Final answer:
Atoms are not indivisible as believed in early atomic theory; they consist of smaller particles like protons, neutrons, and electrons. Discoveries during the nuclear age led to an understanding of isotopes and more subatomic particles, reshaping our conception of matter and nuclear physics.
Step-by-step explanation:
The particle referred to in the student's question is the atom, which was once thought to be indivisible. During the nuclear age, it was discovered that the atom can actually be subdivided into smaller parts, including protons, neutrons, and electrons. This marked a significant shift in our understanding of atomic theory, with key developments like the identification of electrons and the later discovery of the neutron by James Chadwick in 1932.
Our comprehension of atomic structure has also illuminated our understanding of isotopes, which are atoms of the same element that differ in mass due to varying numbers of neutrons. Additionally, advances in technology and physics during the 1920s and beyond have led to the discovery of other subatomic particles such as pions, neutrinos, and quarks. These discoveries have revised the principles of Dalton's atomic theory and led to a deeper understanding of nuclear reactions, which do not adhere to conservation laws in the same way as chemical reactions.
The nuclear age has not only reshaped our view of the atom but also provided insights into the origins and behavior of the universe, indicating that everything was once condensed together at the beginning of time. Basic physical principles still apply, such that as the universe expands, it cools in a similar fashion to a gas expanding from an aerosol can.