<P> Sorting the table by chemical group shows additional patterns, especially with respect to the last two outermost shells . (Elements 57 to 71 belong to the lanthanides, while 89 to 103 are the actinides .) </P> <P> The list below is primarily consistent with the Aufbau principle . However, there are a number of exceptions to the rule; for example palladium (atomic number 46) has no electrons in the fifth shell, unlike other atoms with lower atomic number . Some entries in the table are uncertain, when experimental data is unavailable . (For example, the elements past 108 have such short half - lives that their electron configurations have not yet been measured .) </P> <Table> <Tr> <Th> Z </Th> <Th> Element </Th> <Th> No. of electrons / shell </Th> <Th> Group </Th> </Tr> <Tr> <Td> </Td> <Td> Hydrogen </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> Helium </Td> <Td> </Td> <Td> 18 </Td> </Tr> <Tr> <Td> </Td> <Td> Lithium </Td> <Td> 2, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> Beryllium </Td> <Td> 2, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 5 </Td> <Td> Boron </Td> <Td> 2, 3 </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Carbon </Td> <Td> 2, 4 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Nitrogen </Td> <Td> 2, 5 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Oxygen </Td> <Td> 2, 6 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 9 </Td> <Td> Fluorine </Td> <Td> 2, 7 </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 10 </Td> <Td> Neon </Td> <Td> 2, 8 </Td> <Td> 18 </Td> </Tr> <Tr> <Td> 11 </Td> <Td> Sodium </Td> <Td> 2, 8, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> 12 </Td> <Td> Magnesium </Td> <Td> 2, 8, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 13 </Td> <Td> Aluminium </Td> <Td> 2, 8, 3 </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 14 </Td> <Td> Silicon </Td> <Td> 2, 8, 4 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 15 </Td> <Td> Phosphorus </Td> <Td> 2, 8, 5 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 16 </Td> <Td> Sulfur </Td> <Td> 2, 8, 6 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 17 </Td> <Td> Chlorine </Td> <Td> 2, 8, 7 </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 18 </Td> <Td> Argon </Td> <Td> 2, 8, 8 </Td> <Td> 18 </Td> </Tr> <Tr> <Td> 19 </Td> <Td> Potassium </Td> <Td> 2, 8, 8, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> 20 </Td> <Td> Calcium </Td> <Td> 2, 8, 8, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 21 </Td> <Td> Scandium </Td> <Td> 2, 8, 9, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 22 </Td> <Td> Titanium </Td> <Td> 2, 8, 10, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 23 </Td> <Td> Vanadium </Td> <Td> 2, 8, 11, 2 </Td> <Td> 5 </Td> </Tr> <Tr> <Td> 24 </Td> <Td> Chromium </Td> <Td> 2, 8, 13, 1 </Td> <Td> 6 </Td> </Tr> <Tr> <Td> 