Atomic Theory

  • The Atom
  • Mass Spectrometer
  • Electron Arrangement
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  • Created by: Priscilla
  • Created on: 23-04-11 11:36

The Atom

An atom is made out of:

  • Protons; mass = 1; charge - +1
  • Neutrons; mass = 2; charge - 0
  • Electrons; mass = 1/1840; charge - -1

ISOTOPES - are atoms with the same atomic number but different mass number, this is because there is a different amount of neutrons.

Isotope may differ in physical properties (mass, density, rate of diffusion etc) and radioactivity but not generally in chemical properties.

Radioisotopes are radioactive isotopes. they are used for: 

  • Carbon-14 in radiocarbon dating, 
  • Co-60 in radiotherapy
  • Iodine-131 and Iodine-125 as medical tracers.

RELATIVE ATOMIC MASS: the average of the atomic masses of each isotope multiplied by the isotopes abundance.

RAM = [(mass of Isotope 1 x abundance of Isotope 1) + (mass of Isotope 2 x abundance of isotope 2)] / 100

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Mass Spectrometer

This is used to measure the exact mass of particles.

It can be used on elements to determine the isotopic abundances and to find out the nature of the molecule by looking at the fragmentation pattern of its destructive ionisation.

There are several stages of the operation:

  • Injection of the sample
  • Vaporisation of the sample - this is done by heating the particles.
  • Ionisation of the sample - this is done by bombarding them with fast moving electrons that are accelerated between the two plates ie X + e- => X+ + 2e-
  • Acceleration of the ions - the positive ions are accelerated by the high electrical potential difference between the 2 parallel electrodes.
  • Deflection - the fast moving ions enter a magnetic field produced by an electromagnet, this causes deflection - those with a smaller mass will be be deflected more than those with a greater mass.this means that only particles with a certain mass will be detected.
  • Detection of the ions.

The greater the number of particles of a given mass, the greater the current.

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Electron Arrangement

There are many Spectral Types:

  • Continuous Emission -
  • Continuous Absorption
  • Line Emission
  • Line Absorption

Continuous spectra - shows broad bands of electromagnetic radiation (emission) or shadow (absorption)

A line spectrum contains only some discrete lines of electromagnetic radiation (emission), or shadows (blalck lines superimposed on a continuous spectrum).

Line emission spectra which are produced by excited atoms (heated) promoting electrons to higher energy levels which then emit discrete frequencies of energy when they return to their lower states.

The main electron levels go: 2, 8, 18 if Z = 20

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