Question

Why do elements with similar properties appear at regular intervals in the periodic table?

Answer 1

Because the modern Periodic Table directly reflects electronic structure.....

Explanation:

And electronic structure directly influences the observed chemistry. And thus the Periodic Table is divided into Groups, i.e. the vertical columns, whose members have more or less the same number of valence electrons, and more or less similar chemistry...and more of this will be discussed in your text.