Explain how scientific observations led to the development of, and changes to, the periodic table.

By the observations made by scientists on the properties of the elements that is found in the periodic table, these elements are arranged accordingly depending on their properties. The rows are called as the periods while columns are known as groups. In the case of the size of the atom, they are arranged in a way that from left to right across a period the atomic radius increases as well as going down a group. For reactivity, it increases from left to right and up to down. Also, the elements are arranged with respect to increasing atomic number. It is Dmitri Mendeleev that is one of the chemist  who arranged the periodic table which has been used up to till now.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.

The answers is a nuclear changes