International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

How are the elements in the Periodic Table organized?

1. By increasing atomic mass

2. By increasing atomic number

3. By decreasing atomic radius

4. By their chemical properties

The correct answer is: By increasing atomic number

The elements in the Periodic Table are organized by increasing atomic number, which is the number of protons in the nucleus of an atom. This arrangement allows for a systematic classification of the elements, highlighting their relationships and similarities. As you move across a period from left to right, the atomic number increases sequentially, leading to a progressive change in properties. Organizing the table by atomic number rather than atomic mass provides a clearer understanding of the trends in elemental properties, such as reactivity and electronegativity. For example, elements with similar chemical properties are grouped together in columns known as groups or families, indicating that they have similar outer shell electron configurations. This method of organization also resolves discrepancies that arose when elements were arranged by atomic mass, such as with elements like iodine and tellurium. The correct placement based on atomic number reflects both the underlying structure of atoms and the periodic law, which states that the chemical properties of elements are a periodic function of their atomic numbers.