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    • ...due to the increasing nuclear charge and thus increasing attraction between valence electrons and positive...the the increase in nuclear charge and thus attraction between 4s electrons and nucleus ...

      Edited by UltimaOnline 16 Apr `14, 1:10AM
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    • ... is due to the increasing nuclear charge and thus increasing attraction between valence electrons and positive ... the the increase in nuclear charge and thus attraction between 4s electrons and nucleus is ...

      Edited by UltimaOnline 16 Apr `14, 1:09AM
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    • Ahhh that's good to hear! You're totally welcome to freely post your questions here, but as for 'answering all sorts of chemistry doubts', that's true insofar as : if it's within the H2 Chem syllabus, I'll address it. If it's beyond the H2 Chem syllabus, depending on my personal interest in that particular topic , I'll either decide to comment on it, or if not, I'll direct you to http://www.chemicalforums.com/. Keep enjoying Chemistry! Countdown to 2014 A Levels : just 6 months left! image

      Originally posted by bluepie:

      i'm a frequent lurker of this forum so i'm aware that you'll answer all sorts of chemistry doubts from forum users. i finally decide to create an acc and ask here because i couldn't find any good explanation on the internet haha


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    • ...account is new. You created a SgForum account just to discuss Chemistry here... can discuss Chemistry here at SgForums? Originally posted by bluepie: Yup,...

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    • You already take H3 Org Chem? So you're taking your H2 A levels this year? School candidate or private candidate? So you're a student?

      Originally posted by bluepie:

      Thanks ultimaonline for your guidance and help icon_biggrin.gif i enjoy organic chemistry more than inorganic chemistry though, i've actually study abit of uni organic chem reaction [aldol condensation etc.]


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    • Yes, that's right. Sounds like you'll enjoy Inorganic Chem a lot (take note that if you decide to take up H3 Chem in J2, it'll be Organic Chem rather than Inorganic; have you found out if you love Organic Chem as much as Inorganic?).

      The d block contraction in particular (rather than the lanthanide contraction) could be asked by Cambridge as a challenging question one of these years (I've actually already prepared a BedokFunland JC question on it some time back, will probably upload it sometime in the next few weeks or so).

      Originally posted by bluepie:

      i'm taking the h2 syallbus, though cambridge does not require this knowledge, i am curious in how it works cause i like learning beyond the syllabus icon_biggrin.gif 

      i was serching for anomalies for trends in atomic radius in the periodic table and came across two phenonmenon "d block contraction " and "lanthanide contraction". the explanation imo was unclear to how diffusiveness of orbital affects shielding effect.

      basically is it because there is a larger region of space for electrons to move about and hence the electrons are unable to shield the valence electrons from the positive nuceus as effectively  as compared to say a smaller region of space where electrons has a higher probability of shielding the positive nucleus from the valence electrons? 


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    • Yes, you're right. Imagine it's a freezing icy winter night. Which would keep you warmer (ie. shield you from the cold), a thick (ie. non-diffused) wooly jacket or a thin (ie. diffused) silk shirt?

      For A level purposes (are you a student or a tutor? are you Singaporean taking the Singapore H2 syllabus, or you're a foreigner taking international A levels?), just use any analogy to help yourself understand. Cambridge will *not* require the A level candidate to explain the technical details of how the degree of diffusiveness of the orbital affects the strength of its shielding effect.

      Originally posted by bluepie:

      what does a diffuse orbital mean? does it mean that there is a greater region of space to find an electron?  how does a diffuse orbital cause a poorer shielding effect? 


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    • 4f orbitals have a poorer shielding/screening effect compared to 3d orbitals which have a poorer screening/shielding effect compared to 2p orbitals which have a poorer shielding/screening effect compared to1s orbitals, because of two reasons :

      The further away the (principal quantum) electron shell the orbital belongs to, the larger and hence more diffused the orbital and therefore the poorer the shielding/screening effect.

      The shape of the orbitals also determine their effectiveness of shielding/screening : the shape of the orbitals are such that f orbitals are poorer shielders/screeners compared to d orbitals which are poorer shielders/screeners compared to p orbitals which are poorer shielders/screeners compared to s orbitals, even when in the same (principal quantum) electron shell.

      In the context of A level Chemistry (instead of say, physics or engineering or Marvel's Agents of S.h.i.e.l.d), there is no difference between shielding effect vs screening effect. Cambridge will accept both terms eagerly with open arms.

      Originally posted by bluepie:

      The atomic radius of aluminium is bigger than gallium due to the poor shielding effect of the 3d orbitals.

