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The mass number is a fundamental property of an atom that plays a crucial role in understanding its atomic structure. It is the total number of protons and neutrons found in an atom's nucleus. This value gives insight into the atom's overall mass since both protons and neutrons contribute significantly to the atomic weight. An easy way to remember is:

• Mass Number (A): A = Number of Protons + Number of Neutrons

To put it into perspective, consider the element plutonium in the isotope notation \(^{239}Pu\). Here, 239 represents the mass number. This means there are a total of 239 protons and neutrons combined in its nucleus.
When you know the mass number and the atomic number of an atom, you can quickly figure out the number of neutrons present, which we'll cover later.

The atomic number is a unique identifier for each element and is crucial for distinguishing one element from another. It denotes the number of protons present in the nucleus of an atom. In a neutral atom, the atomic number also equals the number of electrons orbiting the nucleus. For example:

• Atomic Number (Z): Z = Number of Protons

In the case of plutonium (Pu), the atomic number is 94, which means that it has 94 protons in its nucleus. This number is what makes each chemical element unique, as no two elements have the same number of protons.
The atomic number can often be found on the periodic table, giving you quick access to the number of protons, and thus, its identity. This property helps determine the chemical behavior of the atom, as the electrons, matching the number of protons, identify how an atom will interact with others in chemical reactions.

Calculating the number of neutrons in an atom is straightforward once you know the mass number and the atomic number. Neutrons play an essential role in adding mass to the atom and stabilizing the nucleus. The equation to find the number of neutrons is:

• Neutrons (N): N = Mass Number (A) - Atomic Number (Z)

For plutonium with a mass number of 239 and an atomic number of 94, the calculation would be:
\[ N = 239 - 94 \]
This results in 145 neutrons. The calculation reveals the internal composition of the atom and helps understand isotopic differences between atoms of the same element, which can have different numbers of neutrons resulting in different mass numbers.