Physics 30 Modern Physics Unit: Atomic Basics Models of the Atom The Greeks believed that if you kept dividing matter into smaller and smaller pieces, you would eventually come to a bit of matter that could not be divided any further. They called this bit of matter the atom. We still use word today to describe the smallest complete unit of an element. Because it is so small, it is almost impossible to physically see, but we can do experiments, and from the results, we can construct a model of what the atom looks like. Dalton s Model In the early 1800 s John Dalton made a model. He thought an atom o was a solid sphere o was indivisible and had uniform density throughout o reacted with other atoms but were left unchanged Dalton knew nothing about electric charge The Discovery of the Electron William Crooks In 19 th century scientists started doing experiments with vacuum tubes. William Crooks developed a vacuum tube known as a cathode-ray tube (CRT) o This was a vacuum tube with two electrodes (a positive ANODE, and a negative CATHODE) o One end of the tube had a fluorescent screen that glowed when a beam of particles hit it Crooks found that when he created a circuit between the anode and cathode, a beam of particles flowed from the cathode to the anode in a straight line o Crooks assumed the particles were negative because they originated at the negative electrode o He called them CATHODE RAYS These Cathode Rays had the following properties o Cathode rays are emitted from the negative electrode of a highly evacuated tube o Cathode rays from a sharp shadow of any object placed in the tube. This proved that cathode rays travel in a straight line o On striking a solid, the cathode ray transfers energy to it. This is shown by an increase in temperature and by the fluorescence of certain materials
o Cathode rays are deflected by a magnetic field. This shows that they are negatively charged. (Negatively charged particles deflect in the exact same manner as cathode rays when placed in a magnetic field) J.J. Thomson Thomson was able to show that these cathode rays consisted of a material that was the same mass and charge as electrons. This lead him to state that cathode rays were nothing more than fast moving, negatively charged particles called electrons Further experiments with hydrogen gas using a modified CRT produced a new beam of particles o These particles were found to be positively charged protons Robert A. Millikan In 1913, Millikan found a way to measure the charge present on an individual oil drop He found that electric charges had a value of 1.6 x 10-19 C (he discovered elementary charge) o The accepted value for elementary charge is 1.60219 x 10-19 C Millikan determined that this charge was the charge of one electron Once this charge was established, the mass of the electrons was able to be determined o Mass of one electron is equal to 9.1096 x 10-31 kg Thomson s Atomic Model (Plum pudding model) Thomson knew that the atoms as a whole had a neutral charge, but he knew that electrons and protons existed and were part of the atom Thomson decided that an atom must be a sphere of positive charge with electrons embedded in it He believed that the negative charge of the electrons was enough to balance the positive charge of the atom itself and made it neutral Discovery of the Nucleus Rutherford s Experiment In 1911 Ernest Rutherford and his associates experimented with positively charged particles coming out of a radioactive material (material giving out spontaneous emissions) and ultra thin gold foil The energetic radioactive particles were made to strike the gold foil. Rutherford found that: o Most particles passed right through the gold foil without moving o Some of the particles were widely deflected
From this data, Rutherford concluded that the material portion of the atom must be concentrated in a very small region o This very small region he called the nucleus With this idea, he created his own atomic theory Rutherford s Atomic Theory Rutherford proposed that the atom was made up of a massive positively charged nucleus with negatively charged electrons rotating around the nucleus to balance the charge These electrons had to be rotating or orbiting the nucleus or else if they were stationary they would fall into the nucleus because of the electrostatic attraction Discovery of the Neutron In 1932 Chadwick discovered that atoms have neutral particles called neutrons These neutrons had the same mass as protons and were located in the nucleus of the atom o Rutherford had predicted that such a particle would exist before Chadwick discovered it When the neutron was discovered, the Rutherford model of the atom became: o Nuclei of atoms are composed of protons and neutrons (called nucleons) which accounted for the mass of the entire atom o Electrons orbit about the nucleus o The number of electrons outside the nucleus is always equal to the number of protons in the nucleus So with Rutherford s updated model, a helium atom with 2 protons, 2 neutrons, and 2 electrons could be pictured as follows: Atomic Number and Mass Number With this new knowledge of the atom, it became possible to represent the structure of all the known elements in terms of number of protons and neutrons in the nucleus, and the number of electrons which orbit around the nucleus The number of protons in the nucleus of an atom is called the ATOMIC NUMBER o In neutral atoms, the number of electrons equals the number of protons
The total number of protons and neutrons in an atom is equal to the number of MASS NUMBER or ATOMIC MASS NUMBER (Remember that only protons and neutrons contribute to the mass of the atom. The atomic mass of an atom is the AVERAGE mass of the atoms of an element. Below is a summary of the subatomic particles: Electrons, Protons, and Neutrons Electron Proton Neutron Symbol e - p + n Charge -1 +1 0 Mass (u) 1/1837 1 1 Electrons can be removed or added to produce a positive or negative ion. Protons cannot be removed and still retain the characteristics of the atom Bohr s Atomic Model The Rutherford Model had some flaws in describing how electrons worked. Neil Bohr proposed some changes: o Electrons revolving around the nucleus of an atom can do so only in certain allowed orbits o An electron does not radiate energy as long as it stays in the allowed orbits o Electrons can jump from the outer orbit to an inner orbit when this happens, a portion of energy is radiated which is equal to the difference in energy of the 2 orbits This portion of energy is radiated as a package of light called a photon (according to quantum theory) o Electrons can jump from an inner orbit to an outer orbit when some energy is added from an external source
Isotopes Isotopes are atoms of the same element that have different numbers of neutrons. They have the same atomic number but have different masses because of an additional number of neutrons o The chemical properties are similar, but physical properties are different o Examples: Carbon isotopes and Hydrogen isotopes X-rays In 1895, Wilhelm Rontgen produced x-rays by the collision of high-energy electrons with a metallic anode in a vacuum tube X-rays are electromagnetic radiations with short wavelengths, which travel in straight lines at the speed of light. They have high penetrating ability, no charge, effect photographic film and make fluorescent screens glow X-rays are used for medical diagnosis and for the detection of defects in manufactured products