Periodic Table of Elements: History and Classification

Posted on Aug 29, 2025 in Chemistry

The Periodic Table of Elements

The periodic table of elements is an organizational system that distributes various chemical elements based on specific criteria and characteristics. Its development is usually attributed to Dmitri Mendeleev, who ordered elements based on the periodic variation of chemical properties. Julius Lothar Meyer, working separately, developed a similar system based on the physical properties of atoms.

Discovery of Elements

While some elements such as gold (Au), silver (Ag), copper (Cu), lead (Pb), and mercury (Hg) were already known since antiquity, the first scientific discovery of an element occurred in the 17th century when the alchemist Henning Brand discovered phosphorus (P). In the 18th century, many new elements were discovered, most notably gases, with the development of pneumatic chemistry: oxygen (O), hydrogen (H), and nitrogen (N). During these years, the modern concept of an element was consolidated, leading Antoine Lavoisier to compile his famous list of simple substances, which included 33 elements.

In the early 19th century, the application of the electric battery to the study of chemical phenomena led to the discovery of new elements, such as alkali and alkaline earth metals, primarily through the work of Humphry Davy. By 1830, 55 elements were known. Subsequently, in the mid-19th century, with the invention of the spectroscope, new elements were discovered. Many were named after the color of their characteristic spectral lines: cesium (Cs, Latin caesius, blue), thallium (Tl, from thallos, green twig), rubidium (Rb, from rubidus, deep red), etc.

Element Notion and Periodic Properties

Logically, a necessary prerequisite for constructing the periodic table was the discovery of a sufficient number of individual elements to identify patterns in their chemical behavior and properties. Over the next two centuries, significant knowledge about these properties was acquired, and many new elements were discovered.

The word “element” has roots in Greek science, but the modern notion emerged throughout the 17th century, though there is no clear consensus regarding the process that led to its consolidation and widespread use. Some authors cite as a precedent Robert Boyle’s famous book, “The Sceptical Chymist,” where he defined elements as “certain primitive and simple bodies not formed by other bodies, nor by each other, and which are the ingredients that immediately compose, and into which are ultimately resolved, all perfectly mixed bodies.” This phrase actually appears in the context of Boyle’s criticism of the four Aristotelian elements.

Throughout the 18th century, many tables were compiled, reflecting a new understanding of chemical composition, clearly articulated by Lavoisier in his “Elementary Treatise of Chemistry.” This led to differentiating fundamental substances, which were then known as chemical elements, understanding their properties and how to isolate them. The discovery of numerous new elements and the study of their properties revealed similarities among them, sparking chemists’ interest in classification.

Atomic Weights

In the early 19th century, John Dalton (1766-1844) developed a new conception of atomism, stemming from his studies of meteorological and atmospheric gases. His main contribution was the formulation of “chemical atomism,” which integrated the element concept newly defined by Antoine Lavoisier (1743-1794) with the ponderal laws of chemistry (definite proportions, multiple proportions, and reciprocal proportions).

Dalton utilized the knowledge of reacting substance proportions of his time and made assumptions about how atoms combined. He established the mass of a hydrogen atom as a reference unit (though others were suggested at the time) and related the rest of the values to this unit, thus building a system of relative atomic masses. For example, in the case of oxygen, Dalton assumed that water is a binary compound consisting of one atom of hydrogen and one atom of oxygen. He had no way to verify this point, so he accepted this possibility as an a priori hypothesis.

Dalton knew that 1 part of hydrogen combines with 7 parts (today, 8 parts) of oxygen to produce water. Therefore, if the combination occurred atom by atom (i.e., one hydrogen atom combined with one oxygen atom), the relationship between the masses of these atoms should be 1:7 (or 1:8 as calculated today). The result was the first table of relative atomic masses (or “atomic weights,” as Dalton called them), which was subsequently modified and developed in later years. The aforementioned uncertainties led to a series of controversies and disparities in formulas and atomic weights that only began to be overcome, though not entirely, with the Karlsruhe Congress in 1860.

Metals, Nonmetals, and Metalloids

The first classification of known elements was proposed by Antoine Lavoisier, who suggested classifying elements into metals, nonmetals, and metalloids (or transition metals). Although practical and functional for its time, this classification was eventually rejected because it did not adequately account for the many differences in physical and chemical properties.

Döbereiner’s Triads

One of the first attempts to group elements with similar properties and relate them to atomic weights was made by the German chemist Johann Wolfgang Döbereiner (1780-1849). In 1817, he demonstrated a striking similarity between the properties of certain groups of three elements, with a gradual change from the first to the last. Later (1827), he noted the existence of other groups of three elements exhibiting the same ratio: chlorine, bromine, and iodine; sulfur, selenium, and tellurium; and lithium, sodium, and potassium.

