question: How were first elements discovered ? How did they know one thing was an element and other wasn't ?
comment: Which time period? In antiquity certain elements were known. Lead, mercury, silver, tin, copper, and gold come to mind. I suppose back then they were seen as pure metals of some kind rather than how elements are viewed in modern times. How they identified those is likely through material properties and how they separate from composite materials. Fast forward to the 1600s-1900s and we have a developing modern view of elements. It's beyond the scope of a reddit reply to explain each one. An easy example is Hydrogen. In the late 1700s it was identified as the result of a reaction between certain acids and metals. While not the first to synthesize or encounter it, the person attributed with its discovery also figured it was proportioned 2:1 in water. It's difficult to explain without writing a full history, including all scientific knowledge at the time. Electrons were not even explicitly known until 1898. And the modern view of atomic structure wasn't proposed until 1913. Atomic weight was largely used in the 1800s as a classifier. The earliest attempts at periodic tables relied on that. But prior to the discovery of the electron we already had some idea about radiation, could do non-visible light photography, and more. So to answer your question, they sort of didn't know in the sense we did now. But they did know in the context of their time and there are very few false positives. They just knew they had a material with unique properties that didn't further break down with the methods used to break down materials at the time. It also had properties measurable at the time. Periodic tables didn't even come into existence, in any state, until long after many elements were known. And while initial ones were incomplete and had less accurate measurements than today, they were still pretty solid. Apologies for the scattered post. The best way to learn about this is to just read a bunch about the history of chemistry, light/e & m/particle physics, quantum mechanics, and all of that. I don't believe there's a single good consolidated source, and a lot of history in textbooks or articles are just one line "hydrogen was discovered here" without providing the context that it wasn't referred to as hydrogen or known as the "first" element and the discovery didn't discover certain properties of hydrogen that we take for granted today. It'd be an excellent book for someone to write.
passages: The history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal. Alchemists and chemists subsequently identified many more; all of the naturally occurring elements were known by 1950.
   In 1661, Boyle defined an element as "those primitive and simple Bodies of which the mixt ones are said to be composed, and into which they are ultimately resolved."
   Ten materials familiar to various prehistoric cultures are now known to be chemical elements: Carbon, copper, gold, iron, lead, mercury, silver, sulfur, tin, and zinc. Three additional materials now accepted as elements, arsenic, antimony, and bismuth, were recognized as distinct substances prior to 1500 AD. Phosphorus, cobalt, and platinum were isolated before 1750.
  
  Most of the remaining naturally occurring chemical elements were identified and characterized by 1900, including:
  
  BULLET::::- Such now-familiar industrial materials as aluminium, silicon, nickel, chromium, magnesium, and tungsten
  
  BULLET::::- Reactive metals such as lithium, sodium, potassium, and calcium
  
  BULLET::::- The halogens fluorine, chlorine, bromine, and iodine
   Modern chemistry emerged from the sixteenth through the eighteenth centuries through the material practices and theories promoted by alchemy, medicine, manufacturing and mining. A decisive moment came when 'chemistry' was distinguished from alchemy by Robert Boyle in his work "The Sceptical Chymist", in 1661; although the alchemical tradition continued for some time after his work. Other important steps included the gravimetric experimental practices of medical chemists like William Cullen, Joseph Black, Torbern Bergman and Pierre Macquer and through the work of Antoine Lavoisier ("father of modern chemistry") on oxygen and the law of conservation of mass, which refuted phlogiston theory. The theory that all matter is made of atoms, which are the smallest constituents of matter that cannot be broken down without losing the basic chemical and physical properties of that matter, was provided by John Dalton in 1803, although the question took a hundred years to settle as proven. Dalton also formulated the law of mass relationships. In 1869, Dmitri Mendeleev composed his periodic table of elements on the basis of Dalton's discoveries.
   By 1914, seventy-two elements were known, all naturally occurring. The remaining naturally occurring elements were discovered or isolated in subsequent decades, and various additional elements have also been produced synthetically, with much of that work pioneered by Glenn T. Seaborg. In 1955, element 101 was discovered and named mendelevium in honor of D.I. Mendeleev, the first to arrange the elements in a periodic manner. Most recently, the synthesis of element 118 (since named oganesson) was reported in October 2006, and the synthesis of element 117 (tennessine) was reported in April 2010.
  
