Short answers, pulled from the story.
The word atom originates from the ancient Greek atomos meaning uncuttable and was proposed by Leucippus and Democritus in ancient Greece. This philosophical concept was entirely divorced from the scientific reality discovered centuries later.
John Dalton compiled experimental data in the early 1800s to prove matter was composed of discrete units. He observed that grey tin oxide contained 13.5 grams of oxygen for every 100 grams of tin while white tin oxide contained 27 grams of oxygen for the same amount of tin establishing a precise 1 to 2 ratio.
J. J. Thomson discovered the electron in 1897 when he found that cathode rays could be deflected by electric and magnetic fields. He measured these particles to be 1,700 times lighter than hydrogen proving that atoms contained smaller negatively charged constituents.
James Chadwick discovered the neutron in 1932 when he observed that beryllium emitted a highly penetrating electrically neutral radiation when bombarded with alpha particles. He deduced that this radiation was composed of electrically neutral particles with a mass similar to that of a proton confirming Rutherford's hypothesis.
The Earth itself contains approximately 10^50 atoms and the universe's first atoms formed during Big Bang nucleosynthesis in about three minutes. Electrons existed since the early stages of the Big Bang and atomic nuclei formed in nucleosynthesis reactions while the universe cooled enough for electrons to attach to nuclei 380,000 years after the Big Bang.
The smallest atom is helium with a radius of 32 picometers. One of the largest atoms is caesium at 225 picometers and a typical human hair is about 1 million carbon atoms in width.