Mendeleev's Periodic Table

In 1869, the Russian chemist Dmitri Ivanovich Mendeleev published a systematic arrangement of the 63 elements then known, ordering them by increasing atomic mass and noticing that chemical and physical properties repeated in a periodic pattern. He stated his finding as the Periodic Law: 'The properties of elements are a periodic function of their atomic masses.' This was one of the most powerful generalizations in the history of chemistry, revealing hidden order in what had previously seemed like an arbitrary collection of substances. Mendeleev's genius lay not just in organizing known elements but in having the courage to leave deliberate gaps in his table when no known element fit a position. He predicted the existence, atomic mass, density, valency, and oxide formula of several undiscovered elements — naming them eka-boron, eka-aluminium, and eka-silicon (using the Sanskrit prefix 'eka' meaning 'one beyond'). The subsequent discovery of scandium (1879), gallium (1875), and germanium (1886) with properties startlingly close to his predictions provided spectacular validation of the periodic table and persuaded the scientific community of its fundamental truth. Despite its triumphs, Mendeleev's table had limitations. Ordering by atomic mass occasionally forced him to reverse adjacent elements (for example, tellurium and iodine) to preserve chemical similarity, a contradiction he could not resolve. The discovery of noble gases, isotopes, and sub-atomic structure eventually showed that atomic number — not atomic mass — is the true organising principle, leading to the Modern Periodic Law. Nevertheless, Mendeleev's table remains the conceptual foundation of all modern versions, and every periodic trend studied today (atomic radius, ionization enthalpy, electronegativity) traces back to the framework he established.

Key takeaways

Collecting Atomic Mass Data
Identifying Periodicity
Leaving Deliberate Gaps
Predicting Eka-Aluminium (Gallium)
Anomalies and Inversions
Scope and Limitation of Mendeleev's Periodic Law

Collecting Atomic Mass Data

Mendeleev compiled atomic masses from the best available experimental data, largely from Cannizzaro's standardized values presented at the 1860 Karlsruhe Congress. He wrote each element's name, atomic mass, valency, and key compound formulas on separate cards and physically rearranged them, much like a card game, until patterns emerged. Atomic mass was the sole numerical criterion for ordering elements along the horizontal rows.

Identifying Periodicity

When elements are listed in order of increasing atomic mass, properties such as valency, oxide formula (R₂O, RO, R₂O₃, RO₂, R₂O₅, RO₃, R₂O₇), and physical state repeat after a certain interval. Mendeleev placed elements with similar chemical behaviour into vertical columns called groups. Elements in the same group share the same highest valency and form analogous compounds — e.g., Li, Na, K all form monovalent chlorides (LiCl, NaCl, KCl) and oxides of formula R₂O.

Leaving Deliberate Gaps

Where no known element possessed the properties required by the table's logic, Mendeleev left a blank space rather than forcing a misfit element into the slot. For example, between calcium (A_r = 40) and titanium (A_r = 48) he left a gap for eka-scandium (his eka-boron, predicted A_r ≈ 44). He predicted this element would have oxide formula Eb₂O₃, density ≈ 3.5 g cm⁻³, and metallic character. Scandium, discovered in 1879, has A_r = 44.96, forms Sc₂O₃, and has density 2.99 g cm⁻³ — close enough to be a stunning confirmation.

Predicting Eka-Aluminium (Gallium)

Mendeleev's most celebrated prediction concerned eka-aluminium, placed below aluminium in Group III. His predicted properties versus the observed properties of gallium (discovered 1875 by Lecoq de Boisbaudran) are compared below. Predicted atomic mass: 68; observed: 69.72. Predicted density: 5.9 g cm⁻³; observed: 5.91 g cm⁻³. Predicted melting point: low; observed: 29.76 °C (melts in the hand). Predicted oxide formula: Ea₂O₃; observed: Ga₂O₃. The match was so precise that it converted most remaining sceptics.

Anomalies and Inversions

Strict ordering by atomic mass creates several contradictions. Argon (A_r = 39.95) would appear after potassium (A_r = 39.10) if placed by mass, yet argon is chemically inert (Group 18) while potassium is an alkali metal (Group 1). Similarly, cobalt (A_r = 58.93) precedes nickel (A_r = 58.69) in the modern table, the reverse of mass order. Mendeleev had to swap tellurium (A_r = 127.6) and iodine (A_r = 126.9) to keep iodine with the halogens. These inversions are fully explained only by the concept of atomic number Z, which always increases without exception across the modern table.

