Table:
Z | Element | Observed or predicted | Isolated (widely known) | Observer | First isolator | Notes
15.0 | Phosphorus | 1669 | 1669.0 | H. Brand | H. Brand | Prepared from urine, it was the first element to be chemically discovered.
27.0 | Cobalt | 1732 |  | G. Brandt |  | Proved that the blue color of glass is due to a new kind of metal and not bismuth as thought previously.
78.0 | Platinum | 1735 | 1735.0 | A. de Ulloa | A. de Ulloa | First description of a metal found in South American gold was in 1557 by Julius Caesar Scaliger. Ulloa published his findings in 1748, but Sir Charles Wood also investigated the metal in 1741. First reference to it as a new metal was made by William Brownrigg in 1750.
28.0 | Nickel | 1751 | 1751.0 | F. Cronstedt | F. Cronstedt | Found by attempting to extract copper from the mineral known as fake copper (now known as niccolite).
83.0 | Bismuth | 1753 |  | C.F. Geoffroy |  | Definitively identified by Claude François Geoffroy in 1753.
12.0 | Magnesium | 1755 | 1808.0 | J. Black | H. Davy | Black observed that magnesia alba (MgO) was not quicklime (CaO). Davy isolated the metal electrochemically from magnesia.
1.0 | Hydrogen | 1766 | 1500.0 | H. Cavendish | Paracelsus | Cavendish was the first to distinguish H / 2 from other gases, although Paracelsus around 1500, Robert Boyle, and Joseph Priestley had observed its production by reacting strong acids with metals. Lavoisier named it in 1793.
8.0 | Oxygen | 1771 | 1771.0 | W. Scheele | W. Scheele | Obtained it by heating mercuric oxide and nitrates in 1771, but did not publish his findings until 1777. Joseph Priestley also prepared this new air by 1774, but only Lavoisier recognized it as a true element; he named it in 1777.
7.0 | Nitrogen | 1772 | 1772.0 | D. Rutherford | D. Rutherford | He discovered Nitrogen while he was studying at the University of Edinburgh. He showed that the air in which animals had breathed, even after removal of the exhaled carbon dioxide, was no longer able to burn a candle. Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley also studied the element at about the same time, and Lavoisier named it in 1775-6.
17.0 | Chlorine | 1774 | 1774.0 | W. Scheele | W. Scheele | Obtained it from hydrochloric acid, but thought it was an oxide. Only in 1808 did Humphry Davy recognize it as an element.
25.0 | Manganese | 1774 | 1774.0 | W. Scheele | G. Gahn | Distinguished pyrolusite as the calx of a new metal. Ignatius Gottfred Kaim also discovered the new metal in 1770, as did Scheele in 1774. It was isolated by reduction of manganese dioxide with carbon.
56.0 | Barium | 1772 | 1808.0 | W. Scheele | H. Davy | Scheele distinguished a new earth (BaO) in pyrolusite and Davy isolated the metal by electrolysis.
42.0 | Molybdenum | 1778 | 1781.0 | W. Scheele | J. Hjelm | Scheele recognised the metal as a constituent of molybdena.
52.0 | Tellurium | 1782 |  | F.-J.M. von Reichenstein | H. Klaproth | Muller observed it as an impurity in gold ores from Transylvania.
74.0 | Tungsten | 1781 | 1783.0 | T. Bergman | J. and F. Elhuyar | Bergman obtained from scheelite an oxide of a new element. The Elhuyars obtained tungstic acid from wolframite and reduced it with charcoal.
38.0 | Strontium | 1787 | 1808.0 | W. Cruikshank | H. Davy | Cruikshank and Adair Crawford in 1790 concluded that strontianite contained a new earth. It was eventually isolated electrochemically in 1808 by Humphry Davy.
 |  | 1789 |  | A. Lavoisier |  | The first modern list of chemical elements – containing, among others, 23 elements of those known then. He also redefined the term "element". Until then, no metals except mercury were considered elements.