Döbereiner’s Triads Examples

These groups of three elements are called triads. By 1850, approximately 20 triads had been identified, indicating a certain regularity among chemical elements. Döbereiner attempted to relate the chemical properties of these elements (and their compounds) with their atomic weights, noting a strong analogy and a gradual change from the first to the last element in each triad. In his classification of triads, Döbereiner observed that the average atomic weight of the two extreme elements in a triad was approximately equal to the atomic weight of the middle element.

For example, in the chlorine, bromine, iodine triad, their atomic weights are approximately 35.5, 80, and 127, respectively. If we average the atomic weights of chlorine and iodine (35.5 + 127) / 2 = 81.25, which is very close to bromine’s atomic weight of 80. This demonstrated an apparent order within the triads.

Chancourtois’ Telluric Screw

In 1862, Alexandre-Émile Béguyer de Chancourtois created a ‘telluric screw’ (or vis tellurique), a paper helix on which known elements were arranged by increasing atomic weight, wound around a vertical cylinder. He noted that elements with similar properties appeared vertically aligned, separated by approximately 16 atomic mass units. Similar elements were virtually on the same generatrix (a line on the cylinder), indicating a certain periodicity. However, his diagram appeared very complicated and received little attention.

Newlands’ Law of Octaves

In 1864, the English chemist John Alexander Reina Newlands presented his observation to the Royal College of Chemistry: when elements were arranged in increasing order of their atomic weights (excluding hydrogen), every eighth element exhibited properties very similar to the first. At this time, the noble gases had not yet been discovered.

Newlands’ Law of Octaves Example

Newlands arranged elements in rows of seven, similar to musical octaves:

This law demonstrated a certain ordering of elements into families (groups) with very similar properties, and into periods consisting of eight elements whose properties varied progressively. The name ‘Law of Octaves’ reflects Newlands’ intention to relate these properties to the scale of musical notes. However, this rule began to break down after calcium, leading the scientific community to initially scorn and ridicule his arrangement. It was only 23 years later that his work was recognized by the Royal Society, which awarded Newlands its highest honor, the Davy Medal.

Mendeleev’s Periodic Table

For more details, see: Mendeleev’s Periodic Table.

The periodic table of elements was independently proposed by Dmitri Mendeleev and Julius Lothar Meyer. Working separately, they arranged all 64 known elements based on the variation of chemical properties (Mendeleev) and physical properties (Meyer) with respect to their atomic masses. Unlike Newlands’ proposal, in Mendeleev’s periodic table, periods (horizontal rows) were not always the same length. Instead, there was a gradual variation of properties along each period, ensuring that elements within the same group or family exhibited consistent properties across different periods. This table, published in 1869, was based on the principle that the properties of elements are a periodic function of their atomic weights.

Atomic Number and Quantum Mechanics

Mendeleev’s periodic table, while groundbreaking, exhibited some irregularities and problems. In the decades that followed, it became necessary to integrate the discoveries of noble gases, rare earth elements, and radioactive elements. An additional challenge was reconciling the order of increasing atomic weight with the grouping of families sharing common chemical properties. Examples of this difficulty are found in pairs like tellurium-iodine, argon-potassium, and cobalt-nickel, where it was necessary to deviate from the strict order of increasing atomic weights to group elements with similar chemical properties.

For some time, this issue could not be satisfactorily resolved until Henry Moseley (1887-1915) conducted a study on X-ray spectra in 1913. Moseley found that when plotting the square root of the frequency of characteristic X-ray radiation against the sequence number in the periodic system, a straight line was obtained. This suggested that the order was not accidental but reflected a fundamental property of atomic structure. Today, we know that this property is the atomic number (Z), or the number of positive charges (protons) in the nucleus.

The currently accepted explanation of the “periodic law,” discovered by chemists in the mid-19th century, emerged from theoretical developments produced in the first third of the 20th century. Thanks to this research and subsequent developments, it is now accepted that the ordering of elements in the periodic table is directly related to the electronic structure of their atoms, allowing us to predict their diverse chemical properties.

Elements of the Periodic Table

Below is a list of elements with their atomic numbers and symbols, reflecting the structure of the periodic table:

