The correct option is **B. The number of protons in the nucleus of an atom**. ### Detailed Explanation: 1. **Understanding Atomic Number**: - The atomic number is a fundamental property of an element that defines its identity. It is denoted by the symbol \( Z \). - The atomic number is specifically defined as the number of protons found in the nucleus of an atom. For example, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 because it has six protons. 2. **Role in the Periodic Table**: - The periodic table is organized primarily by atomic number. Elements are arranged in order of increasing atomic number from left to right and top to bottom. - This arrangement reflects the periodic law, which states that the properties of elements are a periodic function of their atomic numbers. 3. **Why Option B is Correct**: - Since the atomic number is defined as the number of protons, option B accurately describes this concept. The number of protons determines the element's chemical properties and its place in the periodic table. ### Analysis of Other Options: - **Option A: The total number of protons and neutrons in an atom**: - This option describes the **mass number** (denoted as \( A \)), not the atomic number. The mass number is the sum of protons and neutrons in the nucleus and is not used to define the identity of an element in the periodic table. - **Option C: The average mass of an element's isotopes**: - This option refers to the **atomic mass** or **atomic weight**, which is a weighted average of the masses of an element's isotopes based on their natural abundance. It does not relate to the atomic number. - **Option D: The number of electrons in a neutral atom**: - While a neutral atom has the same number of electrons as protons, this does not define the atomic number. The atomic number specifically refers to protons, and the number of electrons can vary in ions (charged atoms). ### Summary of Key Points: - The atomic number (Z) is the number of protons in an atom's nucleus. - It determines the element's identity and its position in the periodic table. - The mass number is the sum of protons and neutrons, while atomic mass is the average mass of isotopes. - The number of electrons in a neutral atom equals the number of protons but does not define the atomic number. ### Revision Summary: - **Atomic Number (Z)**: Number of protons in the nucleus of an atom. - **Periodic Table Organization**: Elements are arranged by increasing atomic number. - **Mass Number (A)**: Total number of protons and neutrons; not the same as atomic number. - **Atomic Mass**: Average mass of an element's isotopes; distinct from atomic number.