Neils Henrik Bohr (1885-1962) was a Danish physicist. He made fundamental contributions to the understanding of the structure of the atom and to quantum mechanics. For his work in quantum mechanics he received the Nobel Prize in 1922. He later worked on the Manhattan Project that produced the first atomic weapon.
Bohr improved Ernest Rutherford’s hypothesis on the structure of the atom; in 1913 he published a paper showing that the electron circled the nucleus of the atom; he further showed the chemical properties of each element is largely determined by the number of electrons circling its nucleus; he also introduced the idea that electrons do drop from higher orbits to lower ones and in the process emitting energy, photon, (light quantum). This discovery became the basis of quantum mechanics.
Bohr introduced the idea of complementarity which states that the seeming contradictory properties of an item should be analyzed separately.
For example, particles have both wave and particular functions; particles should be analyzed from the perspective of particles and waves should be analyzed from the perspective of waves function.
Bohr introduced the idea of probabilistic function of particles; that is, that the location of particles cannot be predicted with absolute certainty; if you predict the location of particles in their wave function you cannot simultaneously do so for their particular function, and vice versa.
Bohr established what was called the Copenhagen school of Quantum Mechanics; a school that stressed the probabilistic nature of particles, a view vehemently rejected by the religion inclined Albert Einstein.
Bohr made other contributions to science but the above appear to be his most important ones.
Neils Bohr. On the Constitution of Atoms and Molecules. (1913)
Neils Bohr. Atomic Physics and Human Knowledge. (1958)