By definition an atomic mass unit (u), also called a Dalton, is equal to 1/12 of the mass of the carbon atom with 6 protons, 6 electrons and 6 neutrons. This particular atom has a mass of 1.99268 x 10 -23 grams. The carbon atom selected is the carbon atom with 6 protons, 6 electrons and 6 neutrons. Relative to what you might ask? The atomic mass is relative to a particular carbon atom. It turns out the atomic mass number, the number written below the symbol on the periodic table, is a 'relative' mass. Now this is interesting.? The two numbers are different! The first thing we notice is the number below the symbol does not have any units. For example the mass of a hydrogen atom is 1.6736 x 10 -24 grams. When we hear the term atomic mass we think of the mass of an atom. The atomic mass number is located below the symbol of the element in the periodic table. To obtain the number of neutrons in an atom we need to firstdiscuss the atomic mass number. How many protons and how many electrons in each of the following elements? a) C b) Fe c) Hg d) U Answers The number of electrons equal the number of protons for aneutral atom. So the number above the symbol of each element in the periodic table is called the atomic number and equals the number of protons in that element. Moseley suggested that the elements in the periodic table could be ordered using an 'atomic number', a whole number representing the number of protons in the atom. We will not go into the details of how it was discovered, but a scientist named Moseley discovered a correlation between the X-rays released by an atom and the number of protons in the atom. It turns out that information in the periodic table tells us the number of electrons, protons and neutrons in each atom of an element. Knowing that all atoms contain electrons, protons and neutrons we might ask how many of each of these particles are found in an atom? The answer to that question depends on the element. His model, which we currently hold, locates almost allof the mass of the atom in the nucleus with the electron locatedoutside the nucleus. The behavior of thescattered particles lead Rutherford to postulate a new model ofthe atom. In hisexperiment alpha particles (which he had characterized by 1908)were 'shot' at a thin piece of gold foil. Structure of the Atom Our current view of the structure of the atom was describedas a result of experimentsperformed under the direction of Ernest Rutherford. Theneutron has almost the same mass as the proton and no charge. Neutron The neutron was characterized by James Chadwick in 1932. Themass was measured as 1.673 x 10 -27 kg.
In 1919 Ernest Rutherfordcharacterized the proton as a particle with a charge equal inmagnitude to that of the electron but with the opposite sign. Proton Thomson experimentally determined the existence of positivelycharged particles in the cathode ray tube, but he was unable tocharacterize these particles further. UsingThomson's charge-to-mass ratio the mass of an electron had avalue of 9.109 x 10 -31 kg. He experimentallymeasured a value of -1.6022 x 10 -19 Coulombs. It was not until 12 years later that Robert Millikan was ableto determine the charge of an electron.
The value he obtained was independent of the gasused in the cathode ray tube. Thomson found the particle to be negatively charged.He was also able to measure the charge-to-mass ratio of thecathode rays. He characterized the properties of cathoderays, as a stream of negatively charged particles orelectrons. Electron This was the first atomic particle discovered by J.J. The electron is located outside the nucleus. The electron, the proton and the neutron.Protons and neutrons are together and make up the nucleus of anatom. Most of us know the three fundamental particles of which allatoms are composed. The periodic table is a very important tool which contains a very large quantity of information.
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