Introduction

By the mid-1800s, more than 60 elements had been discovered. Although scientists such as Johann Döbereiner and John Newlands had recognised important patterns, no classification system could successfully organise all the known elements.

In 1869, the Russian chemist Dmitri Mendeleev developed a periodic table that transformed chemistry. His arrangement not only grouped similar elements together but also predicted the existence and properties of elements that had not yet been discovered. His work became the foundation of the modern periodic table used today.

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Who Was Dmitri Mendeleev?

Dmitri Mendeleev was a Russian chemist and teacher.

While preparing a chemistry textbook, he realised that the known elements could be organised in a logical way based on their properties.

His goal was to create a system that:

• organised all known elements,
• grouped similar elements together,
• explained repeating chemical patterns,
• helped predict the properties of undiscovered elements.

His periodic table became one of the greatest achievements in the history of science.

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How Did Mendeleev Organise the Elements?

Mendeleev arranged the known elements in order of increasing relative atomic mass.

However, unlike earlier scientists, he did not follow atomic mass blindly.

Whenever the chemical properties suggested a better arrangement, he moved an element to a different position.

This allowed elements with similar properties to appear in the same vertical columns.

For example:

• lithium, sodium, and potassium were placed together,
• fluorine, chlorine, bromine, and iodine were grouped together.

Mendeleev believed that chemical properties were more important than strictly following atomic mass.

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Recurring Patterns in Properties

As Mendeleev arranged the elements, he noticed that their physical and chemical properties repeated at regular intervals.

This repeating pattern became known as periodicity.

Elements in the same group often shared similar:

• chemical reactions,
• valency,
• types of compounds formed,
• physical properties.

For example:

Recognising these recurring patterns allowed Mendeleev to organise the elements much more successfully than previous scientists.

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Key Features of Mendeleev's Periodic Table

Mendeleev's table included several important features.

1. Elements Arranged by Increasing Atomic Mass

Most elements were ordered according to increasing relative atomic mass.

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2. Similar Elements Were Grouped Together

Elements with similar chemical properties appeared in the same vertical columns.

This made it easier to compare their behaviour.

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3. Periodic Patterns Were Recognised

The properties of the elements repeated in a regular pattern.

This gave the periodic table its name.

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4. Gaps Were Left for Undiscovered Elements

Instead of forcing every element into the table, Mendeleev deliberately left empty spaces.

He believed these spaces represented elements that had not yet been discovered.

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5. Some Elements Were Reordered

In a few cases, Mendeleev placed an element slightly out of atomic mass order because its chemical properties clearly matched another group.

Although this seemed unusual at the time, later discoveries showed that his decisions were correct.

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Why Did Mendeleev Leave Gaps?

One of Mendeleev's greatest achievements was his confidence to leave blank spaces in his table.

He realised that:

• not all elements had been discovered,
• forcing elements into incorrect positions would destroy the repeating patterns.

Instead, he predicted that future scientists would eventually discover the missing elements.

Even more remarkably, he predicted:

• their approximate atomic masses,
• their physical properties,
• their chemical behaviour.

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Predictions That Came True

Several years after Mendeleev published his periodic table, scientists discovered new elements that fit perfectly into the gaps.

One famous example is gallium.

Before it was discovered, Mendeleev predicted:

• its approximate atomic mass,
• its density,
• its melting point,
• the formula of its oxide.

When gallium was discovered in 1875, its properties closely matched Mendeleev's predictions.

Later discoveries of scandium and germanium also confirmed his predictions.

These successes convinced scientists that Mendeleev's periodic table was correct.

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Strengths of Mendeleev's Periodic Table

Mendeleev's table:

• organised nearly all known elements,
• grouped similar elements together,
• recognised periodic patterns,
• predicted undiscovered elements,
• correctly predicted many element properties,
• became increasingly accurate as new elements were discovered.

These achievements made his periodic table far superior to earlier classification systems.

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Limitations of Mendeleev's Table

Although revolutionary, Mendeleev's table was not perfect.

Some limitations included:

• It was arranged by atomic mass rather than atomic number.
• Some elements appeared out of atomic mass order.
• Isotopes were not yet understood.
• Noble gases had not yet been discovered.
• The reason for periodicity was not fully understood.

These issues were later resolved through the work of scientists such as Henry Moseley.

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Comparing Mendeleev's Table with the Modern Periodic Table

Despite these differences, the overall organisation of groups and periods remains remarkably similar.

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Worked Example 1

A student says:

"Mendeleev left gaps because he forgot to include some elements."

Question

Is this correct?

Solution

No.

Mendeleev deliberately left gaps because he believed undiscovered elements existed. He even predicted many of their properties before they were found.

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Worked Example 2

Why did Mendeleev sometimes place elements out of atomic mass order?

A. He made calculation mistakes.

B. He wanted similar chemical properties to stay together.

C. He did not know the atomic masses.

D. He arranged elements alphabetically.

Answer

B. He wanted similar chemical properties to stay together.

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Worked Example 3

Complete the table.

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Real-World Connection

The periodic table remains one of the most valuable tools in chemistry. Scientists use it to predict how elements will react, design new materials, develop medicines, and create advanced technologies. Mendeleev's willingness to trust scientific patterns rather than forcing the data to fit existing knowledge is an excellent example of how scientific theories evolve through observation, evidence, and prediction.

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Did You Know?

When Mendeleev presented his periodic table, some scientists were sceptical because it contained empty spaces. However, when elements such as gallium, scandium, and germanium were later discovered with properties that closely matched his predictions, his periodic table gained worldwide acceptance. Today, Mendeleev is often called the "Father of the Periodic Table."

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Key Terms

• Periodic table — an organised arrangement of the chemical elements.
• Relative atomic mass — the average mass of an atom of an element compared with carbon-12.
• Periodicity — the repeating pattern of physical and chemical properties among the elements.
• Group — a vertical column of elements with similar chemical properties.
• Period — a horizontal row of elements.
• Prediction — a statement about future discoveries based on scientific evidence and patterns.

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Key Takeaways

• Dmitri Mendeleev developed the first widely accepted periodic table in 1869.
• He arranged elements mainly by increasing relative atomic mass, while prioritising similarities in their chemical properties.
• Mendeleev recognised that the properties of elements repeat in a regular pattern, known as periodicity.
• He deliberately left gaps for undiscovered elements and accurately predicted many of their properties.
• The later discoveries of gallium, scandium, and germanium strongly supported his ideas.
• Although the modern periodic table is arranged by atomic number, it is built upon the foundation established by Mendeleev's pioneering work.