Question

The mass number of common krypton is $84$ , and its atomic number is $36$ . Krypton $-84$ has how many neutrons within each isotope?

Answer 1

Hint: The word isotope is derived from the Greek words isos and topos, which both mean "the same place." Isotopes are different forms of the same element with the same number of protons but different numbers of neutrons in the atom.

Complete answer:
The mass number, $A$ , of a given isotope tells us the number of protons, $Z$ , which is given by the atomic number and neutrons it has in its nucleus.
 $A=Z+$ number of neutrons
In your case, krypton $-84$ is said to have a mass number equal to
 $A=84$
This means that the nucleus of a krypton $-84$ isotope contains a total of $84$ protons and neutrons.
As we know, the element has an atomic number equal to $36$ , it follows that it will contain $36$ protons in its nucleus. Therefore, the number of neutrons present in the nucleus of a krypton $-84$ isotope will be
Number of neutrons $=A-Z$
Number of neutrons $=84-36\Rightarrow 48$
Hence, the number of neutrons within each isotope is $48$ .

Additional Information:
Because they are obtained by counting whole items, atomic and mass numbers are always whole numbers (protons, neutrons, and electrons). The total number of subatomic particles contained in an atom ( $A-Z$ ) is equal to the sum of the mass number and the atomic number. The mass number indicates the atomic nucleus' mass in atomic mass units (amu).

Note:
Isotopes of elements frequently have different masses due to unequal quantities of neutrons. Elements with odd atomic numbers often have one or two stable isotopes, whereas elements with even atomic numbers typically have three or more stable isotopes. There are, however, some exceptions, such as carbon, helium, and beryllium.