40.0 | Zirconium | 1789 | 1824.0 | H. Klaproth | J. Berzelius | Klaproth identified a new element in zirconia.
92.0 | Uranium | 1789 | 1841.0 | H. Klaproth | E.-M. Péligot | Mistakenly identified a uranium oxide obtained from pitchblende as the element itself and named it after the recently discovered planet Uranus.
22.0 | Titanium | 1791 | 1825.0 | W. Gregor | J. Berzelius | Gregor found an oxide of a new metal in ilmenite; Martin Heinrich Klaproth independently discovered the element in rutile in 1795 and named it. The pure metallic form was only obtained in 1910 by Matthew A. Hunter.
39.0 | Yttrium | 1794 | 1840.0 | J. Gadolin | G. Mosander | Discovered in gadolinite, but Mosander showed later that its ore, yttria, contained more elements.
4.0 | Beryllium | 1798 | 1828.0 | N. Vauquelin | F. Wöhler and A. Bussy | Vauquelin discovered the oxide in beryl and emerald, and Klaproth suggested the present name around 1808.
23.0 | Vanadium | 1801 | 1830.0 | M. del Río | N.G.Sefström | Río found the metal in vanadinite but retracted the claim after Hippolyte Victor Collet-Descotils disputed it. Sefström isolated and named it, and later it was shown that Río had been right in the first place.
41.0 | Niobium | 1801 | 1864.0 | C. Hatchett | W. Blomstrand | Hatchett found the element in columbite ore and named it columbium. Heinrich Rose proved in 1844 that the element is distinct from tantalum, and renamed it niobium which was officially accepted in 1949.
73.0 | Tantalum | 1802 |  | G. Ekeberg |  | Ekeberg found another element in minerals similar to columbite and in 1844, Heinrich Rose proved that it was distinct from niobium.
46.0 | Palladium | 1803 | 1803.0 | H. Wollaston | H. Wollaston | Wollaston discovered it in samples of platinum from South America, but did not publish his results immediately. He had intended to name it after the newly discovered asteroid, Ceres, but by the time he published his results in 1804, cerium had taken that name. Wollaston named it after the more recently discovered asteroid Pallas.
58.0 | Cerium | 1803 | 1839.0 | H. Klaproth, J. Berzelius, and W. Hisinger | G. Mosander | Berzelius and Hisinger discovered the element in ceria and named it after the newly discovered asteroid (then considered a planet), Ceres. Klaproth discovered it simultaneously and independently in some tantalum samples. Mosander proved later that the samples of all three researchers had at least another element in them, lanthanum.
76.0 | Osmium | 1803 | 1803.0 | S. Tennant | S. Tennant | Tennant had been working on samples of South American platinum in parallel with Wollaston and discovered two new elements, which he named osmium and iridium.
77.0 | Iridium | 1803 | 1803.0 | S. Tennant | S. Tennant | Tennant had been working on samples of South American platinum in parallel with Wollaston and discovered two new elements, which he named osmium and iridium, and published the iridium results in 1804.
45.0 | Rhodium | 1804 | 1804.0 | H. Wollaston | H. Wollaston | Wollaston discovered and isolated it from crude platinum samples from South America.
19.0 | Potassium | 1807 | 1807.0 | H. Davy | H. Davy | Davy discovered it by using electrolysis on potash.
11.0 | Sodium | 1807 | 1807.0 | H. Davy | H. Davy | Davy discovered it a few days after potassium, by using electrolysis on sodium hydroxide.
20.0 | Calcium | 1808 | 1808.0 | H. Davy | H. Davy | Davy discovered the metal by electrolysis of quicklime.
5.0 | Boron | 1808 | 1808.0 | L. Gay-Lussac and L.J. Thénard | H. Davy | On June 21, 1808, Lussac and Thénard announced a new element in sedative salt, Davy announced the isolation of a new substance from boracic acid soon afterwards.
