Democritus first theorised that everything is composed of atoms which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size.
Aristotle believed that there were only five elements: air which was light, earth which was cool and heavy, water which was wet, fire which was hot, and Aether which he viewed as a divine substance which made up the stars and planets. Aristotle believed that all matter was made up either of one of the elements of water air earth and fire or combinations of them, with the exception of stars and planets which were made of aether. However, this was a negative contribution because you only need to look at the periodic table to know how many elements more there is than just five.
Sometimes called the Father of Chemistry, Boyle mainly worked with gases. Whilst he did discuss the possibility of atoms existing, he was mainly stopped by the Church. Unlike the greek philosophers, he was doing physical experiments. From his expiraments he concluded that gases are made up of tiny particles that group together to make different substances.
Dalton proposed an “atomic theory”, with spherical solid atoms based upon measurable properties of mass. He claimed the reason elements combined was because all elements are made up of atoms. Finally, Dalton published a three part theory, which said that all particles were made up of atoms and were therefore indestructible, that atoms of the same elements are identical diferent element's atoms are different, and finally that atoms join with other substances to create new and different substances. This means that we were starting to progress towards the modern conception of atoms, especially the latter part of his three part theory.
Goldstein used a CRT to help discover “canal rays”, which, having completely opposite electrical and magnetic properties to an electron, were beams of positive ions. Importantly, Goldstein discovered protons as well as positive ions. Goldstein concluded that in addition to the electrons, or cathode rays, that travel from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction, from the anode toward the cathode, and these were positive ions. This contributed HUGELY to the Atomic Structure as we see it today, because if we didn’t know about protons, frankly we’d be lost.
Becquerel is credited with the discovery of radioactivity, when he exposed a crystal that contained uranium to the Sun. After the crystal had soaked up some sunshine, Antoine Becquerel placed it on a photographic plate. The uranium crystal imprinted its image on to the photographic plate, leading Becquerel to the conclusion that the uranium was releasing the absorbed energy of the sun in the form of an x-ray. However, it also imprinted onto the photographic plate even without sunlight, and this greatly surprised Becquerel since there was no energy involved. He assumed the image was caused by a spontaneous emission from the uranium, but he had in fact discovered radioactivity.
Thomson used a CRT to determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram. Thomson also studied "canal rays" and found they were associated with the proton H +. Finally, Thomson inferred that there are small particles within each atom, which proved Dalton’s theory wrong, in that atoms CAN, in fact, be divided.
Curie discovered the elements radium and polonium, as well as studied uranium and thorium. Curie ended up calling their spontaneous decay process "radioactivity”. Interestingly, Marie was prohibited from Higher Education in her native Poland, and ended up studying in France.
Moseley is credited with the ordering of elements in the periodic table through “atomic numbers”, or the number of protons in the nucleus. This system of ordering elements became widely favoured among chemists and is now, mostly, the sole system of ordering elements.
Between 1908 and 1913, Hans Geiger and Ernest Marsden conducted a series of experiments called the Rutherford gold experiments. The outcome of these experiments was that the nucleus is positively charged, and that the atom consists of mainly empty space, save a cloud of electrons, with all the positive charge being centred in the nucleus. This largely created the image of the Atomic Structure we see today.
Rutherford directed the aforementioned Geiger-Marsden, or Rutherford gold experiments. He concluded that Thomsons “plum pudding” atomic model was wrong. He thought if the particles were soft as Thomson's plum pudding model had suggested than they would pass Through and continue in a straight line, which most did.
Bohr suggested that electrons travel around the nucleus in definite paths, and that these paths are a certain “level” away from the nucleus, thus creating the idea of energy levels within an atom. This hugely contributed to the way we see atoms.
Chadwick famously discovered the neutron, when he discovered a particle with a mass close to a proton, that had no charge whatsoever. This discovery now shows that atoms have no overall charge, that only ions do, and thus bringing us to how we see the atom today.