Classification of Elements and Periodicity in Properties - Genesis of Periodic Classification
Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The genesis of periodic classification began with the need to systemize the study of elements as their number increased. Early attempts were based on atomic weights.
Döbereiner's Triads (1817): Johann Wolfgang Döbereiner arranged elements with similar properties into groups of three called triads. He observed that the atomic weight of the middle element was approximately the arithmetic mean of the weights of the other two elements.
Newlands' Law of Octaves (1866): John Alexander Newlands arranged elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element, analogous to the octaves in music. This law was valid only up to Calcium ().
Lothar Meyer's Curves: Lothar Meyer plotted physical properties like atomic volume, melting point, and boiling point against atomic weight. He observed that elements with similar properties occupied similar positions on the curve (e.g., alkali metals occupied the peaks).
Mendeleev's Periodic Law: Dmitri Mendeleev stated that 'The physical and chemical properties of the elements are periodic functions of their atomic weights.'
Mendeleev's Periodic Table: Mendeleev arranged elements in horizontal rows (periods) and vertical columns (groups). He left gaps for undiscovered elements like Eka-Aluminium (later discovered as Gallium, ) and Eka-Silicon (later discovered as Germanium, ).
📐Formulae
💡Examples
Problem 1:
In a Döbereiner's triad consisting of Lithium (), Sodium (), and Potassium (), the atomic weights of Lithium and Potassium are and respectively. Calculate the theoretical atomic weight of Sodium and compare it with the actual value ().
Solution:
According to Döbereiner's Law of Triads, the atomic weight of Sodium () should be the arithmetic mean of and : Using vertical addition: Now, dividing by : The theoretical value is exactly .
Explanation:
The calculated average atomic weight of the first and third elements matches the known atomic weight of the middle element (Sodium), confirming the triad relationship.
Problem 2:
Mendeleev predicted the existence of an element he called 'Eka-Aluminium'. What is the modern name of this element and what is its approximate atomic weight based on Mendeleev's prediction?
Solution:
The element predicted as Eka-Aluminium () is now known as Gallium (). Mendeleev predicted its atomic weight to be approximately . Its actual atomic weight is .
Explanation:
Mendeleev used the properties of surrounding elements in his table to predict the properties of gaps, demonstrating the power of periodic classification.