25 </Td> <Td> Manganese </Td> <Td> 2, 8, 13, 2 </Td> <Td> 7 </Td> </Tr> <Tr> <Td> 26 </Td> <Td> Iron </Td> <Td> 2, 8, 14, 2 </Td> <Td> 8 </Td> </Tr> <Tr> <Td> 27 </Td> <Td> Cobalt </Td> <Td> 2, 8, 15, 2 </Td> <Td> 9 </Td> </Tr> <Tr> <Td> 28 </Td> <Td> Nickel </Td> <Td> 2, 8, 16, 2 </Td> <Td> 10 </Td> </Tr> <Tr> <Td> 29 </Td> <Td> Copper </Td> <Td> 2, 8, 18, 1 </Td> <Td> 11 </Td> </Tr> <Tr> <Td> 30 </Td> <Td> Zinc </Td> <Td> 2, 8, 18, 2 </Td> <Td> 12 </Td> </Tr> <Tr> <Td> 31 </Td> <Td> Gallium </Td> <Td> 2, 8, 18, 3 </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 32 </Td> <Td> Germanium </Td> <Td> 2, 8, 18, 4 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 33 </Td> <Td> Arsenic </Td> <Td> 2, 8, 18, 5 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 34 </Td> <Td> Selenium </Td> <Td> 2, 8, 18, 6 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 35 </Td> <Td> Bromine </Td> <Td> 2, 8, 18, 7 </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 36 </Td> <Td> Krypton </Td> <Td> 2, 8, 18, 8 </Td> <Td> 18 </Td> </Tr> <Tr> <Td> 37 </Td> <Td> Rubidium </Td> <Td> 2, 8, 18, 8, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> 38 </Td> <Td> Strontium </Td> <Td> 2, 8, 18, 8, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 39 </Td> <Td> Yttrium </Td> <Td> 2, 8, 18, 9, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 40 </Td> <Td> Zirconium </Td> <Td> 2, 8, 18, 10, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 41 </Td> <Td> Niobium </Td> <Td> 2, 8, 18, 12, 1 </Td> <Td> 5 </Td> </Tr> <Tr> <Td> 42 </Td> <Td> Molybdenum </Td> <Td> 2, 8, 18, 13, 1 </Td> <Td> 6 </Td> </Tr> <Tr> <Td> 43 </Td> <Td> Technetium </Td> <Td> 2, 8, 18, 13, 2 </Td> <Td> 7 </Td> </Tr> <Tr> <Td> 44 </Td> <Td> Ruthenium </Td> <Td> 2, 8, 18, 15, 1 </Td> <Td> 8 </Td> </Tr> <Tr> <Td> 45 </Td> <Td> Rhodium </Td> <Td> 2, 8, 18, 16, 1 </Td> <Td> 9 </Td> </Tr> <Tr> <Td> 46 </Td> <Td> Palladium </Td> <Td> 2, 8, 18, 18 </Td> <Td> 10 </Td> </Tr> <Tr> <Td> 47 </Td> <Td> Silver </Td> <Td> 2, 8, 18, 18, 1 </Td> <Td> 11 </Td> </Tr> <Tr> <Td> 48 </Td> <Td> Cadmium </Td> <Td> 2, 8, 18, 18, 2 </Td> <Td> 12 </Td> </Tr> <Tr> <Td> 49 </Td> <Td> Indium </Td> <Td> 2, 8, 18, 18, 3 </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 50 </Td> <Td> Tin </Td> <Td> 2, 8, 18, 18, 4 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 51 </Td> <Td> Antimony </Td> <Td> 2, 8, 18, 18, 5 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 52 </Td> <Td> Tellurium </Td> <Td> 2, 8, 18, 18, 6 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 53 </Td> <Td> Iodine </Td> <Td> 2, 8, 18, 18, 7 </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 54 </Td> <Td> Xenon </Td> <Td> 2, 8, 18, 18, 8 </Td> <Td> 18 </Td> </Tr> <Tr> <Td> 55 </Td> <Td> Caesium </Td> <Td> 2, 8, 18, 18, 8, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> 56 </Td> <Td> Barium </Td> <Td> 2, 8, 18, 18, 8, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 57 </Td> <Td> Lanthanum </Td> <Td> 2, 8, 18, 18, 9, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 58 </Td> <Td> Cerium </Td> <Td> 2, 8, 18, 19, 9, 2 </Td> </Tr> <Tr> <Td> 59 </Td> <Td> Praseodymium </Td> <Td> 2, 8, 18, 21, 8, 2 </Td> </Tr> <Tr> <Td> 60 </Td> <Td> Neodymium </Td> <Td> 2, 8, 18, 22, 8, 2 </Td> </Tr> <Tr> <Td> 61 </Td> <Td> Promethium </Td> <Td> 2, 8, 18, 23, 8, 2 </Td> </Tr> <Tr> <Td> 62 </Td> <Td> Samarium </Td> <Td> 2, 8, 18, 24, 8, 2 </Td> </Tr> <Tr> <Td> 63 </Td> <Td> Europium </Td> <Td> 2, 8, 18, 25, 8, 2 </Td> </Tr> <Tr> <Td> 64 </Td> <Td> Gadolinium </Td> <Td> 2, 8, 18, 25, 9, 2 </Td> </Tr> <Tr> <Td> 65 </Td> <Td> Terbium </Td> <Td> 2, 8, 18, 27, 8, 2 </Td> </Tr> <Tr> <Td> 66 </Td> <Td> Dysprosium </Td> <Td> 2, 8, 18, 28, 8, 2 </Td> </Tr> <Tr> <Td> 67 </Td> <Td> Holmium </Td> <Td> 2, 8, 18, 29, 8, 2 </Td> </Tr> <Tr> <Td> 68 </Td> <Td> Erbium </Td> <Td> 2, 8, 18, 30, 8, 2 </Td> </Tr> <Tr> <Td> 69 </Td> <Td> Thulium </Td> <Td> 2, 8, 18, 31, 8, 2 </Td> </Tr> <Tr> <Td> 70 </Td> <Td> Ytterbium </Td> <Td> 2, 8, 18, 32, 8, 2 </Td> </Tr> <Tr> <Td> 71 </Td> <Td> Lutetium </Td> <Td> 2, 8, 18, 32, 9, 2 </Td> </Tr> <Tr> <Td> 72 </Td> <Td> Hafnium </Td> <Td> 2, 8, 18, 32, 10, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 73 </Td> <Td> Tantalum </Td> <Td> 2, 8, 18, 32, 11, 2 </Td> <Td> 5 </Td> </Tr> <Tr> <Td> 74 </Td> <Td> Tungsten </Td> <Td> 2, 8, 18, 32, 12, 2 </Td> <Td> 6 </Td> </Tr> <Tr> <Td> 75 </Td> <Td> Rhenium </Td> <Td> 2, 8, 18, 32, 13, 2 </Td> <Td> 7 </Td> </Tr> <Tr> <Td> 76 </Td> <Td> Osmium </Td> <Td> 2, 8, 18, 32, 14, 2 </Td> <Td> 8 </Td> </Tr> <Tr> <Td> 77 </Td> <Td> Iridium </Td> <Td> 2, 8, 18, 32, 15, 2 </Td> <Td> 9 </Td> </Tr> <Tr> <Td> 78 </Td> <Td> Platinum </Td> <Td> 2, 8, 18, 32, 17, 1 </Td> <Td> 10 </Td> </Tr> <Tr> <Td> 79 </Td> <Td> Gold </Td> <Td> 2, 8, 18, 32, 18, 1 </Td> <Td> 11 </Td> </Tr> <Tr> <Td> 80 </Td> <Td> Mercury </Td> <Td> 2, 8, 18, 32, 18, 2 </Td> <Td> 12 </Td> </Tr> <Tr> <Td> 81 </Td> <Td> Thallium </Td> <Td> 2, 8, 18, 32, 18, 3 </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 82 </Td> <Td> Lead </Td> <Td> 2, 8, 18, 32, 18, 4 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 83 </Td> <Td> Bismuth </Td> <Td> 2, 8, 18, 32, 18, 5 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 84 </Td> <Td> Polonium </Td> <Td> 2, 8, 18, 32, 18, 6 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 85 </Td> <Td> Astatine </Td> <Td> 2, 8, 18, 32, 18, 7 </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 86 </Td> <Td> Radon </Td> <Td> 2, 8, 18, 32, 18, 8 </Td> <Td> 18 </Td> </Tr> <Tr> <Td> 87 </Td> <Td> Francium </Td> <Td> 2, 8, 18, 32, 18, 8, 1 </Td> <Td> </Td> </Tr> <Tr> <Td> 88 </Td> <Td> Radium </Td> <Td> 2, 8, 18, 32, 18, 8, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 