Periodic Classification of Elements

Lesson

Part 1 — how chemists turned a growing list of elements into an orderly table, one attempt at a time. (This chapter is assessed through school-based activities and projects, not the board theory paper.)

In this lesson you will

Explain why scientists looked for a way to classify elements
Explain Döbereiner’s triads and Newlands’ law of octaves, with their limitations
Describe the basis of Mendeléev’s periodic table
State the achievements and limitations of Mendeléev’s table

Concept

Why classify elements?

By the early 1800s, dozens of elements were known, and new ones kept being discovered. Sorting similar elements together makes their properties easier to study, remember and predict. Today 118 elements are known.

Table contrasting studying elements one by one with studying them in an orderly arrangement
Without an arrangement	With an arrangement
Each element studied separately	Study one element, learn about its whole family
No way to guess unknown properties	Properties can be predicted from position
New discoveries do not fit anywhere	New elements slot into a place

Concept

Döbereiner’s triads

In the early 1800s, the German chemist Johann Döbereiner noticed groups of three elements with similar properties. Arranged by increasing atomic mass, the middle element’s atomic mass was close to the average of the other two. He called these groups triads.

Atomic masses are modern values, rounded to one decimal place.
Triad	Atomic masses (u)	Mean of first and third	Middle element
Lithium, sodium, potassium	6.9, 23.0, 39.1	23.0	Sodium 23.0
Calcium, strontium, barium	40.1, 87.6, 137.3	88.7	Strontium 87.6
Chlorine, bromine, iodine	35.5, 79.9, 126.9	81.2	Bromine 79.9

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