![]() ![]() ![]() AD 1100) onward, many chemical reactions were studied. Aristotle ’s teachings against the idea of Democritus ’s atom were so powerful that the idea of the atom fell out of philosophical fashion for the next 2,000 years.Īlthough atomic theory was abandoned for this long period, scientific experimentation, especially in chemistry, flourished. Aristotle said there were only four elements ( earth, air, fire, water) and that these had some smallest unit that made up all matter. Aristotle, one of the most influential philosophers of that time, believed in some kind of “smallest part ” of matter, but did not believe these parts followed Democritus ’s description. In his theory, different objects looked different because of the way the atoms were arranged. ” The atoms Democritus envisioned differed only in shape and size. The word atomos has been changed in modern times to “atom. This tiniest building block that could no longer be cut he named atomos, Greek for “no-cut ” (indivisible). He imagined starting with a large piece of matter and gradually cutting it into smaller and smaller pieces, finally reaching a smallest possible piece. One of these philosophers was named Democritus. ![]() If they could see small enough things, they would find that the same “building blocks ” they started with were still there. Still others believed that whatever comprised matter, it must be something that could not be destroyed but only recombined into new forms. Others believed that matter was made entirely of fire in ever-changing forms. Some said everything was made of water, which comes in three forms (solid ice, liquid water, and gaseous steam). Historyīeginning in about 600 BC, many Greek philosophers struggled to understand the nature of matter. Quarks are elementary because quarks cannot be broken down any further.Atomic theory is the description of atoms, the smallest units of elements. Protons and neutrons are also not elementary particles because they are made up of even smaller particles called quarks joined together by other particles called gluons (because they "glue" the quarks together in the atom). Modern physicsĪtoms are not elementary particles, because they are made of subatomic particles like protons and neutrons. Some idea of present-day atomic physics can be found in the links in the table below. Although this model was well understood, modern physics has developed further, and present-day ideas cannot be made easy to understand. Isotopes vary in the number of neutrons present in the nucleus. This experiment was called the Geiger–Marsden experiment or the Gold Foil Experimentīy this stage the main elements of the atom were clear, plus the discovery that atoms of an element may occur in isotopes. Rutherford showed this when he used an alpha radiation source (from helium) to hit the very thin gold sheets, surrounded by a Zinc sulphide lampshade that produced visible light when hit by alpha emissions. In 1910, the New Zealand physicist Ernest Rutherford put forward the idea that the positive charges of the atom were found mostly in its center, in the nucleus, and the electrons ( e-) around it. Lord Ernest Rutherford later named these positively charged particles protons Rutherford's atomic modelĪtomic experiment of Lord Ernest Rutherford ![]() In the same time, experiments by Eugene Goldstein in 1886 with cathode discharge tubes allowed him to establish that the positive charges had a mass of 1.6726 * 10 −27 kg and an electrical charge of +1,6 * 10 −19 C. In 1906, Robert Millikan determined that the electrons had a Coulomb (C) charge of -1.6 * 10 −19, something that allowed calculation of its mass as tiny, equal to 9.109 * 10 −31 kg. The negative charges were named electrons ( e-).Īccording to the assumptions established about the atoms neutral charge, Thomson proposed the first atomic model, that was described as a positively-charged sphere in which the electrons were inlaid (with negative charges). Thomson knew that the atoms were electrically neutral, but he established that, for this to occur, an atom should have the same quantity of negative and positive charges. Crookes named the emission ' cathode rays'.Īfter the cathode ray experiments, Sir Joseph John Thomson established that the emitted ray was formed by negative charges, because they were attracted by the positive pole. When creating a vacuum in the tube, a light discharge can be seen that goes from the cathode (negatively-charged electrode) to the anode (positively-charged electrode). In 1850, Sir William Crookes constructed a ' discharge tube', that is a glass tube with the air removed and metallic electrodes at its ends, connected to a high voltage source. Schematic representation of the Thomson model. ![]()
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