The correct option is **A. 1 proton only**. ### Detailed Explanation: 1. **Understanding the Structure of a Hydrogen Atom**: - A hydrogen atom is the simplest and most abundant element in the universe. It consists of one proton in its nucleus and one electron orbiting around that nucleus. - The nucleus of an atom is the central part that contains protons and neutrons. Protons are positively charged particles, while neutrons are neutral (having no charge). 2. **Composition of Hydrogen**: - Hydrogen has an atomic number of 1, which means it has one proton in its nucleus. This is a defining characteristic of hydrogen. - In its most common isotope (protium), hydrogen does not have any neutrons. There are other isotopes of hydrogen, such as deuterium (which has one neutron) and tritium (which has two neutrons), but these are less common and not what is being asked in this question. 3. **Why Option A is Correct**: - Since the question specifically asks about the nucleus of a hydrogen atom, and the most common form of hydrogen (protium) has only one proton and no neutrons, option A is the correct answer. The nucleus of hydrogen consists solely of that one proton. ### Why the Other Options are Incorrect: - **Option B: 1 proton, 2 neutrons**: - This option describes a hypothetical nucleus that does not exist for hydrogen. The presence of two neutrons would make it a heavier isotope, which is not the case for the most common hydrogen isotope (protium). This configuration would actually correspond to a different element or isotope, not hydrogen. - **Option C: 1 neutron only**: - This option is incorrect because hydrogen does not have a neutron in its most common form (protium). While deuterium, another isotope of hydrogen, has one neutron, the question is specifically about the nucleus of a hydrogen atom, which is typically understood to refer to the most common isotope. - **Option D: 1 electron only**: - This option is incorrect because electrons are not part of the nucleus. Electrons are negatively charged particles that orbit the nucleus of an atom. The nucleus itself is made up of protons and neutrons only. ### Summary of Key Points: - The nucleus of a hydrogen atom consists of **1 proton only**. - Hydrogen is the simplest element with an atomic number of 1, indicating one proton and no neutrons in its most common form. - Other isotopes of hydrogen exist (like deuterium and tritium), but they are not the focus of this question. - Electrons are not part of the nucleus; they orbit around it. This understanding of atomic structure is fundamental in chemistry and helps in grasping more complex concepts in atomic theory and chemical bonding.