  Section::::History.:Discovery and recognition of various elements.
   Hydrogen: Cavendish, in 1766, was the first to distinguish hydrogen from other gases, although Paracelsus around 1500, Robert Boyle (1670), and Joseph Priestley (?) had observed its production by reacting strong acids with metals. Lavoisier named it in 1793. Oxygen: Carl Wilhelm Scheele obtained oxygen by heating mercuric oxide and nitrates in 1771, but did not publish his findings until 1777. Priestley also prepared this new "air" by 1774, but only Lavoisier recognized it as a true element; he named it in 1777. Nitrogen: Rutherford discovered nitrogen while he was studying at the University of Edinburgh. He showed that the air in which animals breathed, after removal of exhaled carbon dioxide, was no longer able to burn a candle. Scheele, Henry Cavendish, and Priestley also studied this element at about the same time; Lavoisier named it in 1775 or 1776. Tellurium: In 1783, Franz-Joseph Müller von Reichenstein, who was then serving as the Austrian chief inspector of mines in Transylvania, concluded that a new element was present in a gold ore from the mines in Zlatna, near today's city of Alba Iulia, Romania. In 1789, a Hungarian scientist, Pál Kitaibel, discovered the element independently in an ore from Deutsch-Pilsen that had been regarded as argentiferous molybdenite, but later he gave the credit to Müller. In 1798, it was named by Martin Heinrich Klaproth, who had earlier isolated it from the mineral calaverite. Chlorine: In 1774, Scheele obtained chlorine from hydrochloric acid but thought it was an oxide. Only in 1808 did Humphry Davy recognize it as an element.
   Timeline of chemical element discoveries
  
  The discovery of the 118 chemical elements known to exist as of 2019 is presented in chronological order. The elements are listed generally in the order in which each was first defined as the pure element, as the exact date of discovery of most elements cannot be accurately determined. There are plans to synthesise more elements, and it is not known how many elements are possible.
  
  Each element's name, atomic number, year of first report, name of the discoverer, and notes related to the discovery are listed.
  
  Section::::See also.
  
  BULLET::::- History of the periodic table
   At the beginning of the 19th century only 55 of the 92 naturally occurring elements had been discovered. Scientists had no idea how many more they might find, or indeed if there were an infinite number of elements. They also sought to answer a fundamental question, namely: is there a pattern to the elements?
  
  Section::::Episode 2: The Order of the Elements.:John Dalton's atoms.
   BULLET::::- Periodic table
  
  BULLET::::- Extended periodic table
  
  BULLET::::- "The Mystery of Matter: Search for the Elements" (2014/2015 PBS film)
  
  BULLET::::- Transfermium Wars
  
  Section::::External links.
  
  BULLET::::- History of the Origin of the Chemical Elements and Their Discoverers Last updated by Boris Pritychenko on March 30, 2004
  
  BULLET::::- History of Elements of the Periodic Table
  
  BULLET::::- Timeline of Element Discoveries
  
  BULLET::::- The Historyscoper
  
  BULLET::::- Discovery of the Elements - The Movie - YouTube (1:18)
  
  BULLET::::- The History Of Metals Timeline. A timeline showing the discovery of metals and the development of metallurgy.
   Classical elements typically refer to the concepts in ancient Greece of earth, water, air, fire, and (later) aether, which were proposed to explain the nature and complexity of all matter in terms of simpler substances. Ancient cultures in Persia, Greece, Babylonia, Japan, Tibet, and India had similar lists , sometimes referring in local languages to "air" as "wind" and the fifth element as "void". The Chinese Wu Xing system lists Wood (木 "mù"), Fire (火 "huǒ"), Earth (土 "tǔ"), Metal (金 "jīn"), and Water (水 "shuǐ"), though these are described more as energies or transitions rather than as types of material.
   BULLET::::- 1868: French astronomer Pierre Janssen and English astronomer Norman Lockyer independently discovered evidence in the solar spectrum for a new element that Lockyer named "helium". (The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite.)
  