9.0 | Fluorine | 1810 | 1886.0 | A.-M. Ampère | H. Moissan | André-Marie Ampère predicted an element analogous to chlorine obtainable from hydrofluoric acid, and between 1812 and 1886 many researchers tried to obtain this element. It was eventually isolated by Moissan.
53.0 | Iodine | 1811 | 1811.0 | B. Courtois | B. Courtois | Courtois discovered it in the ashes of seaweed.
3.0 | Lithium | 1817 | 1821.0 | A. Arfwedson | W. T. Brande | Arfwedson discovered the alkali in petalite.
48.0 | Cadmium | 1817 | 1817.0 | S. L Hermann, F. Stromeyer, and J.C.H. Roloff | S. L Hermann, F. Stromeyer, and J.C.H. Roloff | All three found an unknown metal in a sample of zinc oxide from Silesia, but the name that Stromeyer gave became the accepted one.
34.0 | Selenium | 1817 | 1817.0 | J. Berzelius and G. Gahn | J. Berzelius and G. Gahn | While working with lead they discovered a substance that they thought was tellurium, but realized after more investigation that it is different.
14.0 | Silicon | 1824 | 1824.0 | J. Berzelius | J. Berzelius | Humphry Davy thought in 1800 that silica was an element, not a compound, and in 1808 suggested the present name. In 1811 Louis-Joseph Gay-Lussac and Louis-Jacques Thénard probably prepared impure silicon, but Berzelius is credited with the discovery for obtaining the pure element in 1824.
13.0 | Aluminium | 1825 | 1825.0 | H.C.Ørsted | H.C.Ørsted | Antoine Lavoisier predicted in 1787 that alumine is the oxide of an undiscovered element, and in 1808 Humphry Davy tried to decompose it. Although he failed, he suggested the present name. Hans Christian Ørsted was the first to isolate metallic aluminium in 1825.
35.0 | Bromine | 1825 | 1825.0 | J. Balard and L. Gmelin | J. Balard and L. Gmelin | They both discovered the element in the autumn of 1825 and published the results the next year.
90.0 | Thorium | 1829 |  | J. Berzelius |  | Berzelius obtained the oxide of a new earth in thorite.
57.0 | Lanthanum | 1838 |  | G. Mosander |  | Mosander found a new element in samples of ceria and published his results in 1842, but later he showed that this lanthana contained four more elements.
68.0 | Erbium | 1842 |  | G. Mosander |  | Mosander managed to split the old yttria into yttria proper and erbia, and later terbia too.
65.0 | Terbium | 1842 | 1842.0 | G. Mosander | G. Mosander | In 1842 Mosander split yttria into two more earths, erbia and terbia
44.0 | Ruthenium | 1807 | 1844.0 | J. Sniadecki | J. Sniadecki | Sniadecki isolated the element in 1807, but his work was not ratified. Gottfried Wilhelm Osann thought that he found three new metals in Russian platinum samples, and in 1844 Karl Karlovich Klaus confirmed that there was a new element. Klaus is usually recognized as the discoverer of the element.
55.0 | Caesium | 1860 | 1882.0 | R. Bunsen and R. Kirchhoff | C. Setterberg | Bunsen and Kirchhoff were the first to suggest finding new elements by spectrum analysis. They discovered caesium by its two blue emission lines in a sample of Dürkheim mineral water. The pure metal was eventually isolated in 1882 by Setterberg.
37.0 | Rubidium | 1861 |  | R. Bunsen and G. R. Kirchhoff | R. Bunsen | Bunsen and Kirchhoff discovered it just a few months after caesium, by observing new spectral lines in the mineral lepidolite. Bunsen never obtained a pure sample of the metal, which was later obtained by Hervesy.
81.0 | Thallium | 1861 | 1862.0 | W. Crookes | C.-A. Lamy | Shortly after the discovery of rubidium, Crookes found a new green line in a selenium sample; later that year, Lamy found the element to be metallic.
