Chapter 2 Atoms and the Periodic Table I Atoms First Terms Atom is the smallest quantity of matter that retains the properties of matter Element is a substance that cannot be decomposed into simpler substances by physical or chemical means Beginnings of Atomic Theory Democritus and John Dalton believed that matter consisted of small indivisible particles that were called atoms Subatomic particles and isotopes were not known at that time Subatomic Particles and Atomic Structure Discovery of the Electron by J J Thomson Cathode rays a emanated from a metal plate connected to a high voltage source moved toward a positively charged plate called the anode therefore these rays had to be negatively charged were deflected by both electric and magnetic fields Cathode rays Bending of cathode rays by opposite poles of a magnet were the same regardless of the metal used as the cathode were found to be fundamental and common to all matter were named electrons After the discovery of electrons JJ Thomson measured the charge to mass ratio of these tiny particles and found them to be 1 76 x 108 C g R A Millikan determined the charge on the electron by his ingenious oil drop experiment finding that the charge was a multiple of 1 6022 x 10 19 C coulombs b c d e f II A 1 2 3 1 Radioactivity R ntgen discovered that when cathode rays strike a metal surface radiation is emitted He named this radiation X rays X rays were able to penetrate matter Were not deflected by electric or magnetic fields indicating that they did not carry a charge Radioactivity refers to the spontaneous emission of radiation The radiation emitted by radioactive elements are Alpha rays positively charged particles Beta rays negatively charged electrons Gamma rays rays similar to x ray having no mass or charge a b a b c The Proton and the Nuclear Model of the Atom From J J Thomson s studies it was known that atoms were made of negatively charged electrons and because atoms were electrically neutral there had to be something positive in the atom to balance the negative charges The plum pudding model was proposed that described the atom as a sphere of positivelycharged matter in which the electrons were embedded as plums in plum pudding 2 Ernest Rutherford with his gold foil experiment demonstrated that the atom had an extremely dense central core that he named the nucleus The positively charged particles in the nucleus were named protons Protons carry the same magnitude of charge as the electron but are of opposite sign Most of the mass of the atom is in the nucleus and most of its volume is due to the electrons that surround the nucleus Atomic radius is expressed in terms of picometers pm and the radius of a typical atom is about 100pm while the radius of the nucleus is only about 5 x 10 3 pm 1 pm 1 x 10 12 m B 1 2 C 1 3 4 5 2 The Neutron Because the mass of the atom was larger than the sum of the masses of the protons and electrons it was postulated that there had to be some other particle in the nucleus James Chadwick discovered the third subatomic particle and named it the neutron because it was electrically neutral The neutron is slightly heavier than the proton There are other subatomic particles however the three presented here the electron proton and neutron are sufficient to explain chemical properties and chemical reactions III Atomic Number Mass Number and Isotopes Atomic Number Z the number of protons in the nucleus of an atom Mass Number A the total number of protons and neutrons in the nucleus of an atom Isotopes atoms that have the same atomic number but different mass number Representation of an atom A mass number X Z atomic number E Exercises 1 How many protons neutrons and electrons are in the following atoms U Cu p n p n e e e U Fe p n p n e D 1 2 3 A B C D 3 Give the complete symbol for the element whose atomic number is 42 and has 54 neutrons Give the complete symbol for chlorine 35 and chlorine 37 Average Atomic Mass Definitions 1 2 Atomic mass mass of an atom expressed in atomic mass units amu 1 amu 1 6605378 x 10 24 g Average atomic mass the weighted average of all the naturally occurring isotopes of an element B Calculating average atomic mass 1 Average atomic mass abundance 100 mass of isotope 1 2 Sample calculations a Silicon has three isotopes Symbol Abundance Mass amu Si Si Si 92 21 4 70 3 09 27 9769 28 9766 29 9738 Calculate the average atomic mass of silicon Rubidium has two naturally occurring isotopes 2 3 IV A b 4 Symbol Rb Rb Abundance Mass amu 72 17 27 83 84 9118 If the average atomic mass of rubidium is 85 4678 amu what is the atomic mass of Rb c Magnesium has three isotopes Mg 24 Mg 25 26 and Mg Calculate the percent abundance of Mg abundance of Mg 24 3051 amu The mass of each isotope is given below and Mg 25 26 24 is 78 99 and the average atomic mass of magnesium is given that the percent magnesium 24 magnesium 25 magnesium 26 23 9850 amu 24 9858 amu 25 9826 amu The Periodic Table V 5 A Arrangement of the Periodic table 1 2 3 The periodic table is arranged according to increasing atomic number Periods horizontal rows that end with a noble gas Groups vertical columns of elements that have similar properties Types of elements Metals most of the elements in the periodic table metals metals are good conductors of heat and electricity Nonmetals are poor conductors of heat and electricity are Metalloids are elements with properties that are intermediate between metals and nonmetals B Si Ge As Sb Te Po and At Special Names for Certain Groups Alkali metals Group 1A Alkaline earth metals Group 2A Halogens Group 7A Noble gases Group 8A Transition metals Group 1B and Groups 3B 8B Which of the following elements exhibit similar chemical properties I Ba O Br S and Ca VI Mole and Molar Mass The Mole The mole is a collective term for 6 022 x 1023 particles This number is known as Avogadro s number NA 1 mol of Na atoms 6 022 x 1023 Na atoms 1 mol of Zn atoms 6 022 x 1023 Zn atoms 23 Na atoms 6 022 x 10 1 mole Na atoms Molar Mass B 1 2 3 C 1 2 3 4 5 D A 1 2 B 6 1 2 3 C 7 Molar mass is the mass in grams of one mole of substance 1 mole Ca 40 08g Ca 6 022 x 1023 Ca atoms 1 mole Na 22 99g Na 6 022 x 1023 Na atoms 1 mole Cl2 2 35 5g 71 0g 6 022 x 1023 Cl2 molecules Units for molar mass g mol Sample calculations a 82 0 grams of …