Niels Bohr: his model of the atom, 1913. This paper is at the Hathi Trust.
Niels Bohr: 1921 excerpt on the "correspondence principle" of quantum theory.
Niels Bohr: 1921 paper on electron configurations and atomic structure. This paper is at the ChemTeam site.
Niels Bohr and Dirk Coster: excerpt on electron configurations and atomic structure backed by X-ray spectra (1923).
Frederick S. Brackett: 1922 paper listing new members of the Paschen series of hydrogen spectral lines along with members of a new series (now known as the Brackett series) characterized by Bohr's formulas for hydrogen spectra. This paper is at Google Books.
Charles R. Bury: 1921 paper on the arrangement of electrons in atoms; gives electron configurations for most of the periodic table. This paper is at the ChemTeam site. View page images of original (in English).
Clinton Davisson and Lester Germer: 1927 paper on the diffraction of electrons. This paper is at Physical Review. (Link to a biographical sketch of Davisson and one of Germer.)
John Edward Lennard-Jones: 1929 paper on molecular orbital descriptions of diatomic molecules, including paramagnetism of oxygen. This paper is at the ChemTeam site. (View biographical paragraph on Lennard-Jones.)
Robert Millikan: 1913 paper on the elementary electrical charge and Avogadro's constant (excerpt). This paper is at Google Books. See a biographical sketch of Millikan.
Edmund Clifton Stoner: 1924 paper on "The Distribution of Electrons among Atomic Levels". This paper is at the ChemTeam site. (Link to biographical information on Stoner.
George Johnstone Stoney (1894): asserts priority for suggesting that electric charge comes in discrete packages, and proposes the term "electron" for the "atom of electricity". This paper is at the ChemTeam site. View page images of original. See biographical information on Stoney.
J. J. Thomson: 1899 paper further characterizing cathode ray corpuscles by identifying them with thermoelectric, photoelectric, and radioactivity phenomena and measuring their mass. This paper is at the ChemTeam site.
J. J. Thomson: excerpt from "On the Structure of the Atom ..." (1904), elaborating the "plum pudding" model. This paper is at the ChemTeam site.
J. J. Thomson: excerpt from "On the Number of Corpuscles [i.e., electrons] in an Atom" (1906). The number is of the same order as the atomic weight, not thousands of times that number. This paper is at the ChemTeam site.
J. J. Thomson: Nobel Prize in Physics Award Address, 1906, on the characterization of the electron.
J. J. Thomson: on the positive rays of electric discharge tubes (1913), recognizing them as atoms and molecules stripped of one or more electrons, describing essentially an early mass spectrometer, and giving evidence for a heavy isotope of neon. View page images of full original.
Pieter Zeeman (1897): description of the magnetic splitting of spectral lines now named after him; includes measurement of the charge-to-mass ratio of what we now call the electron, independent of Thomson's cathode-ray research. This paper is at the ChemTeam site. (Link to a biographical sketch of Zeeman.)
Aristotle: On Generation and Corruption. This work is at the Internet Classics Archive at MIT. The first five parts of Book II in particular discuss elements, and in particular the system of four elements that predates Aristotle.
Robert Boyle: The Sceptical Chymist (1661), page images at University of Pennsylvania. Boyle does not know how many elements there are or what those elements may be; however, he knows that those who believe the elements to be earth, air, fire, and water or mercury, sulfur, and salt do so on an insufficient basis. See HTML excerpts at this site (Classic Chemistry) and annotations [pdf] here. (Link to the Robert Boyle Project.)
Antoine Lavoisier (1783): maybe not the first to recognize that water was a compound and not an element, but he certainly had a clearer command of the phenomenon than his English phlogistonist contemporaries, Cavendish and Watt. View page images of original (in French).
Antoine Lavoisier: Table of simple substances (elements) from Elements of Chemistry (1789); includes his criterion for considering a substance elementary. See page images at Google Books.
Antoine Lavoisier: Oeuvres, (Paris, 1862-1893, 6 vols.): searchable electronic edition at CNRS (French national center for scientific research)
Lars Nilson: two excerpts (1879, 1880) on the discovery of scandium. This paper is at the ChemTeam site. View page images of originals (in French) of 1879 and 1880 papers. See biographical sketch of Nilson.
