**Correct Option: A. Atoms are the smallest units of matter that retain the properties of an element.** ### Detailed Explanation: 1. **Understanding Atoms:** - An atom is the fundamental building block of matter. It is the smallest unit of an element that retains all the chemical properties of that element. For example, a single carbon atom has the properties of carbon, such as its ability to form four bonds with other atoms. 2. **Structure of an Atom:** - Atoms consist of three main subatomic particles: protons, neutrons, and electrons. - **Protons** are positively charged particles found in the nucleus (the center) of the atom. - **Neutrons** are neutral particles (no charge) also located in the nucleus. - **Electrons** are negatively charged particles that orbit the nucleus in various energy levels or shells. - The number of protons in an atom defines the element (e.g., hydrogen has 1 proton, carbon has 6 protons). 3. **Why Option A is Correct:** - Option A accurately describes the essence of an atom. It emphasizes that atoms are the smallest units of matter that still exhibit the characteristics of the element they represent. This is crucial because it highlights the role of atoms in chemistry and physics, where the behavior of substances is fundamentally linked to their atomic structure. ### Why the Other Options are Incorrect: - **Option B: Atoms are composed solely of protons and electrons.** - This statement is incorrect because it omits neutrons, which are a critical component of most atoms. While protons and electrons are essential for defining the charge and chemical behavior of an atom, neutrons play a vital role in the stability of the nucleus. For example, carbon has 6 protons and 6 neutrons, and its stability is due to the presence of neutrons. - **Option C: Atoms cannot combine to form molecules.** - This statement is false. Atoms can and do combine to form molecules through chemical bonds. For instance, two hydrogen atoms can bond with one oxygen atom to form water (H₂O). This process is fundamental to chemistry and the formation of compounds. - **Option D: Atoms are always in a fixed position and do not move.** - This statement is misleading. While atoms in a solid may have limited movement (vibrating in place), atoms in liquids and gases are in constant motion. The kinetic theory of matter explains that the particles in liquids and gases move freely, which is essential for understanding states of matter and temperature effects. ### Formulas and Concepts: - **Atomic Structure:** - The atomic number (Z) is the number of protons in the nucleus and defines the element. - The mass number (A) is the total number of protons and neutrons in the nucleus (A = Z + N, where N is the number of neutrons). - **Chemical Bonds:** - Atoms combine through ionic or covalent bonds to form molecules. For example: - **Covalent Bonding:** Sharing of electrons (e.g., H₂O). - **Ionic Bonding:** Transfer of electrons (e.g., NaCl). ### Common Pitfalls: - Confusing the roles of protons, neutrons, and electrons. - Misunderstanding the concept of atomic movement in different states of matter. - Overlooking the importance of atomic combinations in forming compounds. ### Revision Summary: - Atoms are the smallest units of matter that retain the properties of an element. - Atoms consist of protons, neutrons, and electrons, with protons defining the element. - Atoms can combine to form molecules through chemical bonds. - The movement of atoms varies with the state of matter (solid, liquid, gas).