What is the definition of isomer?

The definition of an isomer pertains specifically to atoms or molecules that exist in different meta-stable energy states. This concept is especially relevant in nuclear chemistry, where an isomer may refer to the same chemical element but in different excited states. Isomers can have identical atomic numbers and mass numbers but differ in energy levels, which can lead to variations in stability and behavior. This property is crucial in fields such as radiopharmaceuticals and radioactive decay, where the isomeric state can significantly influence the application and safety measures required for handling the substance.

The other definitions provided do not accurately encapsulate the specific characteristics of isomers. Atoms with the same mass number and atomic numbers would simply refer to the same isotope, not capturing the essence of different energy states. Similarly, atoms with the same number of protons and neutrons would also imply isotopic similarities, without consideration for the meta-stable configurations that define isomerism. Lastly, the notion of belonging to different chemical groups refers to the chemical behavior and properties of elements rather than their energetic state. Isomers are primarily focused on variations in energy levels irrespective of the chemical classification.