49.0 | Indium | 1863 | 1867.0 | F. Reich and T. Richter | T. Richter | Reich and Richter First identified it in sphalerite by its bright indigo-blue spectroscopic emission line. Richter isolated the metal several years later.
2.0 | Helium | 1868 | 1895.0 | P. Janssen and N. Lockyer | W. Ramsay, T. Cleve, and N. Langlet | Janssen and Lockyer observed independently a yellow line in the solar spectrum that did not match any other element. / Years later, Ramsay, Cleve, and Langlet observed independently the element trapped in cleveite about the same time.
 |  | 1869 |  | D. I. Mendeleev |  | Mendeleev arranges the 63 elements known at that time into the first modern periodic table and correctly predicts several others.
31.0 | Gallium | 1875 |  | P. E. L. de Boisbaudran | P. E. L. de Boisbaudran | Boisbaudran observed on a pyrenea blende sample some emission lines corresponding to the eka-aluminium that was predicted by Mendeleev in 1871 and subsequently isolated the element by electrolysis.
70.0 | Ytterbium | 1878 | 1907.0 | J.C.G. de Marignac | G. Urbain | On October 22, 1878, Marignac reported splitting terbia into two new earths, terbia proper and ytterbia.
67.0 | Holmium | 1878 |  | M. Delafontaine |  | Delafontaine found it in samarskite and next year, Per Teodor Cleve split Marignac's erbia into erbia proper and two new elements, thulium and holmium.
69.0 | Thulium | 1879 | 1879.0 | T. Cleve | T. Cleve | Cleve split Marignac's erbia into erbia proper and two new elements, thulium and holmium.
21.0 | Scandium | 1879 | 1879.0 | F. Nilson | F. Nilson | Nilson split Marignac's ytterbia into pure ytterbia and a new element that matched 1871 Mendeleev's predicted eka-boron.
62.0 | Samarium | 1879 | 1879.0 | P.E.L. de Boisbaudran | P.E.L. de Boisbaudran | Boisbaudran noted a new earth in samarskite and named it samaria after the mineral.
64.0 | Gadolinium | 1880 | 1886.0 | J. C. G. de Marignac | F. L. de Boisbaudran | Marignac initially observed the new earth in terbia, and later Boisbaudran obtained a pure sample from samarskite.
59.0 | Praseodymium | 1885 |  | A. von Welsbach |  | Von Welsbach discovered two new distinct elements in ceria: praseodymium and neodymium.
60.0 | Neodymium | 1885 |  | A. von Welsbach |  | Von Welsbach discovered two new distinct elements in ceria: praseodymium and neodymium.
66.0 | Dysprosium | 1886 |  | P.E.L. de Boisbaudran |  | De Boisbaudran found a new earth in erbia.
32.0 | Germanium | 1886 |  | A. Winkler |  | In February 1886 Winkler found in argyrodite the eka-silicon that Mendeleev had predicted in 1871.
18.0 | Argon | 1894 | 1894.0 | Lord Rayleigh and W. Ramsay | Lord Rayleigh and W. Ramsay | They discovered the gas by comparing the molecular weights of nitrogen prepared by liquefaction from air and nitrogen prepared by chemical means. It is the first noble gas to be isolated.
36.0 | Krypton | 1898 | 1898.0 | W. Ramsay and W. Travers | W. Ramsay and W. Travers | On May 30, 1898, Ramsay separated a third noble gas from liquid argon by difference in boiling point.
10.0 | Neon | 1898 | 1898.0 | W. Ramsay and W. Travers | W. Ramsay and W. Travers | In June 1898 Ramsay separated a new noble gas from liquid argon by difference in boiling point.
54.0 | Xenon | 1898 | 1898.0 | W. Ramsay and W. Travers | W. Ramsay and W. Travers | On July 12, 1898 Ramsay separated a third noble gas within three weeks, from liquid argon by difference in boiling point.
