Periodic Classification of Elements — Part 2: The modern periodic table and its trends: visual lesson, CBSE Class 10 Science

Lesson

The modern periodic table

Part 2 — one number, the atomic number, decides where every element sits and explains the patterns in its properties.

In this lesson you will

State the modern periodic law and explain how it removes the limitations of Mendeléev’s table
Locate an element in the modern table from its atomic number
Predict trends in valency and atomic size across periods and down groups
Predict trends in metallic and non-metallic character

Concept

The modern periodic law

Modern periodic law: the properties of elements are a periodic function of their atomic numbers. The atomic number (number of protons) is a more basic property than atomic mass, and it rises by exactly one from each element to the next.

Table comparing Mendeléev’s table and the modern periodic table on basis, steps, number of groups and noble gases
	Mendeléev’s table	Modern periodic table
Basis	Atomic mass	Atomic number
Steps between elements	Irregular	Exactly one proton each time
Number of groups	Eight (I to VIII)	Eighteen (1 to 18)
Noble gases	Added later as a new group	Group 18 from the start

Concept

Old problems, solved

Arranging by atomic number removes Mendeléev’s main difficulties.

Table showing how arranging by atomic number solves the isotope problem and the order of tellurium and iodine, cobalt and nickel, and argon and potassium
Old problem	How atomic number solves it
Isotopes have different masses	Isotopes have the same atomic number, so they share one place
Tellurium (52) before iodine (53)	Atomic numbers are in the right order, so no swap is needed
Cobalt (27) before nickel (28)	Again in atomic-number order
Argon (18) before potassium (19)	Argon is heavier, but has the smaller atomic number

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