9.1 The periodic table: chemical periodicity

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6 thoughts on “9.1 The periodic table: chemical periodicity

    • Hi Azfar. Often, there are better ways of ‘remembering’ things other than simply memorising. Perhaps, The best way to learn these things is to learn general trends. For example, bonding characteristics of the chlorides going across period 3: The general trend is that they go from ionic to covalent. The dividing line is between Mg and Al. Often, these general trends are based on something really simple that we have already learned and already absorbed/accepted. E.g. elements on the LSH of the table are metals and metals give away electrons, hence they tend to form cations. So Na and Mg are ionic. Elements on the RHS of the table are non-metals and on the extreme RHS, tend to form anions, hence Cl- is a common anion. So it was easy to guess that NaCl and MgCl2 are ionic.

      Learning trends using general rules is good because it covers many things straight away and then we can just learn any few exceptions to the rule.

      The generalisation above can also be applied to the oxides. Na2O and MgO are ionic, P2O5 SiO2 SO2 & SO3 are covalent. Straight away then we have covered many cases. The only different thing we have to learn here is that now the dividing line between ionic and covalent compounds for the oxides, is between Al and Si. (Al2O3 is ionic).

      This ionic to covalent generalisation also helps us with the acid/base nature of the oxides when added to water! The ionic oxides are basic, the covalent oxides are acidic. Again, we can see that we’ve been able to cover many cases and very quickly by the generalisation. The dividing line this time is on aluminium itself. It is a basic oxide (as Na2O and MgO were) but also it is an acidic oxide (will react with concentrated alkali). SiO2 is an acidic oxide only – it no longer has the basic properties that the previous three elements did.

      I suppose a little bit of memorisation is always necessary, (CIE and other A-level exam boards call it the skill of ‘recall’) but at A-level, the emphasis is much more towards knowledge by understanding and intelligence based “appreciating” / “extrapolation” / “application” of simple principles onto a particular case or reaction. It is much different from SPM – where I am told, a HUGE emphasis is put on memorising the right answer.

      If you don’t aready do this then please try: Constantly re-reading your notes, like all your notes at the end of the day, or certainly at the end of EVERY WEEK. That will expose you the information which will actually help you remember by ‘exposure’ – a bit different from just purely trying to remember things. Also re-reading your notes will actually help you spot any problems/errors/confusing things in the them so you can still take action while the mind is still fresh on that info. In fact I watched a video made by some guy who classed himself as just an ordinary person but had trouble getting top grades. He made that one change… re-reading his notes very frequently and he found he was now scoring the top grades.

      What I mean by re-reading just isn’t reading, but highlighting (or even re-writing), supplementing the notes with book page references or even paragraphs (photocopied, cut out and stuck into your notes?) and so on. When I learned 2nd year organic chem on my degree, I used to ‘highlight’ using colouring pencils – the yellow, green and orange highlighters got a bit boring after a while. Just colouring in various groups etc forced my mind to think about them and ‘read’ my notes. It really helped me.

      Why not make your own “presentation” videos recording yourself talking about a topic that you think you’ve understood. The quick video can just be key points or things that you’ve realised you keep making the same mistake on or something.

      To all: Of course ANY method needs work and commitment. Discipline and creation of good study habits are almost impossible to avoid if you want to score a top grade. Such a grade won’t simply and miraculously float it’s way down on top of you without come good degree of effort. If you are putting in the effort but still the grades are proving elusive, (as happened to me for quite some time with A-level Maths) then you need to change the type of effort you are putting in. My mistake was that I was too embarrassed to ask my classmates for help, and it was their help that I needed. If you have understanding difficulties, then get onto YouTube. There are now hundreds of concept tutoring videos out there for A-level. You will find some that you can quickly relate to.

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  1. Hello again Sir,my question is when aluminium chloride(and magnesium chloride)reacts with water,hexaaquaaluminium ion (Al(H2O)6)3+ is produced but not Al3+ ion and Cl- ion.

    Why is it that 6 molecules of water are attracted to the metal ions by dative bonding???(not sure) then one molecule of water dissociates to become [Mg(H20)5(OH)]+ PLUS H+ .

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    • Another thing is SiCl4 plus water form SiO and HCl(one acid only)but from phosphorus chloride to S2Cl2,two acids are produced instead.

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    • Hi.
      A characteristic behavior of metals is that they give away e- in their reactions. Ionic compounds can form when the recipient of the e- selfishly retains that e- (e.g. a F- anion). The cation and anion are attracted to each other do to their opposite charge.

      The metal having lost its e- in the reaction, forms a cation and now has vacancies for e- in its valence shells.

      If a new, ‘less selfish’ species which can share e- (e.g. a lone pair of e- as present on oxygen in a H2O molecule) happened to get close, then the e- from that species can enter the valence shell of the cation.

      In the case of a H2O molecule, this will always happens (all metal cations are soluble), the lone pair of electrons are shared. A covalent bond occurs, and because both the electrons came from just one of the atoms that are now bonding, then this covalent bond is labeled a ‘dative bond’. The ‘e-sharing species’ is referred to as a ligand (Note: there is a formal definition for ligand).

      How many of ligands interact with the metal ion?
      It depends on a number of factors, the factors usually cited are: the size of the ligand and the number of vacancies the metal has (there are other factors like the Ea,, entropy and delta H of reaction etc). Usually, all things considered, these factors give 6 as the most common numbers of ligands. Sometimes this can be 4 or 2 etc, but mostly ‘six’ is encountered at A-level.

      You can get an ‘appreciation’ of this in two dimensions by putting a coin on a table. You should find that you can get only 6 coins to touch the central coin.

      Why giving off H+ ?
      If the metal ion is small and has a high charge (e.g. Al3+) then the polarizing effect of the metal anion on the ligand weakens the already polarized O-H bonds. In other words, the strongly charged cation pulls e- from the H2O molecule (from the O atom which is datively bonded to the metal). To ‘compensate’, the oxygen, [which is a highly electronegative atom] withdraws e- density from the OH bond more which weakens it allowing it to break.

      I don’t think Mg2+ has enough charge density (charge divided by size) on it to do this. It’s usually only 3+ ions which are noted for doing this. E.g. Al3+, Cr3+, Fe3+ etc. because solutions of Mg salts are neutral, but solutions of Al salts are acidic.
      IF Mg2+ ions do this then it will only happen to a TINY, almost insignificant extent, be very small and the concentration of H+ produced will be significantly less than 1×10-7 mol dm-3. I can say this because the salts of Mg are neutral.

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  2. i went through this information from the blue cie textbook and Advance study guide chemistry but doesnt give a clear answer.Btw not in latest red cie textbook(so wont come out for AS next sem???),so thats why i commented in this 2015 section

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