The correct option is **B**: An atom is the smallest unit of matter that retains the properties of an element, whereas a molecule is formed when two or more atoms bond together. ### Detailed Explanation 1. **Understanding Atoms**: - An atom is the fundamental building block of matter. It is the smallest unit of an element that retains the chemical properties of that element. For example, a single carbon atom (C) has the properties of carbon, such as its ability to form four bonds with other atoms. - Atoms consist of three main subatomic particles: protons (positively charged), neutrons (neutral), and electrons (negatively charged). The number of protons in the nucleus of an atom determines the element (e.g., hydrogen has 1 proton, carbon has 6 protons). 2. **Understanding Molecules**: - A molecule is formed when two or more atoms bond together. These atoms can be of the same element (like O₂, which is a molecule of oxygen) or different elements (like H₂O, which is a water molecule made of hydrogen and oxygen). - The bonds that hold the atoms together in a molecule can be covalent (where atoms share electrons) or ionic (where electrons are transferred from one atom to another). 3. **Why Option B is Correct**: - Option B accurately describes the relationship between atoms and molecules. It highlights that atoms are the smallest units of elements, while molecules are combinations of these atoms. This distinction is fundamental in chemistry and physics, as it lays the groundwork for understanding chemical reactions, bonding, and the properties of substances. ### Why the Other Options are Incorrect - **Option A**: "An atom is made up of two or more elements, while a molecule consists of a single element." - This statement is incorrect because it reverses the definitions. An atom is not made up of multiple elements; it is a single unit of one element. A molecule can consist of multiple atoms, which can be of the same or different elements. - **Option C**: "An atom cannot exist independently, but a molecule can exist alone." - This statement is misleading. Atoms can exist independently (e.g., noble gases like helium and neon exist as single atoms in nature). Molecules can also exist independently, but the statement incorrectly implies that atoms cannot exist alone, which is not true. - **Option D**: "An atom contains electrons only, while a molecule contains protons and neutrons." - This statement is incorrect because it misrepresents the structure of atoms and molecules. Atoms contain protons, neutrons, and electrons. Molecules are made up of atoms, which include protons and neutrons in their nuclei, along with electrons in their outer shells. ### Summary of Key Points - An atom is the smallest unit of an element, retaining its properties. - A molecule is formed when two or more atoms bond together. - Atoms can exist independently, while molecules can consist of different or the same types of atoms. - Understanding the distinction between atoms and molecules is crucial for studying chemistry and physics. This thorough understanding of the differences between atoms and molecules is essential for grasping more complex concepts in chemistry and physics, such as chemical reactions, bonding, and the behavior of matter.