • This is how scientists determine the masses and abundances of the isotopes of an element

Mass Spectrometry & Isotopes

  • Let’s calculate the atomic mass of Ne using the mass-spectrometry data

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

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

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

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)

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

Electromagnetic Radiation

  • Electromagnetic radiation can also be described in terms of “particles” called photons

  • Each photon is a particular amount of energy carried by the wave

  • Planck’s equation relates the energy of the photon to the frequency of radiation:

    • E = h
    • (h is a Planck’s constant, 6.626·10-34 J·s)

Electromagnetic Radiation

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

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