How are elements arranged in the Periodic Table of Elements?

In the Periodic Table of Elements, elements are primarily arranged by the number of protons in the nucleus of their atoms, which is referred to as the atomic number. Each element's atomic number determines its position in the table, leading to a systematic arrangement of elements by their increasing atomic number from left to right.

While the arrangement by atomic weight historically influenced the table's development, the modern Periodic Table is structured around atomic number, making the choice about electron shells relevant. Elements in the same row generally share the same number of electron shells, which contributes to their chemical properties and reactivity.

Other factors like chemical reactivity and physical state are important in categorizing elements but are not the primary basis for the arrangement within the table. Chemical reactivity varies widely across different groups and periods, and elements can exist in different physical states (solid, liquid, gas) without a specific correlation to their placement in the Periodic Table. Thus, the arrangement by the number of electron shells underlines the table's organization based on atomic structure and properties.