One isotope of a metallic element has a mass number of 65 and has 35 neutrons in the nucleus. The cation derived from the isotope has 28 electrons. Write the symbol for this cation.

When we talk about the mass number, we are referring to the total number of protons and neutrons in an atom's nucleus. It's an essential concept in understanding isotopes because, while the number of protons (atomic number) defines what element an atom is, the number of neutrons can vary. This is what forms different isotopes of the same element.

For example, an isotope with a mass number of 65 has a total of 65 protons and neutrons combined. The exact number of protons and neutrons can be found if either the atomic number or the number of neutrons is known. In this case, with 35 neutrons, you subtract from the mass number to find that there are 30 protons, which also gives you the atomic number of the element.

The atomic number is foundational to the organization of the periodic table. It's defined as the number of protons found in the nucleus of an atom and determines the chemical properties of an element and its place on the table. Each element has a unique atomic number, so once you know the atomic number, you can correctly identify the element.

For instance, an element with 30 protons has the atomic number 30. Using the periodic table, we can identify this element as Zinc (Zn). The atomic number is also significant because, in a neutral atom, it tells you the number of electrons orbiting the nucleus.

The periodic table is a marvel of science, organizing all known elements into a tabular display based on their atomic number, electron configurations, and recurring chemical properties. Elements are listed in order of increasing atomic number from left to right and are arranged in rows called periods and columns known as groups or families.

When solving problems involving elements and their ions, the periodic table becomes a crucial tool. It allows us to quickly determine an element's atomic number and its typical properties, such as reactivity or the formation of specific ions like cations or anions. In our example, we used the table to verify that the atomic number 30 corresponds to Zinc (Zn).

Cations are positively charged ions, meaning they have more protons than electrons. The symbol for a cation includes the element's symbol from the periodic table, followed by the charge in superscript. The charge is the difference between the number of protons and electrons.

As in our exercise, a Zinc (Zn) cation with 28 electrons (2 less than its atomic number of 30) will have a charge of +2, represented as the symbol with a superscript plus two, or in chemical notation: