Hey there Sign in to join this conversationNew here? Join for free

Atomic Radius Question!

Announcements Posted on
Study Help needs new mods! 14-04-2014
Post on TSR and win a prize! Find out more... 10-04-2014
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    Nope, effective nuclear charge increases across a period due to increase in Z and increase in valence electron => hence valence electron is more tightly bound, orbital size contracts

    Moreover, the 3d series is poor at shielding as they has 3 - 2 - 1 = 0 radial nodes; no inner maxima to shield increase of Z by 10 across the series
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    Nope, effective nuclear charge increases across a period due to increase in Z and increase in valence electron => hence valence electron is more tightly bound, orbital size contracts

    Moreover, the 3d series is poor at shielding as they has 3 - 2 - 1 = 0 radial nodes; no inner maxima to shield increase of Z by 10 across the series
    See here, it also tells that noble gases have largest radii across their periods. And as of the the largest atomic radii, i know that the elements of last periods have larger radii owing to addition of more number of shells, etc. Plus the data booklet provided during the examinations also gives the radius of argon, i.e a noble gas larger than of all the elements in its period.
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    See here, it also tells that noble gases have largest radii across their periods. And as of the the largest atomic radii, i know that the elements of last periods have larger radii owing to addition of more number of shells, etc. Plus the data booklet provided during the examinations also gives the radius of argon, i.e a noble gas larger than of all the elements in its period.
    there is the metallic, covalent and van der waals radii to consider, do you know which one is quoted in your booklet; they refer to different things really - this was mentioned on chemguide too.

    Argon molecules are just single argon atoms, Ar. The same goes with the other noble gases.

    based on general trend in terms of effective nuclear charge in a single atom across a period, i'd still say Kr is more contracted relative to K.
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    there is the metallic, covalent and van der waals radii to consider, do you know which one is quoted in your booklet; they refer to different things really - this was mentioned on chemguide too.

    Argon molecules are just single argon atoms, Ar. The same goes with the other noble gases.

    based on general trend in terms of effective nuclear charge in a single atom across a period, i'd still say Kr is more contracted relative to K.
    The booklet quotes all the values under the heading of atomic radii. Plus there were also some questions in past papers whose mark schemes gave the noble gases the largest radius.
    • 37 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    The booklet quotes all the values under the heading of atomic radii. Plus there were also some questions in past papers whose mark schemes gave the noble gases the largest radius.
    shengoc is right (tip: he usually is)

    Check out this diagram, or this link
    Attached Thumbnails
    Click image for larger version. 

Name:	radii.gif 
Views:	32 
Size:	19.5 KB 
ID:	98553  
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    (Original post by Plato's Trousers)
    shengoc is right (tip: he usually is)

    Check out this diagram, or this link
    I know noble gases have 'Vander Waal's' Radii and metals have 'metallic' radii. But as the question just quotes atomic radii, then how do I have to answer it?
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    I know noble gases have 'Vander Waal's' Radii and metals have 'metallic' radii. But as the question just quotes atomic radii, then how do I have to answer it?
    i have a gut feeling you sometimes overcomplicate a simple question. excluding metallic or VDW contributions, just think of them as all atoms. They are subject to the standard nuclear charge, what would happen? does that help clarify your point of view?
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    i have a gut feeling you sometimes overcomplicate a simple question. excluding metallic or VDW contributions, just think of them as all atoms. They are subject to the standard nuclear charge, what would happen? does that help clarify your point of view?
    Yep, that clarifies it. And yeah I just go deep into each and every question. The examiners ain't good to us - sometimes in mark schemes they give noble gases as the answers and sometimes just the group 1 metals. Anyways, thank you! I hope to be lucky if this kinda questions come in a real examination.
    • 37 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    Yep, that clarifies it. And yeah I just go deep into each and every question. The examiners ain't good to us - sometimes in mark schemes they give noble gases as the answers and sometimes just the group 1 metals. Anyways, thank you! I hope to be lucky if this kinda questions come in a real examination.
    You are asked about atomic radii so forget the other radii. The diagram I posted above quote clearly shows the atomic radii in Angstroms (10-10m)

    Are you saying your teachers give noble gases as having the largest atomic radius in a given period!? Are you sure about that? They are wrong then. Can you give us an example?
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    Yep, that clarifies it. And yeah I just go deep into each and every question. The examiners ain't good to us - sometimes in mark schemes they give noble gases as the answers and sometimes just the group 1 metals. Anyways, thank you! I hope to be lucky if this kinda questions come in a real examination.
    curiosity is good for science, never lose that. perfection too. but sometimes you just need a different perspective to look at problems and forums tend to help.

    well even if different sort of questions come up, you'd just have to use all your chemical knowledge to work them out. i mean by then, you'd have the knowledge, it is only about applying them to different sort of problems.
    • 9 followers
    Offline

    ReputationRep:
    we are here a lot, eh Plato? oh dear, i should really be revising my stat mech right now...
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    (Original post by Plato's Trousers)
    You are asked about atomic radii so forget the other radii. The diagram I posted above quote clearly shows the atomic radii in Angstroms (10-10m)

    Are you saying your teachers give noble gases as having the largest atomic radius in a given period!? Are you sure about that? They are wrong then. Can you give us an example?

