Atomic Structure

Atomic Structure

1. Fundamental Particles

   • Atoms are made up of protons, neutrons and electrons
     • The protons and neutrons form the nucleus and the electrons surround this core in shells.
       • The nucleons (protons + neutrons) are held together by a force called strong nuclear attraction.

2. Protons, Neutrons and Electrons

   • There were various models that were used to illustrate the atomic structure of an atom e.g. Bohr's model - which consisted of a tiny positive nucleus in which was orbited by negatively charged neutrons in order to form a tiny solar system atom
     • There was also Thomson's plum pudding model which proposed the idea that an atom comprised of electrons surrounded by a soup of protons to balance its negative charges.
   • Electron Shells- the number of electrons in each shell = 2n squared, where the n is the number of the shell. e.g. first shell holds 2 electrons, second shell holds up to 8 electrons ,the third shell holds up to 18 electrons and so on....

3. Mass Number and Atomic number

   • The mass number is the number of protons and neutrons present in the nucleus.
   • The atomic number is the number of protons present in the nucleus of an atom.

4. Isotopes

   • Isotopes are atoms with the same number of protons and different numbers of neutrons. Different isotopes of the exactly same element react simiilarly chemically. Their mass numbers, however, vary because of the difference in the number of neutrons.
   • Mass Spectrometer
     • Gives accurate information about relative isotopic mass and also about the relative abundance of isotopes
       • It is kept under high vacuum so the ions do not collide with air molecules, which can prevent them from reaching the detector
     • The sample is investigated whilst in its gaseous state. It is injected into the mass spectrometer, directly. If it is a solid it first needs tio be vapourised via heating.
       • x(g) ---> x+ (g) + e-
     • 1. IONISATION- a beam of electrons from a high energy electron gun knocks out electrons from atoms of the sample, forming positive ions. Most form 1+ ion charges but a small number form 2+ charge ions.
       • 2. ACCELERATION- these positive ions are attracted towards negatively charged plates and are then focussed into a beam, whilst being accelerated at high speed. Their speed depends on their mass- the lighter m/z ratio the faster, the heavier m/z ratio the slower. Som eions pass through slits in the plates and form a beam.
     • 3. DEFLECTION- the beam of ions then move into a magnetic field at right angles. The field deflects the beam into an arc. This deflection depends on their mass- the lighter ones are deflected less and the heavier ones are deflected more. The deflection also depends on the magnetic strength of the magnet- the stronger it is the greater the deflection, may be.
     • 4. DETECTION- the field is gradually increased, so that ions of increasing mass enter the detector one after the other. Ions strike the detector, accept electrons, lose their charge and create a current which is proportional to the abundance of each ion.
     • From the strength of the magnetic field at which a particular ion hits the detector, a computer works out the value of the mass to charge ratio (m/z) of the original ion. A mass spectrum is produced.
     • RAM= (percentage abundance x m/z) + (percentage abundance x m/z)/100
     • The spectrum may also show peaks caused by ionised molecules (molecular ions). Hence, it can be used to determine RAM. e.g. in chlorine 35 and 37 there wil be three m/z's

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