Coordination no. in crystals (conceptual)

  1. what does coordination no in crystal structures really mean?
    i know it's no of nearest neighbors but has it significantly to do with bonding of electrons.

    actually my doubt is that -
    I read Be can't show coordination no of 6 and is allowed to show only coordination no 4 due to lack of orbitals. Just few minutes ago i read that it is packed in HCP type of lattice

    "Beryllium is a steel gray and hard metal that is brittle at room temperature and has a close-packed hexagonal crystal structure." source wikipedia

    we know in HCP coordination no is 12

    So in crystals what feeling of coordination no. must i get???
     
  2. jcsd
  3. DrDu

    DrDu 4,352
    Science Advisor

    Please cite your source.
     
  4. "jd lee" Wiley india edition page 332 fifth edition,
    also in ncert chemistry textbook(indian board publication)

    Now can anyone help me with my problem
     
    Last edited: May 22, 2014
  5. DrDu

    DrDu 4,352
    Science Advisor

    I don't have access to this book but the statement seems not correct.
    In salts and the like Be is often only coordinated by 4 anions, as it is a very small cation.
    However, this is not due to the number of orbitals.

    Given that anions are much larger than neutral Be atoms, this argument does not hold in Be metal and coordination can be higher.
     
  6. I couldn't access page 332 of JD Lee from Google Books.

    Please point the page number from the NCERT chemistry textbook.
     
  7. beryllium does not exhibit coordination no. more than 4 as in it's valance shell there are only four orbitals .......

    page no. 302 'chemistry class 11' 'part 2' , first column last paragraph
     
  8. "Coordination Number" is used in two different main contexts: in a crystal, or in a molecule, molecular complex, or some type of coordinate-covalent structure (usually as a solute). Until relatively recently, it was difficult to use ab initio quantum mechanical calculations to accurately predict/model ELECTRON DENSITY which is the most preferred way to estimate bonding between atoms. In molecules, coordination number is ONLY about the bonding. In crystals the distance between atoms was/is used as a proxy for bonding and it was assumed that the bonding only occurred from one atom to its "nearest" neighbors. (Which often could be determined by crystallographic methods). If you think about it, the term "nearest" is very unsatisfactory as a descriptor. Let me give you two examples: Ex.1. Lets say in a crystal structure Be has 4 neighbors we'll call A, all at the same (nearest) distance, and 2 neighbors, C, both 2% farther from the Be atom than the A's are. Question: what is the coordination number for the Be - 4 or 6 ? Ex. 2. Lets say that the Be has 6 neighbors, each of which is at a (slightly) different distance from it. It therefore only has one "nearest" neighbor. Is it correct to claim that the coordination number is 1? That sounds pedantic (and incorrect) to me. There are a variety of properties from which we can infer bonding type interactions (conductivity and electrical properties being a major type). Now days we can determine which atoms are bonding by calculation with good accuracy (not perfect, still some unknown territory out there). So, no solid state chemist worth her salt would claim that the coordination number should ignore significant bonding interactions simply because the two atoms aren't "nearest" neighbors.
    ---------------
    You should not ascribe covalent character to metal bonding. Metallic Be has no simple relationship to the covalent chemistry of Be (all Be compounds are (mostly) covalent). Hence the fact that each atom in metallic Be has 12 nearest neighbors elicits a "so what?" from me. You need to consider the context of what ever you are reading, otherwise understanding will be difficult.
    Be will tend to have a formal + charge, which means it would be a very small cation (if it were actually ionized in its compounds). Most anions are quite large, meaning getting 6 of them close to a Be cation would be difficult (at normal pressures, but what about at 10,000 gravities?) There is nothing contradictory in saying that the chemistry of Be is generally tetrahedral (or better and more general has coordination number 4) and the element is HCP in the solid state crystal.
     
  9. thanks abitslow, you really worked hard over the text but your work was tooo.. scientific and high fi, i mean that I AM JUST in 11th AND FOR SURE THE INITIAL LINES WERE GONE OVER MY BRAIN (over that quantum part and mot..)

    also my question was that what was the difference between the two types of coordination no ie. you said it is foolish to compare Be in crystal and covalent bonding, but i think why.

    I accept that i am conceptually not clear over part and i again have no mean that you had done bad work but just a bit simple language will help better.
     
  10. DrDu

    DrDu 4,352
    Science Advisor

    The point is that salts are made up of small cations and large anions, while covalent and metallic compounds are made up of atoms of rather comparable size. Hence in a salt, the cation (Be in this case) can coordinate much less anions around it than a Be atom can coordinate other Be atoms in the metal.
    This has little to do with the number of orbitals. So either your text is not right here, or the half-sentence you have given us is quite out of context.
     
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