What is the significance of atomic number in the periodic table?

The atomic number is fundamental to understanding the behavior and identity of elements in the periodic table. It specifically denotes the number of protons present in the nucleus of an atom. Since each element has a unique atomic number, it serves as a defining characteristic that differentiates one element from another. The number of protons also determines the element's chemical properties, as protons influence the arrangement of electrons in atomic orbitals, which ultimately influences how an element interacts with others in chemical reactions.

Understanding the atomic number is essential because it not only helps in identifying the element but also plays a critical role in determining the element's position in the periodic table and its relationships with other elements. This organizational structure is built logically upon the increasing atomic number, allowing for insights into periodic trends such as electronegativity, atomic radius, and ionization energy.

While the other options might hint at aspects related to atomic structure or properties, they do not accurately encapsulate the atomic number's primary significance in the context of the periodic table. For instance, the number of electrons usually equals the atomic number in a neutral atom, but the atomic number itself is specifically about protons. Similarly, relative mass pertains to isotopes and averages, which are not directly defined by atomic number.