89 </Td> <Td> Actinium </Td> <Td> 2, 8, 18, 32, 18, 9, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 90 </Td> <Td> Thorium </Td> <Td> 2, 8, 18, 32, 18, 10, 2 </Td> </Tr> <Tr> <Td> 91 </Td> <Td> Protactinium </Td> <Td> 2, 8, 18, 32, 20, 9, 2 </Td> </Tr> <Tr> <Td> 92 </Td> <Td> Uranium </Td> <Td> 2, 8, 18, 32, 21, 9, 2 </Td> </Tr> <Tr> <Td> 93 </Td> <Td> Neptunium </Td> <Td> 2, 8, 18, 32, 22, 9, 2 </Td> </Tr> <Tr> <Td> 94 </Td> <Td> Plutonium </Td> <Td> 2, 8, 18, 32, 24, 8, 2 </Td> </Tr> <Tr> <Td> 95 </Td> <Td> Americium </Td> <Td> 2, 8, 18, 32, 25, 8, 2 </Td> </Tr> <Tr> <Td> 96 </Td> <Td> Curium </Td> <Td> 2, 8, 18, 32, 25, 9, 2 </Td> </Tr> <Tr> <Td> 97 </Td> <Td> Berkelium </Td> <Td> 2, 8, 18, 32, 27, 8, 2 </Td> </Tr> <Tr> <Td> 98 </Td> <Td> Californium </Td> <Td> 2, 8, 18, 32, 28, 8, 2 </Td> </Tr> <Tr> <Td> 99 </Td> <Td> Einsteinium </Td> <Td> 2, 8, 18, 32, 29, 8, 2 </Td> </Tr> <Tr> <Td> 100 </Td> <Td> Fermium </Td> <Td> 2, 8, 18, 32, 30, 8, 2 </Td> </Tr> <Tr> <Td> 101 </Td> <Td> Mendelevium </Td> <Td> 2, 8, 18, 32, 31, 8, 2 </Td> </Tr> <Tr> <Td> 102 </Td> <Td> Nobelium </Td> <Td> 2, 8, 18, 32, 32, 8, 2 </Td> </Tr> <Tr> <Td> 103 </Td> <Td> Lawrencium </Td> <Td> 2, 8, 18, 32, 32, 8, 3 </Td> </Tr> <Tr> <Td> 104 </Td> <Td> Rutherfordium </Td> <Td> 2, 8, 18, 32, 32, 10, 2 </Td> <Td> </Td> </Tr> <Tr> <Td> 105 </Td> <Td> Dubnium </Td> <Td> 2, 8, 18, 32, 32, 11, 2 </Td> <Td> 5 </Td> </Tr> <Tr> <Td> 106 </Td> <Td> Seaborgium </Td> <Td> 2, 8, 18, 32, 32, 12, 2 </Td> <Td> 6 </Td> </Tr> <Tr> <Td> 107 </Td> <Td> Bohrium </Td> <Td> 2, 8, 18, 32, 32, 13, 2 </Td> <Td> 7 </Td> </Tr> <Tr> <Td> 108 </Td> <Td> Hassium </Td> <Td> 2, 8, 18, 32, 32, 14, 2 </Td> <Td> 8 </Td> </Tr> <Tr> <Td> 109 </Td> <Td> Meitnerium </Td> <Td> 2, 8, 18, 32, 32, 15, 2 (?) </Td> <Td> 9 </Td> </Tr> <Tr> <Td> 110 </Td> <Td> Darmstadtium </Td> <Td> 2, 8, 18, 32, 32, 16, 2 (?) </Td> <Td> 10 </Td> </Tr> <Tr> <Td> 111 </Td> <Td> Roentgenium </Td> <Td> 2, 8, 18, 32, 32, 17, 2 (?) </Td> <Td> 11 </Td> </Tr> <Tr> <Td> 112 </Td> <Td> Copernicium </Td> <Td> 2, 8, 18, 32, 32, 18, 2 (?) </Td> <Td> 12 </Td> </Tr> <Tr> <Td> 113 </Td> <Td> Nihonium </Td> <Td> 2, 8, 18, 32, 32, 18, 3 (?) </Td> <Td> 13 </Td> </Tr> <Tr> <Td> 114 </Td> <Td> Flerovium </Td> <Td> 2, 8, 18, 32, 32, 18, 4 (?) </Td> <Td> 14 </Td> </Tr> <Tr> <Td> 115 </Td> <Td> Moscovium </Td> <Td> 2, 8, 18, 32, 32, 18, 5 (?) </Td> <Td> 15 </Td> </Tr> <Tr> <Td> 116 </Td> <Td> Livermorium </Td> <Td> 2, 8, 18, 32, 32, 18, 6 (?) </Td> <Td> 16 </Td> </Tr> <Tr> <Td> 117 </Td> <Td> Tennessine </Td> <Td> 2, 8, 18, 32, 32, 18, 7 (?) </Td> <Td> 17 </Td> </Tr> <Tr> <Td> 118 </Td> <Td> Oganesson </Td> <Td> 2, 8, 18, 32, 32, 18, 8 (?) </Td> <Td> 18 </Td> </Tr> </Table> <Tr> <Th> Z </Th> <Th> Element </Th> <Th> No. of electrons / shell </Th> <Th> Group </Th> </Tr>

How many electrons in outer shell of iodine