Here are the essential concepts you must grasp in order to answer the question correctly.
Atomic Structure
Atoms consist of protons, neutrons, and electrons. Protons are positively charged particles found in the nucleus, while electrons are negatively charged and orbit the nucleus. The number of protons defines the element, and in a neutral atom, the number of electrons equals the number of protons.
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Ions
Ions are atoms or molecules that have gained or lost one or more electrons, resulting in a net charge. Cations are positively charged ions formed by losing electrons, while anions are negatively charged ions formed by gaining electrons. The charge of an ion indicates the difference between the number of protons and electrons.
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Gold Ion (Au3+)
The Au3+ ion indicates that a gold atom has lost three electrons, resulting in a +3 charge. Gold has an atomic number of 79, meaning it has 79 protons. Therefore, in the Au3+ ion, there are 79 protons and 76 electrons (79 protons - 3 electrons).
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Rutherford Gold Foil Experiment