      Why does the electron in 3d orbital have a poor shielding effect than say 1s or 2s or 2p?

      on an interesting note, atomic radius of lanthanide series decreases with increasing proton number, also due to the poor shielding effect by 4f electrons.

      again, why does 4f electron have a poorer shielding effect? lastly, what is the difference between shielding effect and screening effect?  

       

      l


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    • 4f orbitals have a poorer shielding/screening effect compared to 3d orbitals which have a poorer screening/shielding effect compared to 2p orbitals which have a poorer shielding/screening effect compared to1s orbitals, because of two reasons :

      The further away the (principal quantum) electron shell the orbital belongs to, the larger and hence more diffused the orbital and therefore the poorer the shielding/screening effect.

      The shape of the orbitals also determine their effectiveness of shielding/screening : the shape of the orbitals are such that f orbitals are poorer shielders/screeners compared to d orbitals which are poorer shielders/screeners compared to p orbitals which are poorer shielders/screeners compared to s orbitals, even when in the same (principal quantum) electron shell.

      In the context of A level Chemistry (instead of say, physics or engineering or Marvel's Agents of S.h.i.e.l.d), there is no difference between shielding effect vs screening effect. Cambridge will accept both terms eagerly with open arms.

      Originally posted by bluepie:

      The atomic radius of aluminium is bigger than gallium due to the poor shielding effect of the 3d orbitals.

      Why does the electron in 3d orbital have a poor shielding effect than say 1s or 2s or 2p?

      on an interesting note, atomic radius of lanthanide series decreases with increasing proton number, also due to the poor shielding effect by 4f electrons.

      again, why does 4f electron have a poorer shielding effect? lastly, what is the difference between shielding effect and screening effect?  

       

      l


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    • Sounds like this chap could well be an international spy ala Jason Bourne (whose trilogy also began with complete memory loss of who he was), being trained in multiple languages and identities, and being involved in dangerous top-secret technologies, abductions "I had on my wrists very big, very bloody traces of plastic strips that they use to tie up cables" and secretly developed mind-altering drugs often employed by spy agencies, that could explain his memory loss, as well as the "interesting" but undisclosed information from the governments of Czech Republic, Norway, and others.

      [Europe] - Mystery man 'Jason Bourne' loses memory of who he is https://sg.news.yahoo.com/robbed-sexually-assaulted-says-norway-mystery-man-183102285.html

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    • Originally posted by atomos:

      Typo: title meant "substitution"

      What would be a conise way to explain why carboxyllic acids undergo nucleophillic subsitution rather than nucleophillic addition?

      My first thoughts would be the stability of the leaving group. Is there anything we can say based on the structure of the orginal molecules ?

       

       




      No, you're totally correct that it's about the stability of the leaving group (which JCs don't even teach, and therefore almost all H2 Chem students (except for H3, Olympiad, and BedokFunland JC students) have no idea *why* in some cases nucleophilic addition occurs, *why* in other caes nucleophilic substitution occurs, they just blindly memorize their schools lecture notes without understanding).

      For instance, for normal ketones and aldehydes, the only possible leaving group (other than the attacking nucleophile itself, which would be the backward reaction) are the H and R (ie. alkyl) groups. And since H- hydride ion and R-/C- carbanions are highly unstable, they just attack right back, and thus no group is willing to leave (ie. be eliminated).

      Hence, the uninegative formal charge remains stuck on the most electronegative O atom for the time being, until it can resolve the negative charge by abstracting a proton (eg. from HCN), generating hydroxynitrile (aka cyanohydrin).

      In contrast, for carboxylic acids electrophiles being attacked by alcohol nucleophiles, the H2O+ (if acid catalyzed) or even OH (if base promoted) leaving group has the electronegative O atom, which can stabilize the negative formal charge better, and hence makes for a more viable leaving group (since H2O and OH- are much more stable compared to H- or R-/C-).

      As a tutor, your/our value is in helping your/our students understand H2 Chemistry by plugging in all the huge gaps that schools leave out (resulting in students blindly memorizing school notes rather than understanding chemistry). Not entirely the school teachers' fault, because (as ex-MOE teachers, you and I can empathize that) MOE teachers are overburdened with much too much non-teaching duties, that inevitably lower the quality of teaching possible, as well as time constraints on the poor JC students (also overburdened with much too much non-learning and time-wasting activities) whose perpetual lack of sleep causes cumulative brain damage and worsens their performance in the A level exams.

      This is why there is, and will always be, a necessary, positive and (even) healthy role for good tuition teachers in Singapore, because the Education System in Singapore is much too flawed, and the MOE teachers and students suffer for it.

      Edited by UltimaOnline 07 Apr `14, 5:33PM
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