ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

The atomic number of an element represents the number of protons found in the nucleus of an atom of that element. This fundamental property uniquely identifies an element, as each element has a distinct number of protons. For instance, hydrogen has an atomic number of 1, indicating it has one proton, while carbon has an atomic number of 6, corresponding to six protons.

The atomic number is crucial for understanding the arrangement of elements in the periodic table, where elements are listed in order of increasing atomic number. This organization reflects the periodic trends and chemical properties associated with specific numbers of protons.

In addition to protons, the atomic number also indirectly provides information about the number of electrons in a neutral atom. In a neutral state, the number of electrons equals the number of protons, but this does not define the atomic number. Thus, while the atomic number relates to electrons, it is specifically defined by protons.

Mass is unrelated to the atomic number because it also considers neutrons and determines the atomic mass or isotopic mass of the atom. Neutrons, while contributing to the mass of an atom, do not influence the atomic number. Therefore, understanding the atomic number is essential for grasping the identity and classification of elements in