English chemist John Dalton developed a more scientific definition of the atom. He stated that atoms of a given element are identical to each other.
English physicist Joseph John Thomson discovered that the atom wasn’t as indivisible as stated by the Greeks. He also discovered that atoms contain electrons. He produced the plum pudding of an atom model
Ernest Rutherford was a physicist from New Zealand and he proposed a model where the electrons orbit the positively charged nucleus. This was a breakthrough as it furthered Thompson's model
Bohr was a Danish physicist and his model suggested the existence of energy levels or shells of electrons around the nucleus.
Austrian physicist Erwin Schrödinger stated that rather than the electrons moving in fixed orbits or shells, the electrons would behave as waves around the nucleus.
English Physicist James Chadwick who ironically was a student of Ernest Rutherford. He discovered the existence of the neutron which finally completed the picture of the subatomic particles that make up an atom.