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Atomic Structure

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Explains the structure of the atom and its discovery
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Atomic Structure

  1. 1. Copyright Sautter 2015
  2. 2. ATOMIC STRUCTURE QUANTUM THEORY & ELECTRONIC CONFIGURATION OF ATOMS
  3. 3. SUBATOMIC STRUCTURE • ALL ATOMS CONSIST OF PROTONS, NEUTRONS AND ELECTRONS • PROTONS ARE POSITIVELY CHARGED PARTICLES THAT RESIDE IN THE NUCLEUS AT THE CENTER OF THE ATOM • NEUTRONS ARE PARTICLES WITH NO CHARGE THAT ALSO ARE CONTAINED IN THE NUCLEUS. • ELECTRONS ARE NEGATIVELY CHARGED PARTICLES THAT ARE EXTERNAL TO THE NUCLEUS.
  4. 4. SUBATOMIC STRUCTURE • The experiments of Ernest Rutherford in the early 1900s, showed that matter consisted of small, highly concentrated, points of matter (the nucleus) surrounded by vast areas of empty space (the areas where electrons exist). • This is called the “Empty Space Model” of the atom. • Rutherford shot high energy alpha particles at a gold foil. The alpha particles acted as “bullets”. Most all the alpha particles went straight through. A few were deflected. • From this experiment, Rutherford concluded that tiny points of concentrated mass, distributed in mostly empty space, was the fundamental structure of matter.
  5. 5. RUTHERFORD’S EXPERIMENT (A CLOSE UP) A FEW ALPHA PARTICLES ARE DEFLECTED GOLD FOIL  PARTICLES MOST TRAVEL STRAIGHT THROUGH THE METAL = NUCLEUS MOST OF THE ATOM IS EMPTY SPACE
  6. 6. RUTHERFORD’S CONCLUSIONS MOST OF EVERYTHING IS NOTHING ! IF THE NUCLEUS OF AN ATOM WAS THE SIZE OF A BASEBALL, THE NEAREST ELECTRONS WOULD BE MILES AWAY ! WHAT IS INBETWEEN ? EMPTY SPACE !
  7. 7. WHAT MAKES ONE ATOM DIFFERENT FROM ANOTHER ? • EACH ELEMENT HAS DIFFERING NUMBERS OF PROTONS. THIS IS WHAT MAKES ELEMENTS DIFFERENT, THE NUMBER OF PROTONS IT CONTAINS. • THE ATOMIC NUMBER OF AN ELEMENT TELLS US THE NUMBER OF PROTONS IT CONTAINS. ELEMENTS ON THE PERIODIC TABLE ARE ARRANGED IN ROWS, LEFT TO RIGHT IN ORDER OF INCREASING ATOMIC NUMBER. THEY START WITH HYDROGEN, ATOMIC NUMBER 1, FOLLOWED BY HELIUM, ATOMIC NUMBER 2, LITHIUM ATOMIC NUMBER 3, ETC.
  8. 8. WHAT ABOUT THE NUMBER OF NEUTRONS IN AN ATOM ? • THE NUMBER OF NEUTRONS VARY FROM ELEMENT TO ELEMENT. SOME ATOMS OF THE SAME ELEMENT MAY HAVE DIFFERENT NUMBERS OF NEUTRONS. ATOMS OF THE SAME ELEMENT WITH DIFFERENT NEUTRON NUMBERS ARE CALLED ISOTOPES. • THE MASS NUMBER TELLS US THE COMBINED NUMBER OF PROTONS AND NEUTRONS IN THE AVERAGE ATOM (ISOTOPE) OF THAT ELEMENT.
  9. 9. DETERMINATION OF ATOMIC MASSES • Atomic mass is measured in “atomic mass units” (amu). • By convention (everyone agrees), one amu equals exactly 1/12 of the mass of a carbon 12 atom. Using this unit, hydrogen 1, the lightest element has an atomic mass of approximately 1 amu. • It is important to realize that atomic mass (atomic weight) and the mass number of an atom are different ! • The mass number equals the number of protons plus neutrons in an isotope. Atomic mass is the weighted average of atomic masses in atomic mass units (amu) based on 1/12 of the mass of carbon 12.
