Answer:

Periodicity refers to trends or recurring variations in element properties with increasing atomic no. Periodicity is caused by regular & predictable variations in element atomic structure.

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Answer:

Catenation occurs most readily in carbon, forming covalent bonds to form longer chains and structures with other carbon atoms. This is why the vast number of organic compounds are found in nature. Carbon is best known for its catenation properties, with the analysis of catenated carbon structures in organic chemistry.

Carbon is by no means the only element capable of forming such catenae, however, and several other main group elements are capable of forming a wide range of catenae, including silicon, sulfur, and boron.