Answer: Principal quantum number of the outermost electron.
- A Principal quantum number of the outermost electron
- B The total number of valence electrons in the outer shell
- C The group number assigned to that element's column
- D The atomic number of the first element in that row
Correct answer: A. Principal quantum number of the outermost electron
Explanation: The period number equals the principal quantum number (n) of the valence shell of that period's elements.
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 →