Atomic number

What is Atomic Number?

The atomic number of a Elements is the number of protons found in the nucleus of an atom of that element. It is denoted by the symbol Z.

Each element has a unique Atomic Number, which determines its identity and position in the Periodic Table of Elements.

Definition

Atomic Number (Z) = Number of Protons in the Nucleus

• For example, all Hydrogen atoms have 1 proton, so the Atomic Number of hydrogen is 1.
• Carbon has 6 protons, so its atomic number is 6.
• Oxygen has 8 protons, so its atomic number is 8.

Role of Atomic Number

The Atomic Number plays a central role in modern Chemistry:

• It defines the element – no two elements share the same Atomic Number.
• It determines the arrangement of elements in the modern Periodic Table.
• It influences an atom’s chemical properties, electron configuration, and place in the periodic trends.
• In a neutral atom, the number of electrons equals the number of protons (i.e., the Atomic Number).

Discovery of Atomic Number

Although early scientists like Dmitri Mendeleev arranged elements by Atomic Mass, inconsistencies in the Periodic Table were only resolved when British physicist Henry Moseley introduced the concept of Atomic Number in 1913.

Using X-ray Spectroscopy, Moseley discovered that each element emits radiation at a frequency directly related to the number of Protons in its nucleus.

This led to the modern Periodic Law:

The properties of elements are a periodic function of their atomic numbers.

Examples of Atomic Numbers

Selected Elements and Their Atomic Numbers

Element	Symbol	Atomic Number
Hydrogen	H	1
Helium	He	2
Carbon	C	6
Oxygen	O	8
Iron	Fe	26
Gold	Au	79
Uranium	U	92
Oganesson	Og	118

Difference Between Atomic Number and Mass Number

• Atomic Number (Z): Number of Protons
• Mass Number (A): Total number of Protons + Neutrons

Example: Carbon-12 → Atomic number = 6, Mass number = 12 So, neutrons = 12 − 6 = 6

Atomic Number and Isotopes

Isotopes are atoms of the same element (same Atomic Number) but with different numbers of Neutrons.

• Example: Hydrogen has three isotopes:
  • Protium (¹H), Deuterium (²H), and Tritium (³H) – all have Atomic Number 1

Importance in Chemistry and Physics

The Atomic Number is essential for:

• Identifying elements
• Understanding Chemical Behavior
• Predicting Reactivity and Bonding
• Nuclear Physics and Radioactivity

See Also

• Periodic Table of Elements – A Complete Guide
• Chemical Elements – A Complete Guide
• Dmitri Mendeleev – The Father of the Periodic Table
• Henry Moseley – The Scientist Who Defined Atomic Number

References and Further Reading

• International Union of Pure and Applied Chemistry (IUPAC)
• Khan Academy – Atomic Structure
• Standard Chemistry Textbooks (NCERT, Oxford, Pearson)

Disclaimer

This article is written for Academic and Educational Purposes only. It is intended to provide a clear explanation of the Atomic Number for students and educators using MediaWiki platforms. For advanced study, consult official scientific publications and curriculum-based textbooks.