  10. 10. CALCULATING THE NUMBER OF NEUTRONS IN AN ATOM • THE ATOMIC NUMBER OF THE ELEMENT SODIUM (Na) IS 11 AND ITS ATOMIC MASS IS 22.99. WE’LL ROUND THIS NUMBER TO 23. SINCE THE ATOMIC NUMBER TELLS THE NUMBER OF PROTONS, SODIUM MUST HAVE 11 PROTONS. THE PROTONS AND NEUTRONS TOGETHER EQUAL 23. BY SUBTRACTION, SODIUM MUST HAVE 12 NEUTRONS IN AN AVERAGE ATOM OF SODIUM. • WHAT ABOUT AN ATOM LIKE CHLORINE? ITS ATOMIC MASS IS 35.45. ITS ATOMIC NUMBER IS 17. IT THEREFORE HAS 17 PROTONS, BUT SUBTRACTING 17 FROM 35.45 GIVES 18.45. HOW COULD AN ATOM HAVE .45 NEUTRONS??
  11. 11. HAVE YOU FIGURED OUT THE ANSWER ?? • REMEMBER, THE ATOMIC MASS IS THE AVERAGE OF THE DIFFERENT ISOTOPES OF THAT ELEMENT. NO ONE ATOM HAS A MASS OF 35.45. INSTEAD, SOME CHLORINE ATOMS HAVE MASSES OF 35, SOME 36, SOME 37. • SO CHLORINE ATOMS OF MASS NUMBER 35 HAVE 18 NEUTRONS, THOSE OF MASS NUMBER 36 HAVE 19 NEUTRONS AND THE MASS NUMBER 37 ATOMS HAVE 20 NEUTRONS. • THERE IS STILL A QUESTION HOWEVER. THE AVERAGE OF 35, 36 AND 37 IS NOT 35.45, IT IS 36. WHY ISN’T 36 LISTED AS THE ATOMIC MASS FOR CHLORINE ?
  12. 12. WHY NOT A MASS NUMBER OF 36 THEN ? • IN ORDER TO GET 36 AS AN AVERAGE THERE MUST BE EQUAL NUMBERS OF ATOMS OF ALL THREE ISOTOPES OF CHLORINE PRESENT. THIS EQUAL DISTRIBUTION IS RARELY TRUE FOR ANY ELEMENT IN NATURE. • HERE IS ANOTHER EXAMPLE. HYDROGEN ATOMS OCCUR AS THREE DIFFERENT ISOTOPES, HYDROGEN WITH AN ATOMIC MASS NUMBER OF 1, DEUTERIUM, THAT IS HYDROGEN WITH A MASS NUJMBER OF 2 AND FINALLY TRITIUM, HYDROGEN WITH A MASS NUMBER OF 3. ON THE PERIODIC TABLE THE ATOMIC MASS OF HYDROGEN IS LISTED AS 1.0079, ALMOST 1. CAN YOU EXPLAIN WHY THE LISTED MASS IS SO CLOSE TO ONE INSTEAD OF THE ARITHEMATIC AVERAGE OF TWO?
  13. 13. ARITHEMATIC AVERAGES AND WEIGHTED AVERAGES ARE DIFFERENT. THE ATOMIC MASS IS A WEIGHTED AVERAGE OF THE ISOTOPES FOR THAT ELEMENT ! • HYDROGEN 1 (IT HAS 1 PROTON AND NO NEUTRONS) REPRESENTS 99.985 % OF ALL HYDROGEN ATOMS • HYDROGEN 2 (IT HAS 1 PROTON AND 1 NEUTRON) REPRESENTS ONLY 0.015 % OF ALL HYDROGEN ATOMS. • HYDROGEN 3 (IT HAS 1 PROTON AND 2 NEUTRONS) REPRESENTS 0.0001 % , ALMOST NO HYDROGEN ATOMS. • THE WEIGHTED AVERAGE CONSIDERS THAT MOST ALL HYDROGEN ATOM HAVE A MASS OF 1 AND THEREFORE THE NUMBER APPEARING ON THE PERIODIC TABLE IS VERY, VERY CLOSE TO 1.00
  14. 14. THE ATOMIC MASS NUMBERS ON THE PERIODIC TABLE THEN ARE WEIGHTED AVERAGES WHICH ACCOUNT NOT ONLY FOR THE MASSES OF DIFFERENT ISOTOPES, BUT ALSO ACCOUNTS FOR ABUNDANCE (PERCENT) AT WHICH THEY OCCUR IN NATURE !! THE AVERAGES WHICH YOU ARE USED TO WORKING WITH IN MATH CLASS ARE ARITHEMATIC AVERAGES WHICH DO NOT ACCOUNT FOR ABUNDANCES.
