Atoms of the same element that possess a different number of neutrons are known as isotopes. For example, all carbon atoms have six protons, but some may have six neutrons (Carbon-12) while others have eight neutrons (Carbon-14). Scientists use a standardized shorthand, known as nuclear notation or the isotope symbol, to communicate the specific composition of any given atomic nucleus.
Decoding the Isotope Symbol
The isotope symbol is a standardized representation that places two numbers immediately to the left of the chemical element’s symbol. This notation follows the general format of A/Z X, where ‘X’ is the one- or two-letter chemical symbol for the element. The atomic number (Z) is located in the bottom-left position as a subscript directly beneath the mass number (A).
To illustrate, the most common form of uranium is written as 238/92 U. In this example, the number 92 occupies the bottom-left subscript position, identifying it as the atomic number. The number 238, positioned above it, represents the total mass of the nucleus.
The Identity Provided by the Atomic Number
The atomic number (Z) is arguably the most significant value in the isotope symbol because it defines the element itself. This number represents the count of protons found within the atom’s nucleus. Every atom of a specific element, whether it is carbon, oxygen, or gold, always contains the exact same number of protons. For instance, every atom with an atomic number of 6 is a carbon atom, and changing the number of protons immediately changes the element.
This number dictates the element’s behavior in chemical reactions, as it determines the number of electrons in a neutral atom. Therefore, the atomic number is the unique identifier that links the isotope symbol to its place on the periodic table.
How Mass Number Completes the Picture
The second number in the notation is the mass number (A), which is located in the top-left position as a superscript. The mass number represents the total count of both protons and neutrons combined within the nucleus. Unlike the atomic number (Z), the mass number (A) is what varies between different isotopes of the same element.
For example, Carbon-12 has a mass number of 12, while Carbon-14 has a mass number of 14, both of which are placed in the superscript position. The difference between the mass number and the atomic number reveals the count of neutrons in the nucleus.