The Atomic Number Revolution
Why protons, not mass, define an element's identity.
What this lesson covers
Why it matters
Imagine the periodic table was a library sorted by book weight instead of author. Heavy encyclopedias would sit next to light novels, making sense of nothing.
Predict first
Why did Mendeleev's mass-based order fail for Argon and Potassium?
Moseley found that X-ray frequencies correlated with proton count, not mass. This 'Atomic Number' is the true fingerprint of an element.
- Argon is a gas and Potassium is a solid
- Atomic number (proton count) dictates chemical properties, not mass — correct
- Mendeleev made a calculation error
What you do
Arrange these four elements in their true periodic-table order. Watch argon and potassium: sorting by mass would swap them, but sorting by atomic number puts every element in its right place.
Check yourself
Which property determines an element's position in the modern periodic table?
The Modern Periodic Law states that properties are a periodic function of atomic numbers (proton count).
Why is Hydrogen placed in Group 1 of the periodic table?
Hydrogen has 1 proton (Z=1) and 1 valence electron, mimicking the ns¹ configuration of alkali metals.
- Atomic Mass
- Atomic Number — correct
- Number of Neutrons
- Valency
- It has one valence electron (1s¹), matching the ns¹ configuration of alkali metals — correct
- It is the lightest element
- It is a gas like Helium