Main Group Elements & Transition Metals

1. H (Hydrogen) 2. He (Helium) 3. Li (Lithium) 4. Be (Beryllium) 5. B (Boron) 6. C (Carbon) 7. N (Nitrogen) 8. O (Oxygen) 9. F (Fluorine) 10. Ne (Neon) 11. Na (Sodium) 12. Mg (Magnesium) 13. Al (Aluminum) 14. Si (Silicon) 15. P (Phosphorus) 16. S (Sulfur) 17. Cl (Chlorine) 18. Ar (Argon) 19. K (Potassium) 20. Ca (Calcium) 21. Sc (Scandium) 22. Ti (Titanium) 23. V (Vanadium) 24. Cr (Chromium) 25. Mn (Manganese) 26. Fe (Iron) 27. Co (Cobalt) 28. Ni (Nickel) 29. Cu (Copper) 30. Zn (Zinc) 31. Ga (Gallium) 32. Ge (Germanium) 33. As (Arsenic) 34. Se (Selenium) 35. Br (Bromine) 36. Kr (Krypton) 37. Rb (Rubidium) 38. Sr (Strontium) 39. Y (Yttrium) 40. Zr (Zirconium) 41. Nb (Niobium) 42. Mo (Molybdenum) 43. Tc (Technetium) 44. Ru (Ruthenium) 45. Rh (Rhodium) 46. Pd (Palladium) 47. Ag (Silver) 48. Cd (Cadmium) 49. In (Indium) 50. Sn (Tin) 51. Sb (Antimony) 52. Te (Tellurium) 53. I (Iodine) 54. Xe (Xenon) 55. Cs (Cesium) 56. Ba (Barium) * (Lanthanides 57-71) 72. Hf (Hafnium) 73. Ta (Tantalum) 74. W (Tungsten) 75. Re (Rhenium) 76. Os (Osmium) 77. Ir (Iridium) 78. Pt (Platinum) 79. Au (Gold) 80. Hg (Mercury) 81. Tl (Thallium) 82. Pb (Lead) 83. Bi (Bismuth) 84. Po (Polonium) 85. At (Astatine) 86. Rn (Radon) 87. Fr (Francium) 88. Ra (Radium) ** (Actinides 89-103) 104. Rf (Rutherfordium) 105. Db (Dubnium) 106. Sg (Seaborgium) 107. Bh (Bohrium) 108. Hs (Hassium) 109. Mt (Meitnerium) 110. Ds (Darmstadtium) 111. Rg (Roentgenium) 112. Cn (Copernicium) 113. Nh (Nihonium) 114. Fl (Flerovium) 115. Mc (Moscovium) 116. Lv (Livermorium) 117. Ts (Tennessine) 118. Og (Oganesson)

Lanthanides (*)

57. La (Lanthanum) 58. Ce (Cerium) 59. Pr (Praseodymium) 60. Nd (Neodymium) 61. Pm (Promethium) 62. Sm (Samarium) 63. Eu (Europium) 64. Gd (Gadolinium) 65. Tb (Terbium) 66. Dy (Dysprosium) 67. Ho (Holmium) 68. Er (Erbium) 69. Tm (Thulium) 70. Yb (Ytterbium) 71. Lu (Lutetium)

Actinides (**)

89. Ac (Actinium) 90. Th (Thorium) 91. Pa (Protactinium) 92. U (Uranium) 93. Np (Neptunium) 94. Pu (Plutonium) 95. Am (Americium) 96. Cm (Curium) 97. Bk (Berkelium) 98. Cf (Californium) 99. Es (Einsteinium) 100. Fm (Fermium) 101. Md (Mendelevium) 102. No (Nobelium) 103. Lr (Lawrencium)

Periodic Table Groups

A vertical column in the periodic table is called a group. All elements belonging to a group share the same valence (number of electrons in their outermost shell), and therefore exhibit similar characteristics or properties. For example, elements in Group 1 (IA) have a valence of 1 (one electron in their outermost energy level) and all tend to lose that electron, forming +1 positive ions. Elements in the last group on the right are the noble gases, which have a full outermost energy level (satisfying the octet rule) and are therefore extremely non-reactive.

The groups of the periodic table, numbered from left to right, are:

• Group 1 (IA): The alkali metals
• Group 2 (IIA): The alkaline earth metals
• Group 3 to Group 12: The transition metals, noble metals, and post-transition metals
• Group 13 (IIIA): The boron group
• Group 14 (IVA): The carbon group
• Group 15 (VA): The nitrogen group
• Group 16 (VIA): The chalcogens
• Group 17 (VIIA): The halogens
• Group 18 (VIIIA): The noble gases

Periodic Table Periods

For more details, see: Periods of the Periodic Table.

The horizontal rows of the periodic table are called periods. Unlike groups, elements within the same period generally have different properties but similar atomic masses. All elements in a period share the same number of electron shells (or energy levels). Following this rule, each element is placed according to its electron configuration. The first period has only two members: hydrogen and helium, both having only the 1s orbital occupied.

The periodic table consists of 7 periods:

• Period 1
• Period 2
• Period 3
• Period 4
• Period 5
• Period 6
• Period 7

The table is also divided into four blocks: s, p, d, and f. The s, d, p blocks are arranged from left to right, while the f-block (lanthanides and actinides) is typically placed below the main table. This classification depends on the outermost electron’s orbital type, following the Aufbau principle.