Dobereiner's Triads and Atomic Masses

The video discusses Johann Wolfgang Dobereiner's discovery of triads in 1829, where he identified groups of three elements with similar properties. These triads, when arranged by increasing atomic masses, showed that the atomic mass of the middle element was approximately the mean of the other two. Examples include lithium, sodium, and potassium, among others. However, Dobereiner's system was limited as it could only identify a few triads and not all elements fit the pattern.

Who discovered the groups of three elements known as triads?

• John Dalton

• Henry Moseley

• Johann Wolfgang Dobereiner

• Dmitri Mendeleev

What was the key observation made by Dobereiner about the atomic masses in a triad?

• The atomic mass of the heaviest element is double that of the lightest.

• The atomic mass of the middle element is the mean of the other two.

• The atomic mass of the middle element is the sum of the other two.

• The atomic mass of the lightest element is half that of the heaviest.

Which elements formed the first triad discovered by Dobereiner?

• Sulfur, Tellurium, Selenium

• Chlorine, Iodine, Bromine

• Lithium, Sodium, Potassium

• Calcium, Barium, Strontium

What is the atomic mass of Sodium in the first triad?

• 39

• 23

• 12

• 6.9

Which elements formed the second triad?

• Lithium, Sodium, Potassium

• Calcium, Barium, Strontium

• Chlorine, Iodine, Bromine

• Sulfur, Tellurium, Selenium

Which elements formed the third triad?

• Sulfur, Tellurium, Selenium

• Chlorine, Iodine, Bromine

• Calcium, Barium, Strontium

• Lithium, Sodium, Potassium

Which elements formed the fourth triad?

• Sulfur, Tellurium, Selenium

• Lithium, Sodium, Potassium

• Calcium, Barium, Strontium

• Chlorine, Iodine, Bromine

Why was Dobereiner's system of triads not successful?

• It could not explain the properties of elements.

• It was too complex to understand.

• It was not based on atomic masses.

• It could only identify a few triads from known elements.

Which of the following is NOT a reason for the failure of Dobereiner's triads?

• It was too complex to understand.

• It could not be applied to all known elements.

• Some triads did not follow the mean atomic mass rule.

• Only a few triads could be identified.