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Understanding the atomic number is crucial to comprehend the identity of an element. In essence, the atomic number is the number that represents the amount of protons in the nucleus of an atom. Every atom of a given element has the same atomic number, which is why it's so important: it defines the element. For example, the atomic number of selenium is 34. This unique identifier tells us that every selenium atom contains 34 protons in its nucleus.

As students learn about nuclear composition, recognizing atomic numbers on the periodic table allows them to determine not only the number of protons but also the chemical behavior of the element. These atomic numbers are not arbitrary; they follow a sequential order, starting from hydrogen with atomic number 1 and increasing as you move across and down the periodic table.

Neutrons are one of the three main particles that make up an atom, along with protons and electrons. They reside in the nucleus alongside protons and play a significant role in the stability of atoms. Contrary to protons, neutrons carry no electrical charge, hence their name, neutral. The number of neutrons within an isotope's nucleus can be calculated using the mass number and the atomic number of the element.

In the case of Selenium-75, the mass number is 75 which is the total number of protons and neutrons together. Given the atomic number of selenium is 34 (indicating 34 protons), subtracting it from the mass number gives us the number of neutrons: 75 - 34 = 41 neutrons. This subtraction is simple but vital for understanding atomic structure and nuclide stability, as different numbers of neutrons lead to various isotopes of the same element.

Electrons are subatomic particles with a negative charge that are found in the space surrounding an atom's nucleus. They are critical for chemical reactions because they engage in bonds with other atoms. In a neutral atom, the number of electrons is equal to the number of protons, balancing out the positive charge of the nucleus with their negative charge.

For a neutral Selenium-75 atom, with 34 protons in its nucleus, there are likewise 34 electrons. These surrounding electrons determine the atom's chemical properties and how it will interact with other atoms. It's also important to note that ions are atoms that have gained or lost electrons and, therefore, do not have the same number of protons and electrons, resulting in a net charge. For everyday chemistry, the behavior of these electrons is often the focus, as they are involved in forming chemical bonds and absorb or release energy during chemical reactions.