Elements in the Chart of Elements are classified by what criterion?

The classification of elements in the Chart of Elements, commonly known as the periodic table, is based on the number of protons found in the nucleus of an atom. This fundamental characteristic is referred to as the atomic number. Elements are arranged in order of increasing atomic number, which allows for the organization of elements in a way that reflects their chemical properties and behaviors.

The arrangement by atomic number leads to periodic trends, such as similar reactivity among groups of elements, which is crucial for understanding chemical reactions and bonding. For instance, elements in the same column or group exhibit similar properties due to having the same number of valence electrons, which are influenced by their atomic structure defined by the number of protons.

While other factors such as atomic mass and the number of neutrons can also provide information about elements and their isotopes, they do not determine the identity or classification of elements in the same systematic way that atomic number does. Similarly, energy levels and electron configurations relate to the arrangement of electrons but are secondary to the primary classification by protons, making atomic number the foundational criteria in the periodic table.