Here's why:

* Electron Shells: Each row (period) represents a new electron shell being filled. As you move down, there are more electron shells, which means more orbitals and therefore more elements in each period.

* Subshells: Within each shell, there are subshells (s, p, d, and f). As you move down the table, the d and f subshells become filled, adding more elements to each period.

Specifically:

* Periods 1 and 2 have only 2 and 8 elements respectively, as they only fill the s and p orbitals.

* Period 3 also has 8 elements, but Period 4 starts to fill the d-orbitals, adding 10 more elements.

* Period 5 and 6 also have 18 elements, but period 7 adds the filling of the f-orbitals, resulting in 32 elements.

So, the increasing width of the periodic table is directly linked to the addition of new electron shells and subshells.