TeXes Science Practice Test

What is the primary basis for the arrangement of the modern Periodic Table?

1. Chemical reactivity

2. Molecular weight

3. Atomic number

4. Electron affinity

The correct answer is: Atomic number

The modern Periodic Table is primarily arranged based on atomic number, which is the number of protons found in the nucleus of an atom of an element. This arrangement reflects the periodic law, which states that the properties of elements are a periodic function of their atomic numbers. As you move across the Periodic Table, the atomic number increases sequentially, resulting in a systematic organization of elements that share similar chemical properties and behaviors. This arrangement based on atomic number has significant implications for predicting chemical reactivity and properties. For example, elements in the same group (column) of the Periodic Table exhibit similar properties because they have similar electron configurations, particularly in their valence shells. This organization allows chemists to foresee trends in reactivity, ionization energy, electronegativity, and other characteristics based on an element's position in the table. While molecular weight, chemical reactivity, and electron affinity are important properties of elements, the table's primary structure relies on atomic number. Molecular weight can vary among isotopes of the same element, leading to inconsistencies if used as a primary basis for arrangement. Similarly, chemical reactivity and electron affinity may not reflect a consistent trend throughout the table as effectively as atomic number does. Thus, atomic number serves