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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Rubidium Ion (Rb+)
Rubidium (Rb) is an alkali metal with an atomic number of 37, meaning it has 37 protons and, in its neutral state, 37 electrons. The Rb+ ion indicates that it has lost one electron, resulting in a total of 37 protons and 36 electrons. This loss of an electron gives Rb+ its positive charge.
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