F321 THE STRUCTURE OF ATOMS. ATOMS Atoms consist of a number of fundamental particles, the most important are... in the nucleus of an atom

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1 Atomic Structure F32 TE STRUCTURE OF ATOMS ATOMS Atoms consist of a number of fundamental particles, the most important are... Mass / kg Charge / C Relative mass Relative Charge PROTON NEUTRON ELECTRON MASS NUMBER & ATOMIC NUMBER Atomic Number (Z) Mass Number (A) Number of protons in the nucleus of an atom Sum of the protons and neutrons in the nucleus 23 Na Mass number (protons + neutrons) Atomic number (protons) Q. Protons utrons Electrons Charge Atomic No. Mass No. Symbol A B 20 utral 40 C + 23 D 6 6 utral E 92 utral 235 F G Al 3+ Relative Atomic The mass of an atom relative to the 2 C isotope having a value of Mass (A r ) A r = average mass per atom of an element 2 mass of one atom of carbon-2 Relative Isotopic Mass Similar, but uses the mass of an isotope 238 U Relative Molecular Mass (M r ) Similar, but uses the mass of a molecule CO 2, N 2 Relative Formula Mass Used for any formula of a species or ion NaCl, O

2 2 Atomic Structure F32 ISOTOPES Definition Atoms with... the same atomic number but different mass number or the same number of protons but different numbers of neutrons. Properties Chemical properties of isotopes are identical Theory Relative atomic masses measured by chemical methods rarely produce whole numbers but they should do (allowing for the low relative mass of the electron). This was explained when the mass spectrograph revealed that atoms of the same element could have different masses due to the variation in the number of neutrons in the nucleus. The observed mass was a consequence of the abundance of each type of isotope. 2 3 P N 0 2 Example There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms Cl Cl P N % Method Three out of every four atoms will be chlorine-35 Average = = Method 2 Out of every 00 atoms 75 are 35 Cl 25 are 37 Cl Average = (75 x 35) + (25 x 37) = Q.2 Calculate the average relative atomic mass of sulphur from the following isotopic percentages S 95% 33 S % 34 S 4% Q.3 Bromine has isotopes with mass numbers 79 and 8. If the average relative atomicmass is , calculate the percentage of each isotope present. ed help:- See example calculation on the next page

3 Atomic Structure 3 F32 Mass spectra An early application was the demonstration by Aston, (Nobel Prize, 922), that naturally occurring neon consisted of three isotopes... 20, 2 and % positions of peaks gives atomic mass peak intensity gives relative abundance highest abundance is scaled up to 00% - other values are adjusted accordingly. Abundance 8.82% 0.26% mass to charge ration m/z CALCULATIONS Example Calculate the average relative atomic mass of neon using the above information. Out of every 00 atoms are 20, 0.26 are 2 and 8.82 are 22 Average = (90.92 x 20) + (0.26 x 2) + (8.82 x 22) = Ans. = 20.8 Example 2 Naturally occurring potassium consists of potassium-39 and potassium-4. Calculate the percentage of each isotope present if the average is 39.. Assume there are x nuclei of 39 K in every 00; there will then be (00-x) of 4 K. so 39x + 4 (00-x) = 39. therefore 39 x x = thus - 2x = - 90 so x = 95 ANSWER 95% 39 K and 5% 4 K Q.4 Calculate the average relative atomic mass of an element producing the following peaks in its mass spectrum... m/z Relative intensity Mass spectra can also be used to find the relative molecular mass of compounds

4 4 Atomic Structure F32 MASS SPECTROMETER A mass spectrometer consists of... an ion source, an analyser and a detector. ELECTRON GUN & ION SOURCE ANALYSER DETECTOR Ion source gaseous atoms are bombarded by electrons from a gun and are IONISED sufficient energy is given to form ions of + charge resulting ions can be ACCELERATED out of the ion source by an electric field Analyser charged particles will be DEFLECTED by a magnetic or electric field the radius of the path depends on the value of their mass/charge ratio (m/z) ions of heavier isotopes with larger m/z values follow a larger radius curve as most ions are singly charged (+), the path depends on their mass if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved 75% 75% Abundance 25% 25% 2+ ion + ion Changes when the ion is 2+ M/Z VALUE IS ALVED mass to charge ratio m/z ABUNDANCE IS TE SAME Detector by electric or photographic DETECTION methods mass spectra record the mass/charge values and relative abundance of each ion

5 Atomic Structure 5 F32 ANSWERS TO QUESTIONS Q. Protons utrons Electrons Charge Atomic No. Mass No. Symbol A utral 9 40 B utral C D utral 6 2 E utral F utral 6 3 G K 40 Ca 23 Na + 2 C 235 U 3 C 32 S 2-27 Al 3+ Q Q.3 79 Br 54.6% 8 Br 46.4% Q

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