Scope and Limitation of Mendeleev's Periodic Law

Mendeleev's law states: 'The properties of the elements are a periodic function of their atomic masses.' This is empirically powerful but not exact. Isotopes were unknown to Mendeleev; different isotopes of the same element have different atomic masses but identical chemical properties, directly contradicting the idea that mass alone determines chemistry. The discovery of the electron, proton, and atomic number Z by Moseley (1913) replaced atomic mass with atomic number as the true independent variable, giving the Modern Periodic Law: properties are a periodic function of atomic number.

The Key Equation

\text{Periodic Law (Mendeleev, 1869): } P = f(A_r)

P — Elemental property (varies (e.g., density in g cm⁻³; IE in kJ mol⁻¹)): Any physical or chemical property such as melting point, valency, atomic radius, or ionization enthalpy.
f — Periodic function (dimensionless (functional relationship)): A function that repeats its values at regular intervals of the independent variable; here, properties recur after every completed period in the table.
A_r — Relative atomic mass (dimensionless (ratio relative to ¹²C = 12)): The weighted average mass of all naturally occurring isotopes of an element divided by 1/12 of the mass of one carbon-12 atom. Used as the ordering variable in Mendeleev's table.

Worked Example

Problem. Mendeleev predicted an unknown element he called eka-silicon (Es) sitting below silicon in Group IV. Using periodic trends in that group (C: A_r = 12, Si: A_r = 28, Sn: A_r = 118.7), (a) estimate the atomic mass of eka-silicon by interpolation, and (b) deduce the formula of its highest oxide and its chloride. Compare your answers to the actual properties of germanium.

A_r \approx \frac{28 + 118.7}{2} \approx 73; \quad \text{oxide: EsO}_2; \quad \text{chloride: EsCl}_4

Answer. Predicted A_r ≈ 73; oxide formula EsO₂; chloride formula EsCl₄. Germanium (A_r = 72.63, GeO₂, GeCl₄) confirms all three predictions.

Figure

Diagram

Origins · 1 minDmitri Ivanovich Mendeleev and how this was discovered

Dmitri Ivanovich Mendeleev

1834–1907 · Russian

Mendeleev presented his Periodic Table on 6 March 1869 to the Russian Chemical Society. He arranged 63 known elements by atomic mass, identified periodicity of properties, left deliberate gaps for undiscovered elements, and predicted the properties of three of them (eka-boron, eka-aluminium, eka-silicon) with remarkable quantitative accuracy. His 1869 paper 'On the Relationship of the Properties of the Elements to their Atomic Weights' and his 1871 extended version are landmarks of physical science. Mendeleev reportedly dreamed of his completed periodic table — he claimed the arrangement came to him while he slept after working on the problem for three days straight. Whether legend or fact, he woke and immediately wrote it down, allegedly changing only one element upon later review. He was also an expert on suitcase-making, writing a professional monograph on the subject, and his doctoral thesis was on the nature of solutions of alcohol in water — a topic he reportedly chose partly out of personal interest.

Modern Periodic Law — The Modern Periodic Law (properties are a function of atomic number Z) directly supersedes Mendeleev's law (properties are a function of atomic mass) and resolves the anomalies — such as the Te/I inversion — that his table could not explain.
Structure of Long Form Table — The modern long-form (18-column) periodic table is the direct descendant of Mendeleev's table; understanding how groups, periods, and blocks are defined requires knowing why Mendeleev's original arrangement needed to evolve.
IUPAC Nomenclature (Z > 100) — IUPAC uses systematic three-letter names (e.g., ununseptium) for undiscovered elements above Z = 100, echoing Mendeleev's practice of provisionally naming predicted-but-undiscovered elements using the prefix 'eka-'.
Atomic Radius — Atomic radius is one of the periodic trends whose regular group and period variation was implicit in Mendeleev's groupings; it is now quantified using the modern table he pioneered.
Ionic Radius — Ionic radius trends across periods and down groups reflect the same periodic organisation of elements that Mendeleev first systematized.
Ionization Enthalpy (IE) — The periodic repetition of ionization enthalpy values is a quantitative expression of the periodicity Mendeleev identified qualitatively through valency and oxide-forming behaviour.
Electron Gain Enthalpy (Δ_eg H) — Electron gain enthalpy follows periodic trends rooted in the electronic structure framework that explains why Mendeleev's empirical groupings work.
Electronegativity — Electronegativity periodicity is a direct consequence of the repeating electronic configurations that underlie the periodic table Mendeleev constructed empirically.
Periodic Chemical Properties — Mendeleev's table was built entirely from chemical properties (valency, oxide formulas, reactivity). All periodic chemical properties studied today are extensions of the pattern he first made visible.