  15. 15. MASSES OF SUBATOMIC PARTICLES PARTICLE MASS CHARGE PROTON 1 AMU (1.007277) +1 NEUTRON 1 AMU (1.008665) 0 ELECTRON 1 / 1856 AMU (0.0005486) -1 AMU = ATOMIC MASS UNIT PROTONS & NEUTRONS ARE VERY MASSIVE AS COMPARED TO ELECTRONS. MOST ALL OF THE MASS OF AN ATOM IS IN THE NUCLEUS ! THE MASS OF AN ATOM’S ELECTRONS IS NEGLEGIBLE (SO SMALL THAT IT DOESN’T COUNT)
  16. 16. SO FAR THEN, THE NUMBER OF PROTONS IN THE NUCLEUS OF AN ATOM EQUALS THE ATOMIC NUMBER. THE NUMBER OF NEUTRONS EQUAL THE ATOMIC MASS MINUS THE ATOMIC NUMBER • WHAT ABOUT THE NUMBER OF ELECTRONS ?? • WELL, THIS DEPENDS ON WHETHER WE ARE TALKING ABOUT A NEUTRAL ATOM OR AN ION ! • IN A NEUTRAL ATOM THE NUMBER OF ELECTRONS AND PROTONS ARE EQUAL, SO THE NUMBER OF ELECTRONS EQUALS THE ATOMIC NUMBER OF THAT ATOM. • IN THE CASE OF IONS THE NUMBER OF ELECTRONS IS DIFFERENT THAN THE ATOMIC NUMBER.
  17. 17. WHAT IS AN ION ? WHAT ARE THE TWO KINDS OF IONS ? • IONS ARE ATOMS OR GROUPS OF ATOMS WITH EXTRA OR MISSING ELECTRONS. • ATOMS WITH EXTRA ELECTRONS ARE IONS WITH A NEGATIVE CHARGE AND ARE CALLED ANIONS. • ATOMS WITH MISSING ELECTRONS ARE IONS WITH A POSITIVE CHARGE AND ARE CALLED CATIONS. IT IS IMPORTANT TO UNDERSTAND HERE THAT CATIONS DO NOT HAVE EXTRA PROTONS, THAT IS THEY DO NOT AND CAN NOT GAIN PROTONS. IF THEY COULD THEY WOULD BE COME A DIFFERENT ELEMENT. THE NUMBER OF PROTONS AN ATOM CONTAINS MAKES THE ATOM WHAT IS IT !!
  18. 18. LET’S DETERMINE THE ATOMIC STRUCTURE FOR OXYGEN • OXYGEN IS ELEMENT WITH AN ATOMIC NUMBER OF 8 AND AN ATOMIC MASS NUMBER OF 16 (OXYGEN OCCURS AS THREE DIFFERENT ISOTOPES 16, 17 AND 18. SINCE THE ATOMIC MASS IS SO CLOSE TO 16, MOST ALL OXYGEN ATOMS (99.8 %) ARE ISOTOPE 16). • THE MOST COMMON ATOM OF OXYGEN THEN CONSISTS OF 8 PROTONS AND (16 – 8) OR 8 NEUTRONS IN THE NUCLEUS. IF THE ATOM IS NEUTRAL IT THEN ALSO CONTAINS 8 ELECTRONS EXTERNAL TO THE NUCLEUS.
  19. 19. WHEN ATOMS COMBINE TO FORM MOLECULES THE ELECTRONS OF EACH ATOM MUST FIRST INTERACT. • HOW ARE THE ELECTRONS ARRANGED IN THE ATOM ? UNDERSTANDING THIS HELPS US TO DISCOVER HOW AND WHY ATOMS COMBINE! • ELECTRON ARRANGEMENTS WITHIN ATOMS ARE REFERRED TO AS ELECTRON CONFIGURATIONS. • BEFORE WE CAN UNDERSTAND ELECTRON CONFIGURATIONS, WE MUST FIRST UNDERSTAND HOW THEY WERE DISCOVERED IN THE FIRST PLACE. THIS REQUIRES THE USE OF A SCIENCE CALLED QUANTUM MECHANICS.
  20. 20. BASIC CONCEPTS OF QUANTUM MECHANICS • ELECTRONS EXIST IN AREAS OUTSIDE THE NUCLEUS. THESE AREAS ARE CALLED ENERGY LEVELS. YOU MIGHT HAVE HEARD OF THEM BEFORE AS “SHELLS” (K, L, M etc.) . THERE ARE NUMEROUS ENERGY LEVELS AT WHICH THE ELECTRON CAN BE FOUND, EACH AT A PROGRESSIVE HIGHER ENERGY. • THESE LEVELS ARE ASSIGNED NUMBERS 1,2,3, ETC. AS THE NUMBER INCREASES, THE ENERGY STATE OF THE ELECTRON BECOMES HIGHER.
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