Paracelsus: 16th century on alchemy and the metals. This paper is at the ChemTeam site. (Link to biographical information on Paracelsus.)
Joseph Priestley: a report describing the discovery of oxygen in terms which continue to embrace the phlogiston theory; it is refreshing in Priestley's frank admission of astonishment at the results he describes. View page images of original section, entire volume. See a biographical sketch of Priestley.
Lord Rayleigh (John William Strutt) and William Ramsay, discovery of argon in full lecture to the Royal Society (1895). View page images of original (in English).
Lord Rayleigh (John William Strutt) and William Ramsay, discovery of argon in full lecture to the Royal Society (1895). View page images of original (in English).
Carl Wilhelm Scheele: excerpts on gases from Chemical Treatise on Air and Fire (1777), including recognition that common air is not a single substance (##8-16) and preparation and properties of "fire-air" (oxygen, ##29-50).
Svante Arrhenius, Philosophical Magazine (1896) excerpt. Not a paper about acidity, electrolyte solutions, or the temperature dependence of rate constants, but rather about the greenhouse effect including an attempt to compute temperature effects in a world with twice as much carbon dioxide. (Link to a biographical sketch of Arrhenius or view page images of original.)
Michael Faraday, 1855 letter to The Times on the foul condition of the Thames. While not a formal scientific paper, this letter (at the ChemTeam site) shows Faraday's powers of observation and plain description turned to a topic which continues to engage scientists and policymakers.
Robert Boyle on the relationship between pressure and volume of a gas (Boyle's law), 1662. This excerpt and a facsimile are in a discussion of Boyle's law at the ChemTeam site.
Henry Cavendish: determined that the "phlogisticated" part of the atmosphere (i.e., nitrogen) could be converted to niter, all except possibly a tiny fraction of less than 1% by volume (probably argon). View page images of original. See a biographical sketch of Cavendish.
John Dalton: on gases of the atmosphere, including their partial pressures (read 1802).
John Davy: 1812 paper describes preparation of a new gas, phosgene; describes the product of the reaction of phosgene with ammonia--apparently urea (although he does not identify it) several years before Wöhler. View page images of original.
Peter Debye: excerpt of 1920 paper explaining the origin of cohesive forces in a van der Waals gas. This paper is at the ChemTeam site. (Link to a biographical sketch of Debye.)
Benjamin Franklin. This founding father was a scientist as well as a statesman. In this letter he describes the effects of marsh gas to Joseph Priestley. Link to more on Franklin.
Jacobus van't Hoff: osmosis and the analogy between solutions and gases (1887). This paper is at the Google books. View page images of original (in German).
Antoine Lavoisier (1775-1777): Excerpts from three papers on properties of oxygen at the ChemTeam site. The first identifies oxygen as what combines with metals to make calces (and is available in full here); the second looks at respiration; the third examines burning of candles. View page images of originals 1, 2, and 3 (in French).
Edme Mariotte (c. 1620-1684) on the relationship between air pressure and volume. (Many Europeans know this relationship as Mariotte's law, as opposed to Boyle's law in most English-speaking countries). Link to biographical information on Mariotte.
Joseph Priestley (1772): instructions and observations on making carbonated water. (Page images of this monograph are available at Google Books.)
Lord Rayleigh, Nature (1892). Interesting because of its frank admission of puzzlement and call for assistance in resolving anomalies which would eventually lead to the discovery of argon.
Carl Wilhelm Scheele: excerpts on gases from Chemical Treatise on Air and Fire (1777), including recognition that common air is not a single substance (##8-16) and preparation and properties of "fire-air" (oxygen, ##29-50).
Svante Arrhenius: 1889 paper treats the temperature dependence of the reaction rate of cane sugar inversion, the "Arrhenius equation". View page images of original (in German).
Antoine Lavoisier: Oeuvres, (Paris, 1862-1893, 6 vols.): searchable electronic edition at CNRS (French national center for scientific research) includes complete Traité élémentaire de chimie