The correct option is **A. Actinoids**. ### Detailed Explanation: 1. **Understanding the Context**: The question refers to a specific group of elements in the periodic table. The elements with atomic numbers 90 to 103 are located in the f-block of the periodic table, which is where the actinides and lanthanides are found. 2. **Definition of Actinoids**: The term "actinoids" (or "actinides") refers to the series of 15 elements that follow actinium (Ac, atomic number 89) in the periodic table. These elements include thorium (Th, 90), protactinium (Pa, 91), uranium (U, 92), neptunium (Np, 93), plutonium (Pu, 94), americium (Am, 95), curium (Cm, 96), berkelium (Bk, 97), californium (Cf, 98), einsteinium (Es, 99), fermium (Fm, 100), mendelevium (Md, 101), nobelium (No, 102), and lawrencium (Lr, 103). 3. **Why Option A is Correct**: The actinoids are characterized by their similar properties, which include being radioactive and having a high atomic number. They are all f-block elements, meaning they have electrons filling the 5f subshell. This group is significant in nuclear chemistry and has applications in various fields, including energy production and medical treatments. ### Why the Other Options are Incorrect: - **Option B: Nucleotides**: This term refers to the basic building blocks of nucleic acids (DNA and RNA), which are composed of a sugar, a phosphate group, and a nitrogenous base. Nucleotides are not elements and are unrelated to the periodic table or the classification of elements. - **Option C: Pre-actinum elements**: This term is not a recognized classification in chemistry. The elements before actinium (atomic numbers 1-89) include a variety of elements from different groups in the periodic table, and they do not form a coherent group like the actinoids. - **Option D: Atomic elements**: This term is too vague and does not specifically refer to any group of elements. All elements in the periodic table can be considered "atomic elements," as they are defined by their atomic structure. This option does not provide any meaningful classification. ### Common Pitfalls: - Confusing actinoids with lanthanides: The lanthanides are the series of elements from atomic number 57 (lanthanum) to 71 (lutetium), which are also f-block elements but are distinct from the actinoids. - Misunderstanding the terminology: Terms like "nucleotides" and "atomic elements" can be misleading if one is not familiar with their definitions in the context of chemistry. ### Revision Summary: - The elements with atomic numbers 90-103 are called **actinoids**. - Actinoids are characterized by their radioactive properties and filling of the 5f subshell. - Other options like "nucleotides," "pre-actinum elements," and "atomic elements" do not accurately describe this group of elements. - Understanding the classification of elements in the periodic table is crucial for chemistry studies.