    (Original post by shengoc)
    we are here a lot, eh Plato? oh dear, i should really be revising my stat mech right now...
    Can you please wait 'til I give an example from another past paper question?
    • 37 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    we are here a lot, eh Plato? oh dear, i should really be revising my stat mech right now...
    well yes. But I figure it's my way of paying back for all the help I get in the maths forum.

    Plus, someone's got to help these people discover how wonderful chemistry is and encourage their interest.

    And if you think we are here a lot, think of poor charco. He's got a camp bed and camping stove here!
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    Nvm for getting a bit late, see the attachment.
    Attached Thumbnails
    Click image for larger version. 

Name:	ra.PNG 
Views:	21 
Size:	52.6 KB 
ID:	98554  
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Zishi)
    Nvm for getting a bit late, see the attachment.
    1) This question is a bit cheeky. Ar can't have ionic radii, so they are comparing the ionic radii of the ions relative to the atomic radii of the atoms.

    In terms of atomic size, due to the extra proton charge unbalanced due to loss of electrons, hence overall Zeff increases in the order Ar, K+, Ca2+ and hence size decreases in that order too

    2) I think the confusion between atomic and covalent radii prevails here. Logically speaking, if you think of a single atom, all that matters is effective nuclear charge, and noble gases at the end of the period should suffer the greatest effective nuclear charge and hence the smallest in size.

    See these links that i have randomly googled(don't take it for granted they are true, but....)
    • 9 followers
    Offline

    ReputationRep:
    http://www.webelements.com/argon/atom_sizes.html

    http://www.creative-chemistry.org.uk...e1/trends5.htm
    • Thread Starter
    • 3 followers
    Offline

    ReputationRep:
    Yep, these links kinda helped. Thanks a lot!
    • 0 followers
    Offline

    ReputationRep:
    Hey everyone!

    I know bumping an old topic is annoying, but I thought that this was necessary.
    My school (that is, my Chemistry teachers) were recently stuck on the questions Zishi here has raised. After much thought, we've finally figured out a way to go about these questions.

    I do not think any Chemistry study-guide, as of yet, has this tip in it so read this very carefully.

    You may have noticed in the past-papers that whenever a bond enthalpy, atomic radius or ionization energy value is asked or is needed, it is mentioned in italics above the question that you have to make use of the Data Booklet.

    However, when a question does not mention the use of the Data Booklet, you should not usually refer to it.
    Referring to the Periodic Table to view the order of the first 20 elements and their atomic numbers, is not regarded as a question that requires the Periodic Table, and hence the Data Booklet as you're expected to know that by heart (the first 20 elements only).
    CIE will only ask you to refer to the Data Booklet when you need RAM/RMM values from the Periodic Table.
    (If you did not understand the above part, please try to read it again.)

    Now for some practical application of this. I'll use the same examples used by Zichi here:

    The MCQ) 12 [second attachment] does not state at the beginning that you should use the Data Booklet. If you do use it here, do not look at the atomic/ionic radii values. Look at the Periodic Table. This is what you can deduce from it:
    • Ar - Period 3, 3 quantum shells, proton number 18, electron number 18
    • K - Period 4, 4 quantum shells, proton number 19, electron number 19 (the question is about K+ so that will reduce the quantum shells to 3, and electron number to 18)
    • Ca - Period 4, 4 quantum shells, proton number 20, electron number 20 (the question is about Ca2+ so that will reduce the quantum shells to 3, and electron number to 18).

    Since now everything in the above particles is the same except the proton number (making it a fair comparison), it can be said that Ca2+ has the smallest size since it has the most number of protons, and Ar the largest since it has the least. (Coloumb's law)

    Now come to Q) 3 [the first attachment on this thread], here too the question does not state that it requires the use of the Data Booklet. Hence, you know by looking at the section of the Periodic Table given in the question that Potassium being in the 3rd Period and being the first element there has the longest ionic radius.
    Do not look at the atomic/ionic radii values given in the Data Booklet for this question either.

    --------

    The Q)4 from Paper 2 does state that you need to use the Data Booklet. It is only now that you should look at the atomic and ionic radii values.
    Directly putting the values from the Data Booklet will give you the shape that the Mark Scheme gives. Also note that since the Data Booklet does not have the ionic radius of Ar, the Mark Scheme of this question does not require the graph for ionic radii to extend after Cl.

    I know it's all a bit confusing but it's pretty easy once you get it. Just remember:
    In only the questions that mention the requirement of the Data Booklet must you turn to he atomic/ionic radii pages. Otherwise, refer ONLY to the Periodic Table.

    Kudos!

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?

    this is what you'll be called on TSR

  2. this can't be left blank
    this email is already registered. Forgotten your password?

    never shared and never spammed

  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By completing the slider below you agree to The Student Room's terms & conditions and site rules

  2. Slide the button to the right to create your account

    Slide to join now Processing…

    You don't slide that way? No problem.

Updated: March 14, 2012
Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.