Ontario Grade 9 Science Class Practice Exam

What is an atom?

1. The smallest unit of an element.

2. A device used to measure current.

3. A body that becomes a meteor upon entering Earth's atmosphere.

4. An element in Group 1 of the periodic table.

The correct answer is: The smallest unit of an element.

An atom is defined as the smallest unit of an element that retains all the chemical properties of that element. It consists of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. This fundamental structure is what makes up all matter in the universe. Since each element is characterized by its unique number of protons (the atomic number), the atom serves as the basic building block for all chemical substances. The other options do not accurately describe what an atom is. For instance, a device used to measure current is not related to the structure of matter, and bodies that become meteors upon entering Earth's atmosphere pertain to astronomical phenomena instead of atomic structure. Lastly, elements in Group 1 of the periodic table refer specifically to alkali metals, which are types of atoms, but the term "atom" encompasses all elements, not just those in a specific group.