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= Henry Moseley – The Scientist Who Defined Atomic Number =

== Introduction ==
'''Henry Gwyn Jeffreys Moseley''' (1887–1915) was a British physicist whose groundbreaking work on the atomic structure of elements redefined the **Periodic Table**. He introduced the concept of the **atomic number**, which became the modern basis for arranging elements.

His work corrected flaws in Mendeleev’s table and laid the foundation for modern atomic physics.

== Early Life and Education ==
Henry Moseley was born on '''November 23, 1887''', in Weymouth, England. He was educated at **Eton College** and later studied physics at the **University of Oxford (Trinity College)**.

After graduating, he worked under **Ernest Rutherford** at the University of Manchester, one of the most prominent nuclear physicists of the time.

== Major Scientific Contributions ==

=== X-ray Spectroscopy and Atomic Number ===
In 1913, Moseley conducted experiments using **X-ray spectroscopy** to study the emission spectra of various elements. From this, he made a key discovery:

<blockquote>
''The frequency of X-rays emitted by elements increases in a systematic way as you go up the Periodic Table.''  
</blockquote>

This led him to propose the concept of the **atomic number** (Z) — the number of protons in the nucleus — as a more accurate method of ordering the elements than atomic mass.

=== Moseley's Law ===
Moseley formulated what is now known as **Moseley’s Law**:

<blockquote>
''The square root of the frequency of the emitted X-rays is proportional to the atomic number.''  
</blockquote>

This law helped to resolve ambiguities in Mendeleev’s table and correctly placed elements like:
* **Cobalt (Co)** before **Nickel (Ni)**, despite a higher atomic mass
* **Argon (Ar)** before **Potassium (K)**

His work confirmed the identity and order of known elements and predicted the existence of several undiscovered ones.

== Impact on the Periodic Table ==
Moseley's contributions led to the modern definition of the **Periodic Law**:
<blockquote>
''The properties of elements are periodic functions of their atomic numbers.''  
</blockquote>

Thanks to Moseley, the Periodic Table was revised to be arranged by **atomic number** rather than atomic mass — a correction that brought it into alignment with modern atomic theory.

== Military Service and Death ==
Tragically, Henry Moseley volunteered to serve in the British Army during **World War I**. He was killed in action on '''August 10, 1915''', at the **Battle of Gallipoli**, at the young age of 27.

His early death was considered a great loss to science. Many scientists believe he would have received a **Nobel Prize** had he lived longer.

== Legacy ==
* The element ordering we use today is based on **Moseley’s concept of atomic number**.
* Moseley is honored in scientific history as the person who gave the Periodic Table its final, accurate structure.
* Some scientific institutions and awards are named in his memory.

== See Also ==
* [[Dmitri Mendeleev – The Father of the Periodic Table]]
* [[Periodic Table of Elements – A Complete Guide]]
* [[Atomic Number]]
* [[X-ray Spectroscopy]]

== References and Further Reading ==
* [https://www.britannica.com/biography/Henry-Moseley Britannica – Henry Moseley]
* IUPAC and Royal Society of Chemistry Archives
* Moseley’s original 1913 publication on atomic numbers
* University of Manchester historical resources

== Disclaimer ==
''This article is provided for '''educational and academic purposes only'''. All scientific and historical facts are based on credible sources and public domain materials. It is designed to support school and college-level science education on MediaWiki platforms.''