  BULLET::::- 1869: Dmitri Ivanovich Mendeleyev published his periodic table of chemical elements, and the following year (1870) Julius Lothar Meyer published his independently constructed version.
   The history of the periodic table is also a history of the discovery of the chemical elements. The first person in history to discover a new element was Hennig Brand, a bankrupt German merchant. Brand tried to discover the Philosopher's Stone — a mythical object that was supposed to turn inexpensive base metals into gold. In 1669 (or later), his experiments with distilled human urine resulted in the production of a glowing white substance, which he called "cold fire" ("kaltes Feuer"). He kept his discovery secret until 1680, when Robert Boyle rediscovered phosphorus and published his findings. The discovery of phosphorus helped to raise the question of what it meant for a substance to be an element.
   Frederick Soddy and Kazimierz Fajans found in 1913 that although these substances emitted different radiation, many of these substances were identical in their chemical characteristics, so shared the same place on the periodic table. They became known as isotopes, from the Greek "" ("same place"). Austrian chemist Friedrich Paneth cited a difference between "real elements" (elements) and "simple substances" (isotopes), also determining that the existence of different isotopes was mostly irrelevant in determining chemical properties.
   Section::::Modern astronomy.
  
  In the 19th century it was discovered that (by Joseph von Fraunhofer), when sunlight was dispersed, a multitude of spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases, specific lines corresponding to unique elements. It was proved that the chemical elements found in the Sun (chiefly hydrogen and helium) were also found on Earth.
   BULLET::::- 1940:Edwin McMillan and Philip H. Abelson identify neptunium, the lightest and first synthesized transuranium element, found in the products of uranium fission. McMillan would found a lab at Berkeley that would be involved in the discovery of many new elements and isotopes.
  
  BULLET::::- 1941:Glenn T. Seaborg takes over McMillan's work creating new atomic nuclei. Pioneers method of neutron capture and later through other nuclear reactions. Would become the principal or co-discoverer of nine new chemical elements, and dozens of new isotopes of existing elements.
   Section::::Discovery.:13th century: (As).
  
  Albertus Magnus (Albert the Great, 1193–1280) is believed to have been the first to isolate the element from a compound in 1250, by heating soap together with arsenic trisulfide. If so, it was the first element to be chemically discovered.
  
  Section::::Discovery.:17th century: P.
  
  Phosphorus was prepared from urine, by Hennig Brand, in 1669.
  
  Section::::Discovery.:18th century: H, O, N, (Te), Cl.
   BULLET::::- T. H. Huxley discovers what he thinks is primordial matter and names it "Bathybius haeckelii". He admits his mistake in 1871.
  
  BULLET::::- The Granny Smith apple cultivar originates in Eastwood, New South Wales, Australia, from a chance seedling propagated by Maria Ann Smith (née Sherwood, 1799–1870).
  
  Section::::Chemistry.
  
  BULLET::::- August 18 – The element later named as helium is first detected in the spectrum of the Sun's chromosphere by French astronomer Jules Janssen during a total eclipse in Guntur, India, but assumed to be sodium.
   Modern science recognizes classes of elementary particles which have no substructure (or rather, particles that are not made of other particles) and composite particles having substructure (particles made of other particles).
  
  Section::::Modern history.:Western astrology.
  
  Western astrology uses the four classical elements in connection with astrological charts and horoscopes. The twelve signs of the zodiac are divided into the four elements: Fire signs are Aries, Leo and Sagittarius, Earth signs are Taurus, Virgo and Capricorn, Air signs are Gemini, Libra and Aquarius, and Water signs are Cancer, Scorpio, and Pisces.
  