84.0 | Polonium | 1898 | 1902.0 | P. and M. Curie | W. Marckwald | In an experiment done on July 13, 1898, the Curies noted an increased radioactivity in the uranium obtained from pitchblende, which they ascribed to an unknown element.
88.0 | Radium | 1898 | 1902.0 | P. and M. Curie | M. Curie | The Curies reported on December 26, 1898, a new element different from polonium, which Marie later isolated from uraninite.
86.0 | Radon | 1898 | 1910.0 | E. Dorn | W. Ramsay and R. Whytlaw-Gray | Dorn discovered a radioactive gas resulting from the radioactive decay of radium, isolated later by Ramsay and Gray.
89.0 | Actinium | 1899 | 1899.0 | A.-L. Debierne | A.-L. Debierne | Debierne obtained from pitchblende a substance that had properties similar to those of thorium.
63.0 | Europium | 1896 | 1901.0 | E.-A. Demarçay | E.-A. Demarçay | Demarçay found spectral lines of a new element in Lecoq's samarium, and separated this element several years later.
71.0 | Lutetium | 1906 | 1906.0 | G. Urbain and C.A. von Welsbach | G. Urbain and C.A. von Welsbach | Urbain and von Welsbach proved independently that the old ytterbium also contained a new element.
75.0 | Rhenium | 1908[contradiction] | 1908.0 | M. Ogawa | M. Ogawa | Ogawa found it in thorianite but assigned it as element 43 instead of 75 and named it nipponium. In 1922 Walter Noddack, Ida Eva Tacke and Otto Berg announced its separation from gadolinite and gave it the present name.
72.0 | Hafnium | 1911 | 1922.0 | G. Urbain and V. Vernadsky | D. Coster and G. von Hevesy | Urbain claimed to have found the element in rare-earth residues, while Vernadsky independently found it in orthite. Neither claim was confirmed due to World War I. After the war, Coster and Hevesy found it by X-ray spectroscopic analysis in Norwegian zircon. Hafnium was the next to last element with stable isotopes to be discovered.
91.0 | Protactinium | 1913 |  | O.H.Göhring and K. Fajans |  | The two obtained the first isotope of this element that had been predicted by Mendeleev in 1871 as a member of the natural decay of 238U. Originally isolated in 1900 by William Crookes.
43.0 | Technetium | 1937 | 1937.0 | C. Perrier and E. Segrè | C. Perrier & E.Segrè | The two discovered a new element in a molybdenum sample that was used in a cyclotron, the first synthetic element to be discovered. It had been predicted by Mendeleev in 1871 as eka-manganese.
87.0 | Francium | 1939 |  | M. Perey |  | Perey discovered it as a decay product of 227Ac. Francium is the last element to be discovered in nature, rather than synthesized in the lab, although some of the "synthetic" elements that were discovered later (plutonium, neptunium, astatine) were eventually found in trace amounts in nature as well.
85.0 | Astatine | 1940 |  | R. Corson, R. Mackenzie and E. Segrè |  | Obtained by bombarding bismuth with alpha particles. Later determined to occur naturally in minuscule quantities (<25 grams in earth's crust).
93.0 | Neptunium | 1940 |  | E.M. McMillan and H. Abelson |  | Obtained by irradiating uranium with neutrons, it is the first transuranium element discovered.
94.0 | Plutonium | 1940–1941 |  | Glenn T. Seaborg, Arthur C. Wahl, W. Kennedy and E.M. McMillan |  | Prepared by bombardment of uranium with deuterons.
95.0 | Americium | 1944 |  | G. T. Seaborg, A. James, O. Morgan and A. Ghiorso |  | Prepared by irradiating plutonium with neutrons during the Manhattan Project.
