Answer: atomic mass.
- A atomic number
- B atomic mass
- C density
- D valency
Correct answer: B. atomic mass
Explanation: Mendeleev's periodic law arranged elements by increasing atomic mass; the modern table later used atomic number instead.
The complete modern periodic table (118 elements). Image: Cepheus et al., Public Domain, via Wikimedia Commons.
Concept context
The periodic table organises all 118 elements by atomic number and similar properties. Study periodic trends like atomic radius, ionization energy, and electronegativity, and learn how an element's position predicts its behaviour.
Read the full Classification of Elements and Periodicity in Properties notes →