3. Predicting Missing Elements

Introduction

One of the greatest achievements in the history of science was Dmitri Mendeleev's ability to predict the existence of elements that had not yet been discovered. Rather than forcing all the known elements into his periodic table, he recognised that some spaces were missing. He believed these gaps represented elements that scientists would eventually find.

Even more impressively, Mendeleev predicted many of the physical and chemical properties of these missing elements before anyone had seen them. When these elements were later discovered and closely matched his predictions, scientists gained great confidence in the periodic table. This success showed that the periodic table was not simply a way of organising information—it was a powerful scientific model capable of making accurate predictions.

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Looking for Patterns

Mendeleev arranged the known elements according to:

• increasing relative atomic mass,
• similar chemical properties,
• repeating patterns called periodicity.

As he organised the elements, he noticed that certain positions in the table were empty.

Instead of assuming his table was wrong, he concluded that:

• some elements had not yet been discovered,
• these missing elements would eventually fill the gaps,
• their properties could be estimated from the surrounding elements.

Definition:
A prediction is a statement about something that is expected to happen or be discovered based on scientific evidence and patterns.

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How Did Mendeleev Make His Predictions?

Mendeleev carefully examined the elements:

• above and below each gap,
• to the left and right of each gap.

From these neighbouring elements, he estimated:

• relative atomic mass,
• density,
• melting point,
• chemical reactivity,
• types of compounds formed,
• formulas of oxides and chlorides.

Because elements in the same group have similar properties, he could make surprisingly accurate predictions.

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Example 1 – Eka-Aluminium (Gallium)

One of Mendeleev's most famous predictions concerned an unknown element he called eka-aluminium.

("Eka" is a Sanskrit word meaning one, indicating that the element would appear one place below aluminium.)

He predicted that this element would:

• have an atomic mass of about 68,
• be a metal,
• have a low melting point,
• form the oxide E₂O₃,
• have a density of about 6 g/cm³.

In 1875, French chemist Paul-Émile Lecoq de Boisbaudran discovered gallium.

Its measured properties closely matched Mendeleev's predictions.

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Example 2 – Eka-Boron (Scandium)

Mendeleev also predicted an element below boron.

He called it eka-boron.

He predicted that it would:

• be a light metal,
• have an atomic mass of about 44,
• form an oxide with the formula E₂O₃.

In 1879, Swedish chemist Lars Fredrik Nilson discovered scandium.

Once again, its measured properties closely matched Mendeleev's predictions.

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Example 3 – Eka-Silicon (Germanium)

Mendeleev predicted another missing element below silicon.

He called it eka-silicon.

He predicted:

• atomic mass about 72,
• density around 5.5 g/cm³,
• grey appearance,
• semiconductor-like behaviour (although this term did not yet exist),
• oxide formula EO₂.

In 1886, German chemist Clemens Winkler discovered germanium.

The agreement between prediction and reality was remarkable.

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Comparing Predictions with Discoveries

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The predictions were astonishingly close to the measured values.

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Why Were These Predictions So Important?

Many scientists were initially doubtful about Mendeleev's periodic table.

However, when predicted elements were discovered and matched his forecasts:

• confidence in the periodic table increased,
• scientists accepted periodic patterns as real,
• Mendeleev's classification became widely respected,
• chemistry became much more organised and predictive.

The success of these predictions transformed the periodic table from a useful chart into a powerful scientific theory.

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Using Periodic Patterns to Make Predictions

Even today, scientists use periodic patterns to predict the behaviour of unfamiliar elements.

For example:

If an unknown element is placed below calcium in Group 2, we can predict that it will:

• be a metal,
• lose two electrons,
• form 2+ ions,
• react with water (though the rate may differ),
• form an oxide with the formula EO.

Likewise, if a newly discovered element belongs to the halogens, we can predict that it will:

• be a non-metal,
• form –1 ions,
• react with metals to form salts,
• exist as diatomic molecules.

This ability to predict properties remains one of the greatest strengths of the periodic table.

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Worked Example 1

A student says:

"Mendeleev filled every space in his periodic table, even if the element had not been discovered."

Question

Is this correct?

Solution

No.

Mendeleev deliberately left gaps because he believed undiscovered elements would eventually fill them.

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Worked Example 2

Which element was originally called eka-aluminium?

A. Germanium

B. Gallium

C. Scandium

D. Silicon

Answer

B. Gallium

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Worked Example 3

An unknown element is discovered directly below chlorine in the periodic table.

Predict two of its properties.

Solution

Possible predictions:

• It is a halogen.
• It is a reactive non-metal.
• It forms –1 ions.
• It reacts with metals to form salts.

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Worked Example 4

Why did the discovery of gallium strengthen confidence in Mendeleev's periodic table?

Solution

Gallium's measured properties closely matched Mendeleev's predictions. This showed that the periodic patterns he had identified were real and that his periodic table could successfully predict the existence and properties of undiscovered elements.

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Real-World Connection

Prediction remains an essential part of modern science. Chemists use the periodic table to estimate the properties of newly created elements, while physicists predict the behaviour of subatomic particles before they are observed experimentally. Mendeleev's work demonstrated that a good scientific model does more than organise existing knowledge—it allows scientists to make accurate predictions about discoveries that have not yet been made.

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Did You Know?

When gallium was first discovered, it melted at about 30°C—just above room temperature. If you hold a piece of gallium in your hand (with proper safety precautions), it can melt because of your body heat! This unusual property was not only fascinating but also closely matched Mendeleev's prediction that eka-aluminium would have a relatively low melting point.

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Key Terms

• Prediction — a statement based on scientific evidence about something expected to be discovered or observed.
• Periodicity — the repeating pattern of physical and chemical properties among the elements.
• Eka- — a prefix used by Mendeleev meaning "one," indicating an undiscovered element one place below a known element.
• Gallium (Ga) — the element that matched Mendeleev's predicted eka-aluminium.
• Scandium (Sc) — the element that matched Mendeleev's predicted eka-boron.
• Germanium (Ge) — the element that matched Mendeleev's predicted eka-silicon.

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Key Takeaways

• Mendeleev recognised repeating patterns in the properties of the elements and used these patterns to predict undiscovered elements.
• He deliberately left gaps in his periodic table rather than placing elements in incorrect positions.
• Mendeleev accurately predicted the properties of gallium, scandium, and germanium before they were discovered.
• The close agreement between his predictions and the measured properties of these elements convinced scientists that the periodic table was a reliable scientific model.
• The periodic table continues to be used today to predict the properties and behaviour of elements based on their positions.
• Mendeleev's successful predictions remain one of the strongest examples of the predictive power of scientific theories.