Why is the first member of group IVA and the second member of group VIA show stronger tendencies to catenate compared to the other group members?

catenation is a process by which a molecule or an atom forms a straight or repeating chain with other atoms.in carbon it has 4 electrons in the outer most energy level hence can form chains repeatedly




The ability to catenate appears to be based on several factors:


* The bond energy of the element to itself - stronger bonds promote catenation
* Atom size - smaller atoms promote catenation due to lower steric demand and better orbital overlap (especially p-p)
* Degree to which orbital hybridization is important in the structure of that elements compounds (for example, the bond angle in $\ce{NH3}$ is $\sim 108^\circ$ while that of $\ce{AsH3}$ is more like $90^\circ$
* Electronegativity of that element - atoms with electronegativity in the middle of the scale appear to catenate best.


Here is a plot of bond energy and electronegativity for the first 36 elements. Note that there appears to be a sweet spot of electronegtaivity between ~2 and ~3 that produces high bond energies. In the box I drew are all of the elements that are known to have covalent compounds containing more than 3 catenated atoms of that element. In case you were wondering, I used bond energies for single bonds only so comparison was better.


[![Plot of bond energy vs. electronegativity for the first 36 elements highlighting a sweet spot for catenation between electronegativities of about 2 and about 3 and bond energies above 300 kJ/mol](https://i.stack.imgur.com/Q9FY6.png)](https://i.stack.imgur.com/Q9FY6.png)


Note that hydrogen is in this box. Hydrogen cannot catenate since it has only one valence electron, but if it had electrons in the second shell, it probably would catenate.


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