96.0 | Curium | 1944 |  | G. T. Seaborg, R. A. James and A. Ghiorso |  | Prepared by bombarding plutonium with alpha particles during the Manhattan Project
61.0 | Promethium | 1942 | 1945.0 | S. Wu, E.G. Segrè and A. Bethe | Charles D. Coryell, Jacob A. Marinsky, Lawrence E. Glendenin, and Harold G. Richter | It was probably first prepared in 1942 by bombarding neodymium and praseodymium with neutrons, but separation of the element could not be carried out. Isolation was performed under the Manhattan Project in 1945.
97.0 | Berkelium | 1949 |  | G. Thompson, A. Ghiorso and G. T. Seaborg (University of California, Berkeley) |  | Created by bombardment of americium with alpha particles.
98.0 | Californium | 1950 |  | S. G. Thompson, K. Street,Jr., A. Ghiorso and G. T. Seaborg (University of California, Berkeley) |  | Bombardment of curium with alpha particles.
99.0 | Einsteinium | 1952 | 1952.0 | A. Ghiorso et al. (Argonne Laboratory, Los Alamos Laboratory and University of California, Berkeley) |  | Formed in the first thermonuclear explosion in November 1952, by irradiation of uranium with neutrons; kept secret for several years.
100.0 | Fermium | 1952 |  | A. Ghiorso et al. (Argonne Laboratory, Los Alamos Laboratory and University of California, Berkeley) |  | Formed in the first thermonuclear explosion in November 1952, by irradiation of uranium with neutrons; kept secret for several years.
101.0 | Mendelevium | 1955 |  | A. Ghiorso, G. Harvey, R. Choppin, S. G. Thompson and G. T. Seaborg |  | Prepared by bombardment of einsteinium with helium.
102.0 | Nobelium | 1958 |  | A. Ghiorso, T. Sikkeland, R. Walton and G. T. Seaborg |  | First prepared by bombardment of curium with carbon atoms.
103.0 | Lawrencium | 1961 |  | A. Ghiorso, T. Sikkeland, E. Larsh and M. Latimer |  | First prepared by bombardment of californium with boron atoms.
104.0 | Rutherfordium | 1968 |  | A. Ghiorso, M. Nurmia, J. Harris, K. Eskola and P. Eskola |  | Prepared by bombardment of californium with carbon atoms.
105.0 | Dubnium | 1970 |  | A. Ghiorso, M. Nurmia, K. Eskola, J. Harris and P. Eskola |  | Prepared by bombardment of californium with nitrogen atoms.
106.0 | Seaborgium | 1974 |  | A. Ghiorso, J. Nitschke, J. Alonso, C. Alonso, M. Nurmia, G. T. Seaborg, K. Hulet and W. Lougheed |  | Prepared by collisions of californium-249 with oxygen atoms.
107.0 | Bohrium | 1981 |  | G.Münzenberg et al. (GSI in Darmstadt) |  | Obtained by bombarding bismuth with chromium.
109.0 | Meitnerium | 1982 |  | G. Münzenberg, P. Armbruster et al. (GSI in Darmstadt) |  | Prepared by bombardment of bismuth with iron atoms.
108.0 | Hassium | 1984 |  | G. Münzenberg, P. Armbruster et al. (GSI in Darmstadt) |  | Prepared by bombardment of lead with iron atoms
110.0 | Darmstadtium | 1994 |  | S. Hofmann et al. (GSI in Darmstadt) |  | Prepared by bombardment of lead with nickel.
111.0 | Roentgenium | 1994 |  | S. Hofmann et al. (GSI in Darmstadt) |  | Prepared by bombardment of bismuth with nickel.
112.0 | Copernicium | 1996 |  | S. Hofmann et al. (GSI in Darmstadt) |  | Prepared by bombardment of lead with zinc.
114.0 | Flerovium | 1999 |  | Y. Oganessian et al. (JINR in Dubna) |  | Prepared by bombardment of plutonium with calcium
116.0 | Livermorium | 2000 |  | Y.Oganessian et al. (JINR in Dubna) |  | Prepared by bombardment of curium with calcium

Question: which element was observed or predicted in 1771 by w.scheele?
Answer: Oxygen