As expected, aluminium shows related behaviour that is modified by the larger at
ID: 1063362 • Letter: A
Question
As expected, aluminium shows related behaviour that is modified by the larger atomic radius of this Period 3 element. The compound AIH_3 does not exist as a monomer but forms a polymer in which each relatively large Al atom is surrounded octahedrally by six H atoms. Beryllium, unlike its congeners, exhibits a diagonal relationship with Al and also forms a polymeric covalent hydride BeH_2. Although BH_3 and AlH_3 do not exist as monomers they do form important complex anions in combination with the hydride anion. The common reagents sodium tetrahydridoborate (NaBH_4) and lithium tetrahydridoaluminate (LiAlH_4) are examples of adduct formation between BH_3 or AlH_3, each a Lewis acid, and the Lewis base H^+.Explanation / Answer
Molecular hydride:
Molecules with -H bonds are typically known as hydride.
Consider E-H is a molecule. When it has to undergoe a chemical reactions, the bond must be broken.
The cleavage of bond is of two types:
1. Homolytic cleavage
2. Heterolytic cleavage
1. Home lytic cleavage means both bonded electrons were taken by both the bonded atoms, one each. Resulting, free radicals. Hydrogen free radical will be formed.
Homolytic cleavage occurs readily for some of the hydrogen compounds of p-block elements, especially heavier elements I.e elements of high molecular mass.
One example given above, as going down in group the homolytic cleavage tendency increases. Sn-H bond can be dissociated to free radicals easily than that of Si-H.
Also, here bond strength will be a factor. As the bond strength is low the homolytic cleavage becomes easier.
See the example of trialkylstannane.
Ex: thermal decomposition of molecular hydrides yielding H2 undergoe homolytic cleavage .
If E-H bond energy is high, you need more energy to dissociate the bond. Or you have to take to high temperature.
So, higher the bond enthalpy or energy, difficult for homolytic cleavage . See the examples of AsH3 and H2O.
2. Heterolytic cleavage:
When H is bonded to high electronegative atom, it will give its bond pair of electrons to electronegative atom and release as hydrogen cation(proton).
Due to this tendency to give protons, these molecules act as bronsted acids.as the electronegativity of the atom attached to H is high, the tendency of losing proton is high.
Easier the tendency of losing proton stronger the acid strength .
If we see the electronegativity in a period it increases from left to right. Hence acid strength also.
In a group bond energy decreases , so they can easily lose protons. Hence acid strength increases .
In heterolytic cleavage if the atom attached to H is electrode positive it will result in hydrogen negative ion.
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