AS chemistry unit one part four

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ghoran
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254154
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AS chemistry unit one part four
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2013-12-23 11:45:37
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periodicity
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revision
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  1. how does first ionisation energy generally change across period 3
    it generally increases going across period 3
  2. the trend in first ionisation energy across period 3 needs more detailed consideration than the trend in group 2 because
    • the first ionisation energy drops between magnesium and aluminium before increasing again 
    • the first ionisation energy drops between phosphorous and sulphur before increasing
  3. table of physical data is below using this information draw a graph to show the trend in first ionisation energies across period 3  
  4. the first ionisation energy is
    the energy needed to remove 1 mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive ions
  5. the enthalpy change for first ionisation energy is an ..... process
    endothermic
  6. explain the general trend in first ionisation energy going across period 3
    • there are more protons in each nucleus so the nuclear charge in each element increases 
    • therefore the force of attraction between the nucleus and the outer electron is increased 
    • there is negligible increase in shielding because each successive electron enters the same energy level
    • so more energy is needed to remove the outer electron
  7. explain the first ionisation energy decrease between magnesium and aluminium
    • look at their electron configuration 
    • magnesium : 1s22s22p63s
    • aluminium : 1s22s22p63s23p1
    • the outer electron in aluminium is in the p sub-level . this is higher in energy than the outer electron in magnesium , which is in an s sub level , so less energy is needed to remove it
  8. explain the decrease in first ionisation energy between phosphorous to sulphur
    • look at their electron configurations 
    • phosphorous : 1s22s22p63s23p3
    • sulphur : 1s22s22p63s23p4
    • its not immediately obvious what's going on until we look at the arrangements of the electrons 
    • the 3p electrons in phosphorous are all unpaired . In sulphur two of the 3p electrons are paired . there is some repulsion between paired electrons in the same sub level . this reduces the force of their attraction to the nucleus , so less energy is needed to remove one of these paired electrons than is needed to remove an unpaired electron from phosphorus
  9. how does the atomic radius change across period 3
    decreases
  10. using the table of data below draw a graph to show the trend in atomic radius across period 3 
  11. explain the trend in atomic radius across period 3
    • the number of protons in the nucleus increases 
    • so the nuclear charge increases
    • there are more electrons , but the increase in shielding is negligible because extra electron enters the same principal energy level 
    • therefore the attraction between the nucleus and the electrons increases 
    • so the atomic radius decreases
  12. using the table of physical data draw a graph to show the trend in electro negativity across period 3
  13. how does electro negativity change across period 3
    it increases
  14. electro-negativity is the
    relative ability of an atom to attract the bonding electron pair in a covalent bond (to withdraw electron density) . it cannot be measured directly
  15. in the pauling electronegativity index , the most electronegative element
    fluorine is given an electronegativity value of 4
  16. there are no values of electronegativity for the noble gases as
    they don't usually form covalent bonds as they are inert
  17. explain the trend in electro negativity across period 3
    • going across period 3 the nuclear charge increases 
    • the atomic radius decreases 
    • there are more electrons , but the increase in shielding is negligible because each extra electron enters the same principal energy level
    • so electrons are more attracted to the nucleus
  18. remember that ionisation energies are all endothermic processes (i.e. energy is needed to remove electrons) , so atoms
    with low electro negativities have a tendency to lose electrons and a weaker tendency than others to attract a the bonding pair in a covalent bond
  19. the trends in melting points and boiling points across period 3 are not straightforward , and need more detailed consideration than the trends in group 2 :
    • melting points generally increase going from sodium to silicon , then decrease going to argon (with a "bump" at sulphur)
    • boiling points generally increase going from sodium to silicon , then decrease going to argon (with a "bump" at sulphur
  20. using the table of physical data below draw a graph to show the trend in melting and boiling points across period 3 
  21. explain the trend in melting and boiling points from sodium to magnesium
    • sodium , magnesium and aluminium are all metals . they all have metallic bonding , in which positive metal ions are attracted to de-localised electrons . Going from sodium to aluminium :
    • the charge on the metal ions increases from +1 to +2 to +3
    • the number of de-localised electrons increases 
    •  so the strength of the metallic bonding increases 
    • so the melting points and boiling points increase
  22. silicon is a
    metalloid (an element with some properties of metals and some of the properties of nonmetals)
  23. what bonding does silicon have
    giant covalent . it has a giant lattice structure similar to that of diamond , in which each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement . this extends in 3D to form a giant molecule or macromolecule
  24. silicon has a very high melting point and boiling point because
    • all the silicon atoms are held together by strong covalent bonds 
    • which need a very large amount of energy to be broken
  25. explain the trend in melting and boiling points from phosphorous to argon
    • they all have low melting points because : 
    • when these four substances melt or boil , it is van der waals forces between the molecules that are broken 
    • which are very weak 
    • little energy is needed to overcome them 
    • sulphur has a higher melting point and boiling point than the other three because : 
    • phosphorus exists as Pmolecules
    • sulphur exists as Smolecules 
    • chlorine exists as Cl2 molecules
    • argon exists as individual Ar atoms 
    • the strength of the van der waals forces decreases as the size of the molecule decreases 
    • so the melting points and boiling points decrease in the order S8 > P4 > Cl> Ar
  26. use the table of physical data to plot a graph to show the trends in electrical conductivity across period 3 
  27. for an element to conduct electricity it must
    contain electrons that are free to move . In general metals are good conductors of electricity and non metals are poor conductors of electricty
  28. explain the trend in electrical conductivity from sodium to aluminium
    • sodium , magnesium and aluminium are all metals . they have metallic bonding , in which positive metal ions are attracted to de-localised electrons . the de-localised electrons are free to move going from sodium to aluminium :
    • the number of de-localised electrons increases
    • there are more electrons which can move and carry charge
    • so electrical conductivity increases
  29. explain the electrical conductivity of silicon
    • it is a semi conductor because : 
    • the four outer electrons in each atom are held strongly in covalent bonds 
    • few electrons have enough energy at room temperature to enter the higher energy levels 
    • so there are few de localised electrons and silicon is a poor conductor 
    • but at higher temperatures more electrons are promoted to higher energy levels 
    • so there are many de localised electrons to move
  30. explain why phosphorus , sulphur , chlorine and argon don't have any electrical conductivity
    • the outer electrons aren't free to move and carry charge in P , S and Cl because they are held strongly in covalent bonds
    • in argon (which exists as single atoms) the outer electrons aren't free to move because they are held strongly and stable in the third energy level
  31. where are non metal and metals on the periodic table
    • non metals are on the right 
    • metals are on the left
  32. draw a diagram of the periodic table to show where the s , p , d and f blocks are found

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