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

In the Periodic Table, elements in the same group have the same number of what?

1. Neutrons

2. Protons

3. Electrons in the outer shell

4. Nuclei

The correct answer is: Electrons in the outer shell

Elements in the same group of the Periodic Table share similar chemical properties primarily because they have the same number of electrons in their outer shell, also referred to as valence electrons. This similarity is critical because the outer shell electrons determine how an element interacts with others, influencing its reactivity and the types of bonds it can form. For example, elements in Group 1 (alkali metals) all have one valence electron, which makes them highly reactive and gives them similar characteristics, such as the tendency to readily lose that electron and form positive ions. In contrast, having the same number of neutrons or protons would not apply consistently across a group since different elements have different atomic masses and identities. The nucleus of an atom (containing protons and neutrons) also does not dictate the similarities seen in chemical behavior within a group. Hence, the correct answer points to the electrons in the outer shell as the defining feature that leads to shared properties among group members.