Relative to what you might ask?The atomic mass is relative to a particular carbon atom. It turns out the atomicmass number is a 'relative' mass. Now this is interesting.? The twonumbers are different! The first thing we notice is the numberbelow the symbol does not have any units. For example the mass of a hydrogen atom is 1.6726 x10 -24 grams. When we here the term atomic mass we think of the massof an atom. The atomic mass number is located below the symbol of theelement 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 numberabove the symbol of each elemetn in the periodic table is calledthe atomic number and equals the number of protons in thatelement. Moseley suggested that the elements in the periodictable could be ordered using an 'atomic number', a whole numberrepresenting the number of protons in the atom. Wewill not go into the details of how it was discovered, but ascientist named Moseley discovered a correlation between aproperty of X-rays released by an atom and the number of protonsin the atom. It turns out thatinformation in the periodic table tells us the number ofelectrons, protons and neutrons in each atom of an element. Knowing that all atoms contain electrons, protons andneutrons we might ask how many of each of these particles arefouind in an atom? The answer to that question depends on theelement. 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 metalsare blue and nonmetals are yellow. In the periodioc table below the metals, nonmetals andmetalloids are color coded for easy identification. The metalloids are a small collection of elements that liebetween the metals and the nonmetals in the periodic table andshare some of the properties of metals and nonmetals. are not malleable or ductile and are generally poor conductors (graphite is a very good electrical conductor). lack the remaining properties of the metals, i.e.are found in gas, liquid or solid state.The next largest group are the nonmetals. are good conductors of heat and electricity.The metals are the largest group of elements. The elements in the periodic table can be classified intothree groups based on their physical properties metals,nonmetals and metalloids. We knowthe physical state of all of the elements in the periodic table.We know the reactivity of the alkali metals with water, increasesas you go down the group. The formula is notshown, that is information we need to recall from memory. The symbol for eachelemetn is included in the periodic table. We learned some of theinformation the periodic table reveals to us. The periodic table is a very important tool which contains avery large quantity of information. The periodic table is a very important tool which contains a very large quantity of information.
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