Ontario Grade 9 Science Class Practice Exam 2025 – Your Complete Guide to Exam Success!

The periodic table organizes elements by increasing atomic number, which refers to the number of protons in the nucleus of an atom. This organization reflects the underlying structure of atoms, as each successive element has one more proton than the element before it. Additionally, elements are grouped by similar chemical and physical properties. For instance, elements in the same column, or group, often share similar characteristics, such as valence electron configuration, which significantly influences their reactivity and bonding behavior. This systematic arrangement allows for easier prediction of element behaviors based on their position in the table.

The other options do not accurately capture the essence of how the periodic table is structured. While atomic mass was historically used for organization, modern periodic tables are primarily arranged by atomic number. Grouping solely by the number of electrons or reactivity alone does not encompass the broader systematic organization seen in the periodic table.