What is a period? What do large and small periods have in common and how do they differ?

What is a period? What do large and small periods have in common and how do they differ? What properties of chemical elements regularly change in the period?

A period is a horizontal row of chemical elements arranged in ascending order of their relative atomic masses, starting with an alkali metal and ending with a noble gas.
The common thing for large and small periods is that they begin with an alkali metal (the exception is the first period) and end with a noble gas. The difference is that in small periods there are less than 8 elements, in large ones – more than 18 elements.
In periods, the metallic and non-metallic properties of chemical elements change naturally.

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