Fill in the following table: $$ \begin{array}{lcccc} & { }_{8}^{15} \mathrm{O}^{+} & ? & ? & ? \\ \hline \text { Mass number } & ? & 27 & ? & 58 \\ \text { Atomic number } & ? & ? & 15 & ? \\ \text { Number of protons } & ? & 13 & ? & ? \\ \text { Number of neutrons } & ? & ? & 16 & 30 \\ \text { Number of electrons } & ? & ? & ? & 27 \\ \text { Charge on ion } & ? & +3 & -3 & +1 \\ \hline \end{array} $$

The mass number represents the total number of protons and neutrons in an atom's nucleus. It is denoted by the symbol A. Since protons and neutrons are approximately equal in mass and much heavier than electrons, the mass number closely approximates the atomic weight of an isotope. For example, in an isotope labeled as _{8}^{15}O, the mass number is 15, indicating the sum of protons and neutrons in the oxygen nucleus. This is key information when solving problems involving isotopic notation and understanding nuclear reactions.
For ions, the mass number remains unchanged even when the number of electrons varies, since only protons and neutrons contribute to the mass number. Remember, electrons have a negligible mass compared to protons and neutrons. Therefore, the number of electrons does not affect the mass number of an isotope or ion.

The atomic number, denoted as Z, is the number of protons in an atom's nucleus and defines the element. Each element on the periodic table has a unique atomic number. For instance, the atomic number of oxygen (O) is 8. The atomic number is crucial because it not only tells us the number of protons but also, in a neutral atom, the number of electrons. This is because an atom in its uncharged, or neutral, state will have an equal number of protons and electrons.
Even when an atom becomes an ion, the atomic number does not change; instead, the atom gains or loses electrons, altering its net charge. For example, an oxygen atom that loses an electron will become an O+ ion, with an atomic number of 8 and 7 electrons.

The number of protons in an atom is the defining feature of an element. This number is consistent for all atoms of a particular element and is the same as the atomic number (Z). Protons have a positive charge, and their number in the nucleus dictates the chemical properties of the atom.
For isotopes, which are different forms of the same element, the number of protons remains constant, while the number of neutrons can vary. For ions, the number of protons also does not change. For example, an oxygen atom and an oxygen ion (O+) both have 8 protons, reflecting the element's atomic number.

Neutrons, which are neutral particles residing in the atom's nucleus alongside protons, contribute to an atom's mass but not to its charge. The number of neutrons can vary among the isotopes of an element, which is why we have different mass numbers for isotopes. To calculate the number of neutrons (N), you subtract the atomic number (Z) from the mass number (A): N = A - Z.
For example, in an atom with a mass number of 27 and an atomic number of 13 (_{13}^{27}[Ion]), there are 14 neutrons. The ability to calculate the number of neutrons is fundamental for understanding the stability, radioactivity, and nuclear properties of isotopes.

Electrons are negatively charged particles found in the atom's electron cloud, which orbits the nucleus. In a neutral atom, the number of electrons equals the atomic number, since the charges balance out. When an atom gains or loses electrons, it becomes an ion, and its charge changes respectively. The net charge of an ion indicates whether electrons have been lost (positive charge) or gained (negative charge).
For instance, if an atom with an atomic number of 15 gains three electrons, it becomes a negatively charged ion with 18 electrons (_{15}^{31}[Ion]^{-3}). Understanding electron count is essential for studying chemical reactions and predicting how an atom will interact with others.

The charge on an ion represents the net electrical charge resulting from the gain or loss of electrons. A positive charge (+) indicates that the ion has more protons than electrons, while a negative charge (-) means more electrons than protons. The magnitude of the charge tells us exactly how many electrons have been lost or gained.
For example, an ion with a +3 charge has lost three electrons compared to its neutral state. In contrast, an ion with a -3 charge has gained three additional electrons. This concept is crucial for predicting the behavior of ions in chemical reactions, determining the formulas of compounds, and balancing chemical equations.