Ace the Ontario Grade 9 Science Exam 2025 – Get Ready to Rock Your Science Socks Off!

An atom is defined as the smallest unit of an element that retains its unique properties. This means that each element on the periodic table is made up of atoms, and these atoms are the fundamental building blocks that characterize that element. When you break down an atom into subatomic particles (like protons, neutrons, and electrons), it no longer retains the distinct properties of the element itself; thus, the atom remains the fundamental unit that defines an element's chemical identity.

While the concept of compounds and molecules is important in chemistry, an atom isn't classified as a unit of a compound nor simply a building block for molecules; rather, molecules are combinations of atoms. The largest stable particle notion also does not pertain to atoms, as that would imply either subatomic or more complex structures that do not exhibit the properties of a basic element. Therefore, understanding that an atom uniquely embodies the characteristics of an element is crucial for grasping the foundational concepts of chemistry.