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In the world of chemistry, the mass number is a key concept that helps us understand the structure of an atom. It represents the total number of particles in an atom's nucleus. These particles are called nucleons and include both protons and neutrons. The mass number is given by the sum:

• Mass Number = Number of Protons + Number of Neutrons

The mass number is an important number because it gives us insight into the weight and stability of an atom. Although electrons are present in atoms, their mass is very minimal compared to protons and neutrons, and thus they do not significantly affect the mass number. In the exercise, the known ion has a mass number of 27, meaning there is a combined total of 27 protons and neutrons in its nucleus.

Neutrons are subatomic particles located in the nucleus of an atom, having no electric charge. They play a crucial role in determining the mass number and stability of an atom. Neutrons help keep the nucleus tightly bound together by offsetting the repulsion between like-charged protons. In our example problem, the unknown ion has 14 neutrons, as we determined it to have the same number of neutrons as there are electrons in the \(\text{S}^{2+}\) ion.
Neutrons influence the isotopic identity of an element, meaning different numbers of neutrons in the nucleus result in different isotopes of the same element. However, in this exercise, we are concerned with how the neutron count helps determine the other characteristics of a given ion.

Protons are positively charged particles found in the nucleus of an atom. The number of protons defines the identity of an element, correlating directly with its atomic number. Every element in the periodic table has a unique number of protons, making this a reliable identifier. For example, an atom with 13 protons is uniquely recognized as aluminum since its atomic number is 13. In the exercise, we determine the number of protons in the unknown ion using the mass number rule again. Knowing the mass number (27) and the neutron count (14), the calculation:

• Number of Protons = Mass Number - Number of Neutrons = 27 - 14 = 13

helped us confirm its aluminum identity. Thus, the count and recognition of protons play a crucial part in element identification.

Electrons are negatively charged subatomic particles that orbit the nucleus in various energy levels or shells. The number of electrons in an atom determines many of its chemical behaviors and reactivity, especially in forming ions. In neutral atoms, electrons equal protons, ensuring the whole atom is charge balanced.
When ions form, they typically gain or lose electrons, leading to a net charge difference. For instance, a \(\text{Al}^{3+}\) ion indicates it has lost 3 electrons relative to its neutral state. Since a neutral aluminum atom has 13 electrons, this ion would have 10 electrons:

• Initial Electron Count = 13
• Ion Electron Count = 13 - 3 = 10

Thus, the number of electrons in ions helps in understanding their charge and the way they interact in chemical reactions.