  Section::::Criticism.
   Nonferrous alloys are those which contain no appreciable amounts of iron. Although the first alloys, bronze and brass, were used for thousands of years, along with lead alloys, pewter and others, these were all made from metals that were fairly non-reactive and could be smelted over open flames. In the 18th century, Antoine Lavoisier helped to establish the oxygen theory of combustion, displacing the defunct phlogiston theory that had ruled since the late Middle Ages. The oxygen theory helped correctly explain the phenomenon of things like oxidation of metals (i.e.: rust) and how rocky ores transform into metals when heated. Lavoisier predicted that many of the earths, salts, and alkalis contained metallic bases that were too reactive to oxygen to be smelted in the usual methods; for example in alum, a salt that had been used since antiquity. His work eventually led to the periodic table of elements, which helped confirm the existence of these "missing metals."
   Section::::History.
  
  Section::::History.:Background and early claims.
  
  When the first periodic table of the elements was published by Dmitri Mendeleev in the early 1870s, it showed a " — " in place after uranium similar to several other places for then-undiscovered elements. Other subsequent tables of known elements, including a 1913 publication of the known radioactive isotopes by Kasimir Fajans, also show an empty place after uranium, element 92.
   The development of the electrochemical theory of chemical combinations occurred in the early 19th century as the result of the work of two scientists in particular, J.J. Berzelius and Humphry Davy, made possible by the prior invention of the voltaic pile by Alessandro Volta. Davy discovered nine new elements including the alkali metals by extracting them from their oxides with electric current.
   Lavoisier's "Traité Élémentaire de Chimie" ("Elementary Treatise of Chemistry"), which was written in 1789 and first translated into English by the writer Robert Kerr, is considered to be the first modern textbook about chemistry. Lavoisier defined an element as a substance that cannot be broken down into a simpler substance by a chemical reaction. This simple definition served for a century and lasted until the discovery of subatomic particles. Lavoisier's book contained a list of "simple substances" that Lavoisier believed could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur, which formed the basis for the modern list of elements. Lavoisier's list also included 'light' and 'caloric', which at the time were believed to be material substances. He classified these substances into metals and non metals. While many leading chemists refused to believe Lavoisier's new revelations, the "Elementary Treatise" was written well enough to convince the younger generation. However, Lavoisier's descriptions of his elements lack completeness, as he only classified them as metals and non-metals.
   Jöns Jacob Berzelius was a Swedish chemist who studied medicine at the Univseristy of Uppsala and was a professor of chemistry in Stockholm. He drew on the ideas of both Davy and Dalton to create an electrochemical view of how elements combined together. Berzelius classified elements into two groups, electronegative and electropositive depending which pole of a galvanic battery they were released from when decomposed. He created a scale of charge with oxygen being the most electronegative element and potassium the most electropositive. This scale signified that some elements had positive and negative charges associated with them and the position of an element on this scale and the element's charge determined how that element combined with others. Berzelius's work on electrochemical atomic theory was published in 1818 as "Essai sur la théorie des proportions chimiques et sur l'influence chimique de l'électricité". He also introduced a new chemical nomenclature into chemistry by representing elements with letters and abbreviations, such as O for oxygen and Fe for iron. Combinations of elements were represented as sequences of these symbols and the number of atoms were represented at first by superscripts and then later subscripts.
   On 20 October, Joseph Norman Lockyer in England set up a new, relatively powerful spectroscope. He also observed the emission spectrum of the chromosphere, including the same yellow line. Within a few years, he worked with a chemist and they concluded that it could be caused by an unknown element, after unsuccessfully testing to see if it were some new type of hydrogen. This was the first time a chemical element was discovered on an extraterrestrial body before being found on the earth. Lockyer and the English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος ("helios").
   Section::::19th century.:New elements and gas laws.
  