Final answer:
Atoms contain neutrons, with the exception of most hydrogen atoms. Observations supporting this include the existence of isotopes with the same number of protons but varying numbers of neutrons, and the ability to calculate the number of neutrons by subtracting the atomic number from the mass number.
Step-by-step explanation:
Observations that indicate an atom has neutrons include the existence of isotopes, which are variants of a chemical element that have the same number of protons (atomic number) but a different number of neutrons, and therefore, different mass numbers. Neutrons are electrically neutral particles in the nucleus of an atom, with a mass slightly greater than that of protons. The presence of neutrons in an atom is evident when its mass number, which is the sum of protons and neutrons, is greater than the atomic number, which represents only the number of protons.
Isotopes are a clear indication of the presence of neutrons in an atom since they arise due to the difference in the number of neutrons while maintaining the same atomic number. For example, naturally occurring carbon has isotopes with six, seven, or eight neutrons, while always having six protons. By subtracting the atomic number from the mass number, the presence and quantity of neutrons in an atom can be determined. This reflects the principle that the nucleus of an element usually contains an equal or greater number of neutrons than protons, the latter being signified by the element's atomic number.