Special differences between the second and subsequent Periods

4 Special differences between the second and subsequent Periods 

To what extent is this trend apparent in the known forms of the elements carbon and silicon  

Although molecules containing silicon-silicon and phosphorus-phosphorus multiple bonds are known, they are thermodynamically unstable, and exist only when bulky groups attached to the silicon or phosphorus atoms provide a kinetic barrier to conversion into single-bonded polymers. 

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