Beginner Topic 19 of 1102 Atomic Structure & Periodicity
Atomic Number
The number of protons in an atom's nucleus, symbol Z, which determines the element.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
In-depth explanation of Atomic Number
The atomic number, symbol Z, is the number of protons in an atom's nucleus, and it determines the identity of the element. It also gives the element's position in the periodic table. In 1913 Henry Moseley showed that the frequency of characteristic X-rays varies regularly with Z, providing a physical basis for ordering the elements instead of ordering them by atomic mass. In a neutral atom, Z also equals the number of electrons. Atomic number is always a whole number and is written as a subscript before the element symbol.
Going deeper: Atomic Number
Key ideas: Atomic number Z is the number of protons and fixes the element's position in the periodic table (Moseley, 1913, from X-ray frequencies: square root of frequency is proportional to Z - b). For a neutral atom electrons = Z; for an ion electrons = Z minus charge. Written as a subscript left of the symbol.
Common mistakes and connections: Students confuse Z with atomic mass; Z is always an integer and never shown with decimals. Another mistake is changing Z when making an ion, but only the electron count changes. Moseley's work replaced atomic mass as the ordering principle of the periodic law.
Visualization of Atomic Number
Oxygen has atomic number 8: 8 protons in the nucleus and, when neutral, 8 electrons arranged 2 and 6 in two shells. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Chlorine has Z = 17, so a chloride ion Cl- has 17 protons and 18 electrons.
Frequently asked questions about Atomic Number
What is Atomic Number?
The number of protons in an atom's nucleus, symbol Z, which determines the element.
Is Atomic Number a beginner, intermediate or advanced topic?
Atomic Number is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Atomic Number?
Study these earlier topics in Atomic Structure & Periodicity first: Thomson's Plum Pudding Model, Rutherford's Gold Foil Experiment, Nuclear Atom Model.
What comes after Atomic Number?
The next topics in Atomic Structure & Periodicity are Mass Number and Isotopes.
Can I practise Atomic Number with quizzes?
Yes. The interactive lesson includes a self-check question on Atomic Number, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Atomic Number fits in Atomic Structure & Periodicity
Atomic Number is topic 19 of 115 in Atomic Structure & Periodicity.