Triads, Octaves & The Search
Explore early classification attempts by Dobereiner and Newlands.
What this lesson covers
Why it matters
Before electrons were known, chemists hunted for patterns to tame the chaos of the elements. Doebereiner spotted families of three — 'triads' — whose middle member sat neatly between the others.
Predict first
Why did Doebereiner group lithium, sodium and potassium into one triad?
A Doebereiner triad is three elements with similar properties where the middle one's atomic mass ≈ the average of the other two. Classic triads: (Li, Na, K), (Ca, Sr, Ba), (Cl, Br, I).
- They have similar properties, and sodium's atomic mass is about the average of lithium and potassium — correct
- They are the three lightest metals known
- They were discovered in the same year
What you do
Each of these elements belongs to one of Doebereiner's triads. Sort them into their families of three.
Check yourself
In the triad Li (7), Na (23), K (39), the middle mass 23 is close to…
(7 + 39) / 2 = 23 — the middle element's mass is about the average, the heart of the triad idea.
Newlands' Law of Octaves said that…
Like the eighth note of a scale, Newlands found every eighth element repeated similar properties.
- the average of 7 and 39, which is 23 — correct
- the sum of 7 and 39
- the difference of 7 and 39
- every eighth element had properties like the first, as in a musical scale — correct
- elements come in groups of eight bonded atoms
- there are exactly eight elements