Introduction

Dmitri Mendeleev's periodic table was one of the greatest achievements in chemistry, successfully grouping elements with similar properties and predicting the existence of undiscovered elements. However, there were still a few inconsistencies. Some elements had to be placed out of order when arranged by relative atomic mass so that they matched the chemical properties of their groups.

In the early 1900s, the English physicist Henry Moseley discovered the true basis for organising the elements. He showed that the correct order was based not on atomic mass, but on atomic number—the number of protons in an atom's nucleus. This discovery transformed the periodic table into the version we use today.

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Who Was Henry Moseley?

Henry Moseley was an English physicist who studied the X-rays emitted by different elements.

By measuring these X-rays, he discovered that each element has a unique positive charge in its nucleus.

Today, we know this positive charge comes from the number of protons in the nucleus.

Moseley's work showed that each element has its own unique atomic number.

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What Is Atomic Number?

The atomic number is the number of protons in the nucleus of an atom.

Every element has a unique atomic number.

For example:

No two elements have the same atomic number.

Definition:
The atomic number is the number of protons in the nucleus of an atom. It determines the identity of an element.

Atomic Number vs Atomic Mass

Before Moseley's work, scientists organised elements according to relative atomic mass.

These two terms describe different things.

For example:

• Carbon always has 6 protons, so its atomic number is 6.
• Different carbon isotopes have different numbers of neutrons, giving them different masses, but they are still all carbon because they each contain 6 protons.

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Why Is Atomic Number More Accurate?

Atomic number provides a better method of classification because it is based on the fundamental identity of an element.

The number of protons:

• never changes for a particular element,
• determines the number of electrons in a neutral atom,
• determines the electron arrangement,
• determines the chemical properties of the element.

Because chemical behaviour depends largely on electron arrangement, organising elements by atomic number naturally places elements with similar properties into the same groups.

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Resolving Problems in Mendeleev's Table

Mendeleev occasionally placed elements out of atomic mass order because their chemical properties suggested they belonged in different groups.

At the time, this seemed unusual.

Moseley's discovery explained why.

Example 1: Argon and Potassium

If arranged by atomic mass, potassium would come before argon.

However, chemically:

• Argon is a noble gas.
• Potassium is an alkali metal.

Arranging by atomic number correctly places:

• Argon (18)
• Potassium (19)

This matches both their atomic structure and chemical behaviour.

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Example 2: Cobalt and Nickel

Although cobalt has a slightly greater atomic mass, it correctly comes before nickel because its atomic number is lower.

These apparent "mistakes" disappeared once the table was arranged by atomic number.

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How Moseley's Work Improved the Periodic Table

Moseley's discovery brought several important improvements.

It:

• correctly ordered every element,
• removed inconsistencies caused by atomic mass,
• explained why periodic patterns exist,
• correctly grouped elements with similar chemical properties,
• provided a scientific basis for the periodic table,
• made predictions about new elements even more reliable.

Today, every modern periodic table is arranged by atomic number.

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Why Atomic Number Determines Chemical Properties

The number of protons determines the number of electrons in a neutral atom.

The arrangement of these electrons controls:

• chemical bonding,
• reactivity,
• ion formation,
• physical and chemical properties.

Because elements in the same group have similar outer electron arrangements, they also have similar chemical behaviour.

This explains why the periodic table works so well.

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Comparing Mendeleev's and Moseley's Periodic Tables

Moseley's work did not replace Mendeleev's ideas—it improved and completed them.

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Worked Example 1

A student says:

"Atomic number tells us how heavy an atom is."

Question

Is this correct?

Solution

No.

Atomic number tells us the number of protons in an atom.

The mass of an atom depends on both protons and neutrons.

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Worked Example 2

Which statement correctly explains why the modern periodic table is arranged by atomic number?

A. Atomic number changes as atoms gain electrons.

B. Atomic number determines the identity and chemical properties of an element.

C. Atomic number is always larger than atomic mass.

D. Atomic number depends on the number of neutrons.

Answer

B. Atomic number determines the identity and chemical properties of an element.

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Worked Example 3

Complete the table.

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Worked Example 4

Why are argon and potassium arranged in that order in the modern periodic table, even though argon has a slightly greater relative atomic mass?

Solution

They are arranged according to atomic number, not atomic mass. Argon has atomic number 18, while potassium has atomic number 19, so argon correctly comes first. This arrangement also places them in groups that match their chemical properties.

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Real-World Connection

Atomic number is fundamental to modern chemistry and physics. Scientists use it to identify elements, predict their behaviour, and design new materials. It also plays a key role in medicine, where radioactive isotopes are selected according to the properties of specific elements, and in technology, where semiconductors such as silicon are chosen because of their electron arrangements, which are determined by atomic number.

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Did You Know?

Henry Moseley made his groundbreaking discovery when he was only in his twenties. Sadly, he was killed during the Gallipoli Campaign in 1915 at the age of just 27. Many historians believe that, had he lived, he might have made even greater contributions to physics and chemistry—perhaps even earning a Nobel Prize.

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Key Terms

• Atomic number — the number of protons in the nucleus of an atom.
• Relative atomic mass — the weighted average mass of an element's naturally occurring atoms compared with carbon-12.
• Proton — a positively charged particle found in the nucleus of an atom.
• Periodicity — the repeating pattern of physical and chemical properties among the elements.
• Electron arrangement — the distribution of electrons around the nucleus of an atom.
• Modern periodic table — the arrangement of elements in order of increasing atomic number.

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Key Takeaways

• Henry Moseley showed that elements should be arranged according to atomic number, not relative atomic mass.
• The atomic number is the number of protons in an atom and uniquely identifies each element.
• Atomic number provides a more accurate basis for organising elements because it determines electron arrangement and chemical behaviour.
• Moseley's work resolved inconsistencies in Mendeleev's periodic table, such as the positions of argon and potassium, and cobalt and nickel.
• The modern periodic table is arranged by increasing atomic number, ensuring that elements with similar chemical properties appear in the same groups.
• Moseley's discovery completed the foundation of the periodic table and remains one of the most important advances in the history of chemistry.