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Thread: Group 2- trend in melting points

  1. #1 Group 2- trend in melting points 
    h89
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    Why is it that this trend in mps of G2 is not smooth? As in why is calcium's mp higher than that of magnesium's? Isn't it supposed to be lower?
    Even the density of calcium is supposed be more than that of magnesium's...but its not! why??


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  3. #2  
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    Density isn't something you should consider heavily in melting point. The melting point of any species of atoms or molecules has much more to do with the attractive forces of its molecules.

    I don't specifically remember why calcium has a higher melting point than magnesium, but I'm sure that if you look into the attractive forces between calcium atoms vs. magnesium atoms, you'll find your answer.


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    I've been looking around and I noticed that this isn't as simple as it seems.

    The two relevent attractive forces in judging the melting point of light metals are metalic bonds and covalent bonds. None of the first group elements exhibit covalent bonding, which is why their melting points are pretty straight-forward.

    However, because beryllium and magnesium have high electronegativities, the electrons of their elements can behave covalently to form differently shaped lattices. Since beryllium has a very high electronegativity, it's covalent lattices are much higher in energy and consequently its melting point is very high. Magnesium has a much lower melting point since its electronegativity is substantially lower and consequently its covalent lattice is lower in energy.

    Calcium, strontium, etc. have very low electronegativities, so their lattices are no longer bound by covalent characteristics. That means that starting from calcium, ionic character takes precedence and the general trend of metallic bond melting points takes place like the alkali metals. Because calcium's ionic lattice happens to be higher in energy than magnesium's covalent lattice, calcium has a higher melting point.

    It's confusing looking at the melting points of this group, so there's no guarrantee that my explanation is entirely right. Many of my assumptions were based on the general chemical behaviour of magnesium vs. calcium, and what little I know about lattice structures.

    Feel free to correct me if I'm wrong. :wink:
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  5. #4  
    h89
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    Thank you for your reply! It seems quite rational!
    What about the densities though? Why is that calcium's density is lower than that of magnesiums? The general trend is that as you go down the group2, the densities increase...but there's again an anomaly observed amongst Ca & Mg! How do I account for this paradox?
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    Same difference actually. If I'm not mistaken, metallic bonds are generally longer than covalent bonds, so the distance between each magnesium atom is smaller allowing for more mass to exist in a set volume. Because calcium does not have covalent characteristics, the distance between each of its atoms is larger, allowing for less mass in a fixed volume.

    If you also look at Br, it's density is much larger than magnesium. This is because Br's electronegativity allows for its covalent bond to be shorter and higher in energy, allowing for more atoms to squeeze into a set volume.

    Once again, this is merely my best guess.
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    Although the metallic bond in Mg is stronger than in Ca but we know that Mp depends on two things
    1.Bond nature
    2.Structure of element i.e of the crystal or metal
    so here we have that the atoms in Mg are loosely packed than in Ca . That is what lowers its MP
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  8. #7 Anomalous Low Melting Point of Magnesium in Group 2 
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    Group 2 elements are so weird! Even after I searched through the research papers, i still can't find the reason for the anomalous melting point of magnesium.


    Except for a guess which is not officially publish, stating that Be has stronger covalent properties that it strengthens the metallic bond between atoms by sharing the two delocalised electron fully between the atoms so this increase the strength of metallic bonds significantly and magnesium which has a lower polarising power than Be, exhibits only small amount of covalent characters which disrupt the sea of delocalised electrons, therefore making the metallic bonds in magnesium weaker. So it has an anomalous low melting point.


    Anyone has a better idea?
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