Predicting Unknown Elements: Mendeleev's Successes

Introduction

Any idea can be made to fit facts that are already known. The real test of a scientific theory is whether it can predict something nobody has yet seen. In 1871 Dmitri Mendeleev did exactly that: he described in detail three elements that had never been found. When chemists later discovered them, the match was so close that the periodic table stopped being a clever filing system and became one of the foundations of chemistry.

Core explanation

Gaps on purpose. When Mendeleev arranged the elements in order of atomic mass in 1869, he grouped together elements with similar properties. Where the next known element clearly did not fit the pattern of a column, he left a gap , arguing that an undiscovered element belonged there. He named these missing elements using the Sanskrit word eka , meaning "one", so the element one place below silicon was eka-silicon , and the one below aluminium was eka-aluminium.

Predicting from neighbours. To predict eka-silicon, Mendeleev averaged and extended the properties of the elements around the gap: silicon and tin above and below it in the group, and zinc and arsenic on either side. This is the same reasoning you use when predicting properties from position.

The test. In 1886 the German chemist Clemens Winkler discovered a new element in a silver-rich mineral and named it germanium . Its properties matched eka-silicon remarkably well.

Property Mendeleev's prediction for eka-silicon (1871) Germanium as measured --- --- --- Relative atomic mass about 72 72.6 Density of element 5.5 g/cm³ 5.3 g/cm³ Appearance Dark grey metal Grey-white, metal-like Formula of oxide EsO₂ GeO₂ Formula of chloride EsCl₄, a liquid GeCl₄, a liquid Boiling point of chloride below 100 °C about 86 °C Density of chloride 1.9 g/cm³ 1.88 g/cm³

Two more successes. Eka-aluminium was discovered in 1875 as gallium , and eka-boron in 1879 as scandium . For gallium, the first measured density (4.7 g/cm³) disagreed with Mendeleev's prediction of about 5.9 g/cm³. He suggested the sample was impure; when it was purified, the density turned out to be about 5.9 g/cm³, just as he had forecast.

Why it mattered. A theory that makes risky, precise predictions and survives the test earns great confidence. After these discoveries, the periodic law was accepted by chemists worldwide, and the gaps in the table became a guide for finding the remaining elements.

What Mendeleev did not know. He did not know about protons or electron shells. The modern explanation — that elements in a group share the same number of outer electrons — came decades later, and it confirmed why his arrangement worked.

Step-by-step reasoning

To see how Mendeleev predicted a missing element:

1. Identify the gap and its neighbours above, below and on each side. 2. Note the combining power of the group, which gives the formulae of the oxide and chloride. 3. Average neighbouring values such as atomic mass and density. 4. State the predicted properties clearly so they can be tested. 5. Compare with measurements once the element is discovered.

Visual explanation

Picture a small section of the table as a cross: silicon above, tin below, zinc to the left and arsenic to the right, with an empty square in the middle. Arrows point inwards from all four neighbours into the empty square, showing how their properties were combined to fill it in.

Real-world analogy

Mendeleev was like a detective who studies a family photograph with one person missing. From the heights, faces and ages of the brothers and sisters on either side, the detective describes the missing sibling so precisely that, when that person finally walks in, everyone recognises them at once.

Real-world example

Germanium, once just a gap in a table, became vital in technology. It was used in some of the first transistors in the 1940s and 1950s, and today it is used in fibre-optic cables, infrared camera lenses and high-efficiency solar cells on satellites.

Why?

Why could Mendeleev predict formulae so confidently? Elements in the same group combine with oxygen and chlorine in the same ratios. Silicon forms SiO₂ and SiCl₄ and tin forms SnO₂ and SnCl₄, so the element between them had to form the oxide XO₂ and chloride XCl₄.

Common misconception

"Mendeleev discovered germanium." He never isolated any of the missing elements. His achievement was to predict their existence and properties from the pattern in his table; other chemists made the discoveries that confirmed his predictions.

Worked example

Question: Mendeleev predicted that eka-silicon would have a density of 5.5 g/cm³. Germanium's measured density is 5.3 g/cm³. Calculate the percentage difference relative to the measured value and comment.

Reasoning: Difference = 5.5 − 5.3 = 0.2 g/cm³. Percentage difference = (0.2 ÷ 5.3) × 100 ≈ 3.8%.

Answer: About 4%, a very close prediction for an element no one had ever seen.

Quick check

1. Which element was Mendeleev's "eka-silicon"? Answer: Germanium, discovered by Clemens Winkler in 1886.

Exam focus

Be ready to explain why Mendeleev left gaps and why the discovery of germanium, gallium and scandium led to his table being accepted. Use data from a comparison table to show that predictions and measurements agree, and remember that successful prediction is the key evidence for a scientific theory.

Advanced insight

Mendeleev also made predictions that failed. He proposed elements lighter than hydrogen and an element to explain the solar corona, neither of which exists. Science judges a theory by its overall record: the striking successes, combined with the later explanation from atomic structure, outweighed the misses.

Summary

Mendeleev left gaps in his periodic table for undiscovered elements and predicted their properties from their neighbours. Eka-silicon was found in 1886 as germanium, with atomic mass, density and compound formulae almost exactly as predicted; gallium and scandium also matched their predictions. These successful, testable predictions convinced chemists that the periodic law was real.

Practice questions

1. Why did Mendeleev leave gaps in his periodic table? Answer: Because some known elements did not fit the pattern of properties in a column, so he concluded that undiscovered elements belonged in those places. 2. Mendeleev predicted eka-silicon would form an oxide with the formula EsO₂. Explain how he could make this prediction. Answer: Silicon above and tin below both form dioxides, SiO₂ and SnO₂, so an element in the same group should form an oxide of the same type. 3. Name the elements that were discovered to fill the places of eka-aluminium and eka-boron. Answer: Gallium (eka-aluminium) and scandium (eka-boron). 4. Explain why successful predictions are stronger evidence for a theory than explaining existing facts. Answer: Any arrangement can be adjusted to fit known facts, but a precise prediction of something unknown could have been proved wrong, so when it is confirmed it shows the theory reflects a real pattern.