Chemistry: Structure of the atom today we define an atom as the smallest particle of an element that retains the chemical properties of that element. J.J Thompson 1807 J.J THOMPSON is the responsible for the discovery of the electron. An electron is surrounding the nucleus, it’s a region occupied by negatively charged particles. It is connected to a vacuum pump because charges (Electrons) will flow through gases only at very low pressures. experiments were performed in which electric current was passed through various gases at low pressures. (Gases at atmospheric pressure don’t conduct electricity well.) These experiments were carried out in glass tubes that had been hooked up to a vacuum pump. Such tubes are known as cathode-ray tubes. Investigators noticed that when current was passed through the tube, the surface of the tube directly opposite the cathode glowed. They hypothesized that the glow was caused by a stream of particles, which they called a cathode ray. The ray traveled from the cathode to the anode when current was passed through the tube. J.J THOMPSON measure how much heat the cathode rays generated he estimate the mass of the ray produced which the mass was a thousand time lighter than hydrogen, the smallest bit of matter knows at the time, he concluded that they weren’t “rays” but tactually very light and small negatively charged particles. In one investigation, he was able to measure the ratio of the charge of cathode-ray particles to their mass. He found that this ratio was always the same, regardless of the metal used to make the cathode or the nature of the gas inside the cathode-ray tube. Thomson concluded that all cathode rays must be composed of identical negatively charged particles, which named electrons. Ernest Rutherford ERNEST RUTHERFOR is responsible for the discovery of the proton and the nucleus. While experimenting he used Alpha Particles, which are positively charged particles with about four times the mass of a hydrogen atom. The nucleus is a very small region located at the center of an atom. In every atom, the nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons. Ernest Rutherford design for the experiment used a thin gold sheet and a screen to detect the deflected particles. He bombarded the sheet with Alpha particles (which are positively charged and are produced by the decay of radium). He expected then alpha particle to go thru the foil with no deflection (that mass and charge were uniformly distributed throughout the atoms of the gold foil) and in many try they did just that, BUT in one occasion roughly 1 in 8000 of the alpha particles had actually been deflected back toward the source. He concluded that the force must be caused by a very densely packed bundle of matter with a positive electric charge. Rutherford called this positive bundle of matter the nucleus. Therefore, if alpha particles a positive that means that the nucleus must contain another subatomic particle that has a positive charge (equals repel).MOST OF THE ATOM IS EMPTYSPACE AND MOST OF ITS MASS IS ON THE NUCLEUS (99%). where were the electrons? This question was not answered until Rutherford’s student, Niels Bohr, proposed a model in which electrons surrounded the positively charged nucleus as the planets surround the sun. James Chadwick JAME CHADWICK is the responsible for the discovery of the neutron. the nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons. Since opposites attract and equals repel people thought that there must be some other subatomic particle that “holds the protons together”. This particle was the neutron and it does exactly that. James Chadwick shoots alpha particles to beryllium and he notices that rays get release from the atoms of Beryllium, what he did was that when he shot the alpha particles thru the nuclei of the beryllium he was knocking out the “unknown” particles out into a beam that Chadwick would be able to study, he then used this beam and tested it with the cathode ray, to his surprised the beam did not bend towards any charge when going thru the plates which meant it was neutral particles (0 charge). Nuclear Forces The problem was that in the nucleus there are protons and if we follow the electromagnetic forces, they should repel each other but they were not, in fact they had a strong bond, therefore there must be a force holding them together. How does it work? When protons are really close together instead of repelling each other the created a strong attraction between them, there’s a similar attraction that neutrons close to each other have or when protons and neutrons are very close together the same strong bond forms. If the subatomic particles didn’t have these short-range distance they would repel normally. Properties of the subatomic particles The radii of atoms are expressed in picometers. The radius of an atom is the distance from the center of the nucleus to the outer portion of this electron cloud. Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. This unit is the picometer. The abbreviation for the picometer is pm (1 pm = 10-12 m = 10-10 cm). To get an idea of how small a picometer is, consider that 1 cm is the same fractional part of 103 km (about 600 mi) as 100 pm is of 1 cm. Atomic radii range from about 40 to 270 pm. By contrast, the nuclei of atoms have much smaller radii, about 0.001 pm. Nuclei also have incredibly high densities, about 2 × 108 metric tons/cm3.