The atomic number of materials in the periodic table is based on the number of protons rather than the number of electrons because it is the protons that determine the fundamental properties of an element. The atomic number represents the number of protons in the nucleus of an atom. Since protons have a positive charge and electrons have a negative charge, the number of protons determines the overall charge of the nucleus, which in turn determines the element's chemical properties and its position in the periodic table.
In a neutral atom, the number of protons is balanced by the number of electrons surrounding the nucleus. This balance of positive and negative charges ensures electrical neutrality. While the number of electrons in an atom can vary, the number of protons is constant for a given element. Changing the number of protons would result in a different element altogether.
The arrangement of elements in the periodic table is based on their atomic numbers because elements with similar atomic numbers generally exhibit similar chemical properties. The periodic table is organized in a way that groups elements with similar properties together, making it a useful tool for understanding and predicting the behavior of different elements.
In summary, the atomic number, determined by the number of protons, is used to classify and arrange elements in the periodic table because it reflects the essential characteristic that defines each element's identity and properties.