<Tr> <Td> <Table> <Tr> <Td> <Table> <Tr> <Td> Hydrogen </Td> <Td_colspan="30"> </Td> <Td> Helium </Td> </Tr> <Tr> <Td> Lithium </Td> <Td> Beryllium </Td> <Td_colspan="24"> </Td> <Td> Boron </Td> <Td> Carbon </Td> <Td> Nitrogen </Td> <Td> Oxygen </Td> <Td> Fluorine </Td> <Td> Neon </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> Magnesium </Td> <Td_colspan="24"> </Td> <Td> Aluminium </Td> <Td> Silicon </Td> <Td> Phosphorus </Td> <Td> Sulfur </Td> <Td> Chlorine </Td> <Td> Argon </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> Calcium </Td> <Td> Scandium </Td> <Td_colspan="14"> </Td> <Td> Titanium </Td> <Td> Vanadium </Td> <Td> Chromium </Td> <Td> Manganese </Td> <Td> Iron </Td> <Td> Cobalt </Td> <Td> Nickel </Td> <Td> Copper </Td> <Td> Zinc </Td> <Td> Gallium </Td> <Td> Germanium </Td> <Td> Arsenic </Td> <Td> Selenium </Td> <Td> Bromine </Td> <Td> Krypton </Td> </Tr> <Tr> <Td> Rubidium </Td> <Td> Strontium </Td> <Td> Yttrium </Td> <Td> </Td> <Td_colspan="13"> </Td> <Td> Zirconium </Td> <Td> Niobium </Td> <Td> Molybdenum </Td> <Td> Technetium </Td> <Td> Ruthenium </Td> <Td> Rhodium </Td> <Td> Palladium </Td> <Td> Silver </Td> <Td> Cadmium </Td> <Td> Indium </Td> <Td> Tin </Td> <Td> Antimony </Td> <Td> Tellurium </Td> <Td> Iodine </Td> <Td> Xenon </Td> </Tr> <Tr> <Td> Caesium </Td> <Td> Barium </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> <Td> Hafnium </Td> <Td> Tantalum </Td> <Td> Tungsten </Td> <Td> Rhenium </Td> <Td> Osmium </Td> <Td> Iridium </Td> <Td> Platinum </Td> <Td> Gold </Td> <Td> Mercury (element) </Td> <Td> Thallium </Td> <Td> Lead </Td> <Td> Bismuth </Td> <Td> Polonium </Td> <Td> Astatine </Td> <Td> Radon </Td> </Tr> <Tr> <Td> Francium </Td> <Td> Radium </Td> <Td> Actinium </Td> <Td> Thorium </Td> <Td> Protactinium </Td> <Td> Uranium </Td> <Td> Neptunium </Td> <Td> Plutonium </Td> <Td> Americium </Td> <Td> Curium </Td> <Td> Berkelium </Td> <Td> Californium </Td> <Td> Einsteinium </Td> <Td> Fermium </Td> <Td> Mendelevium </Td> <Td> Nobelium </Td> <Td> Lawrencium </Td> <Td> Rutherfordium </Td> <Td> Dubnium </Td> <Td> Seaborgium </Td> <Td> Bohrium </Td> <Td> Hassium </Td> <Td> Meitnerium </Td> <Td> Darmstadtium </Td> <Td> Roentgenium </Td> <Td> Copernicium </Td> <Td> Nihonium </Td> <Td> Flerovium </Td> <Td> Moscovium </Td> <Td> Livermorium </Td> <Td> Tennessine </Td> <Td> Oganesson </Td> </Tr> </Table> </Td> <Td> Ar ↑ Kr ↓ Xe </Td> </Tr> <Tr> <Td_colspan="2"> bromine ← krypton → rubidium </Td> </Tr> </Table> </Td> </Tr> <Table> <Tr> <Td> <Table> <Tr> <Td> Hydrogen </Td> <Td_colspan="30"> </Td> <Td> Helium </Td> </Tr> <Tr> <Td> Lithium </Td> <Td> Beryllium </Td> <Td_colspan="24"> </Td> <Td> Boron </Td> <Td> Carbon </Td> <Td> Nitrogen </Td> <Td> Oxygen </Td> <Td> Fluorine </Td> <Td> Neon </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> Magnesium </Td> <Td_colspan="24"> </Td> <Td> Aluminium </Td> <Td> Silicon </Td> <Td> Phosphorus </Td> <Td> Sulfur </Td> <Td> Chlorine </Td> <Td> Argon </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> Calcium </Td> <Td> Scandium </Td> <Td_colspan="14"> </Td> <Td> Titanium </Td> <Td> Vanadium </Td> <Td> Chromium </Td> <Td> Manganese </Td> <Td> Iron </Td> <Td> Cobalt </Td> <Td> Nickel </Td> <Td> Copper </Td> <Td> Zinc </Td> <Td> Gallium </Td> <Td> Germanium </Td> <Td> Arsenic </Td> <Td> Selenium </Td> <Td> Bromine </Td> <Td> Krypton </Td> </Tr> <Tr> <Td> Rubidium </Td> <Td> Strontium </Td> <Td> Yttrium </Td> <Td> </Td> <Td_colspan="13"> </Td> <Td> Zirconium </Td> <Td> Niobium </Td> <Td> Molybdenum </Td> <Td> Technetium </Td> <Td> Ruthenium </Td> <Td> Rhodium </Td> <Td> Palladium </Td> <Td> Silver </Td> <Td> Cadmium </Td> <Td> Indium </Td> <Td> Tin </Td> <Td> Antimony </Td> <Td> Tellurium </Td> <Td> Iodine </Td> <Td> Xenon </Td> </Tr> <Tr> <Td> Caesium </Td> <Td> Barium </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> <Td> Hafnium </Td> <Td> Tantalum </Td> <Td> Tungsten </Td> <Td> Rhenium </Td> <Td> Osmium </Td> <Td> Iridium </Td> <Td> Platinum </Td> <Td> Gold </Td> <Td> Mercury (element) </Td> <Td> Thallium </Td> <Td> Lead </Td> <Td> Bismuth </Td> <Td> Polonium </Td> <Td> Astatine </Td> <Td> Radon </Td> </Tr> <Tr> <Td> Francium </Td> <Td> Radium </Td> <Td> Actinium </Td> <Td> Thorium </Td> <Td> Protactinium </Td> <Td> Uranium </Td> <Td> Neptunium </Td> <Td> Plutonium </Td> <Td> Americium </Td> <Td> Curium </Td> <Td> Berkelium </Td> <Td> Californium </Td> <Td> Einsteinium </Td> <Td> Fermium </Td> <Td> Mendelevium </Td> <Td> Nobelium </Td> <Td> Lawrencium </Td> <Td> Rutherfordium </Td> <Td> Dubnium </Td> <Td> Seaborgium </Td> <Td> Bohrium </Td> <Td> Hassium </Td> <Td> Meitnerium </Td> <Td> Darmstadtium </Td> <Td> Roentgenium </Td> <Td> Copernicium </Td> <Td> Nihonium </Td> <Td> Flerovium </Td> <Td> Moscovium </Td> <Td> Livermorium </Td> <Td> Tennessine </Td> <Td> Oganesson </Td> </Tr> </Table> </Td> <Td> Ar ↑ Kr ↓ Xe </Td> </Tr> <Tr> <Td_colspan="2"> bromine ← krypton → rubidium </Td> </Tr> </Table> <Tr> <Td> <Table> <Tr> <Td> Hydrogen </Td> <Td_colspan="30"> </Td> <Td> Helium </Td> </Tr> <Tr> <Td> Lithium </Td> <Td> Beryllium </Td> <Td_colspan="24"> </Td> <Td> Boron </Td> <Td> Carbon </Td> <Td> Nitrogen </Td> <Td> Oxygen </Td> <Td> Fluorine </Td> <Td> Neon </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> Magnesium </Td> <Td_colspan="24"> </Td> <Td> Aluminium </Td> <Td> Silicon </Td> <Td> Phosphorus </Td> <Td> Sulfur </Td> <Td> Chlorine </Td> <Td> Argon </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> Calcium </Td> <Td> Scandium </Td> <Td_colspan="14"> </Td> <Td> Titanium </Td> <Td> Vanadium </Td> <Td> Chromium </Td> <Td> Manganese </Td> <Td> Iron </Td> <Td> Cobalt </Td> <Td> Nickel </Td> <Td> Copper </Td> <Td> Zinc </Td> <Td> Gallium </Td> <Td> Germanium </Td> <Td> Arsenic </Td> <Td> Selenium </Td> <Td> Bromine </Td> <Td> Krypton </Td> </Tr> <Tr> <Td> Rubidium </Td> <Td> Strontium </Td> <Td> Yttrium </Td> <Td> </Td> <Td_colspan="13"> </Td> <Td> Zirconium </Td> <Td> Niobium </Td> <Td> Molybdenum </Td> <Td> Technetium </Td> <Td> Ruthenium </Td> <Td> Rhodium </Td> <Td> Palladium </Td> <Td> Silver </Td> <Td> Cadmium </Td> <Td> Indium </Td> <Td> Tin </Td> <Td> Antimony </Td> <Td> Tellurium </Td> <Td> Iodine </Td> <Td> Xenon </Td> </Tr> <Tr> <Td> Caesium </Td> <Td> Barium </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> <Td> Hafnium </Td> <Td> Tantalum </Td> <Td> Tungsten </Td> <Td> Rhenium </Td> <Td> Osmium </Td> <Td> Iridium </Td> <Td> Platinum </Td> <Td> Gold </Td> <Td> Mercury (element) </Td> <Td> Thallium </Td> <Td> Lead </Td> <Td> Bismuth </Td> <Td> Polonium </Td> <Td> Astatine </Td> <Td> Radon </Td> </Tr> <Tr> <Td> Francium </Td> <Td> Radium </Td> <Td> Actinium </Td> <Td> Thorium </Td> <Td> Protactinium </Td> <Td> Uranium </Td> <Td> Neptunium </Td> <Td> Plutonium </Td> <Td> Americium </Td> <Td> Curium </Td> <Td> Berkelium </Td> <Td> Californium </Td> <Td> Einsteinium </Td> <Td> Fermium </Td> <Td> Mendelevium </Td> <Td> Nobelium </Td> <Td> Lawrencium </Td> <Td> Rutherfordium </Td> <Td> Dubnium </Td> <Td> Seaborgium </Td> <Td> Bohrium </Td> <Td> Hassium </Td> <Td> Meitnerium </Td> <Td> Darmstadtium </Td> <Td> Roentgenium </Td> <Td> Copernicium </Td> <Td> Nihonium </Td> <Td> Flerovium </Td> <Td> Moscovium </Td> <Td> Livermorium </Td> <Td> Tennessine </Td> <Td> Oganesson </Td> </Tr> </Table> </Td> <Td> Ar ↑ Kr ↓ Xe </Td> </Tr> <Table> <Tr> <Td> Hydrogen </Td> <Td_colspan="30"> </Td> <Td> Helium </Td> </Tr> <Tr> <Td> Lithium </Td> <Td> Beryllium </Td> <Td_colspan="24"> </Td> <Td> Boron </Td> <Td> Carbon </Td> <Td> Nitrogen </Td> <Td> Oxygen </Td> <Td> Fluorine </Td> <Td> Neon </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> Magnesium </Td> <Td_colspan="24"> </Td> <Td> Aluminium </Td> <Td> Silicon </Td> <Td> Phosphorus </Td> <Td> Sulfur </Td> <Td> Chlorine </Td> <Td> Argon </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> Calcium </Td> <Td> Scandium </Td> <Td_colspan="14"> </Td> <Td> Titanium </Td> <Td> Vanadium </Td> <Td> Chromium </Td> <Td> Manganese </Td> <Td> Iron </Td> <Td> Cobalt </Td> <Td> Nickel </Td> <Td> Copper </Td> <Td> Zinc </Td> <Td> Gallium </Td> <Td> Germanium </Td> <Td> Arsenic </Td> <Td> Selenium </Td> <Td> Bromine </Td> <Td> Krypton </Td> </Tr> <Tr> <Td> Rubidium </Td> <Td> Strontium </Td> <Td> Yttrium </Td> <Td> </Td> <Td_colspan="13"> </Td> <Td> Zirconium </Td> <Td> Niobium </Td> <Td> Molybdenum </Td> <Td> Technetium </Td> <Td> Ruthenium </Td> <Td> Rhodium </Td> <Td> Palladium </Td> <Td> Silver </Td> <Td> Cadmium </Td> <Td> Indium </Td> <Td> Tin </Td> <Td> Antimony </Td> <Td> Tellurium </Td> <Td> Iodine </Td> <Td> Xenon </Td> </Tr> <Tr> <Td> Caesium </Td> <Td> Barium </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> <Td> Hafnium </Td> <Td> Tantalum </Td> <Td> Tungsten </Td> <Td> Rhenium </Td> <Td> Osmium </Td> <Td> Iridium </Td> <Td> Platinum </Td> <Td> Gold </Td> <Td> Mercury (element) </Td> <Td> Thallium </Td> <Td> Lead </Td> <Td> Bismuth </Td> <Td> Polonium </Td> <Td> Astatine </Td> <Td> Radon </Td> </Tr> <Tr> <Td> Francium </Td> <Td> Radium </Td> <Td> Actinium </Td> <Td> Thorium </Td> <Td> Protactinium </Td> <Td> Uranium </Td> <Td> Neptunium </Td> <Td> Plutonium </Td> <Td> Americium </Td> <Td> Curium </Td> <Td> Berkelium </Td> <Td> Californium </Td> <Td> Einsteinium </Td> <Td> Fermium </Td> <Td> Mendelevium </Td> <Td> Nobelium </Td> <Td> Lawrencium </Td> <Td> Rutherfordium </Td> <Td> Dubnium </Td> <Td> Seaborgium </Td> <Td> Bohrium </Td> <Td> Hassium </Td> <Td> Meitnerium </Td> <Td> Darmstadtium </Td> <Td> Roentgenium </Td> <Td> Copernicium </Td> <Td> Nihonium </Td> <Td> Flerovium </Td> <Td> Moscovium </Td> <Td> Livermorium </Td> <Td> Tennessine </Td> <Td> Oganesson </Td> </Tr> </Table>

When was krypton added to the periodic table