This is due to the modern theory of the structure of the atom. Periods can contain a different number of chemical elements, depending on the number of electrons at the outer level. In the first period, atoms of chemical elements have only one energy level, at which 2 electrons can be maximally located, in the second period – two energy levels and a maximum of 8 electrons.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.