Maharashtra Board Class 10 Science Part 1 Chapter 2 Periodic Classification of Elements PDF Download

Periodic Classification of Elements

Elements and their classification, Dobereiner's Triads, Newlands Law of Octaves, Mendeleev's Periodic Table, Modern Periodic Table

Classification of Elements

We have learnt in the previous standards that all the atoms of an element are of only one type. Today 118 elements are known to the scientific world. However, around year 1800 only about 30 elements were known. More number of elements were discovered in the course of time. More and more information about the properties of these elements was gathered. To ease the study of such a large number of elements, scientists started studying the pattern if any, in the vast information about them. You know that in the initial classification elements were classified into the groups of metals and nonmetals. Later on another class of elements called metalloids was noticed. As the knowledge about elements and their properties went on increasing different scientists started trying out different methods of classification.

Dobereiner's Triads

In the year 1817 a German scientist Dobereiner suggested that properties of elements are related to their atomic masses. He made groups of three elements each, having similar chemical properties and called them triads. He arranged the three elements in a triad in an increasing order of atomic mass and showed that the atomic mass of the middle element was approximately equal to the mean of the atomic masses of the other two elements. However, all the known elements could not be classified into the Dobereiner's triads.

Teacher's Note

Dobereiner found a pattern in three elements. This is like how you can find patterns in your school - tall students, medium students, and short students together. Scientists use patterns to organize things.

Exam Trick

Remember: In a triad, the middle element's atomic mass is the average of the first and third elements. Just like the middle height student's height is between the tallest and shortest!

Points to Remember

Dobereiner made groups of three elements called triads.
Elements in a triad have similar chemical properties.
The middle element's mass is the average of the other two.
Not all elements could fit into triads.
This was the first attempt to classify elements by properties.

Newlands' Law of Octaves

The English scientist John Newlands correlated the atomic masses of elements to their properties in a different way. In the year 1866 Newlands arranged the elements known at that time in an increasing order of their atomic masses. It started with the lightest element hydrogen and ended up with thorium. He found that every eighth element had properties similar to those of the first. For example, sodium is the eighth element from lithium and both have similar properties. Also, magnesium shows similarity to beryllium and chlorine shows similarity with fluorine. Newlands compared this similarity with the octaves in music. He called the similarity observed in the eighth and the first element as the Law of octaves.

Many limitation were found in Newlands' octaves. This law was found to be applicable only upto calcium. Newlands fitted all the known elements in a table of 7 X 8 that is 56 boxes. Newlands placed two elements each in some boxes to accommodate all the known elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some elements with different properties under the same note in the octave. For example, Newlands placed the metals Co and Ni under the note 'Do' along with halogens, while Fe, having similarity with Co and Ni, away from them along with the nonmetals O and S under the note 'Ti'. Also, Newlands' octaves did not have provision to accommodate the newly discovered elements. The properties of the new elements discovered later on did not fit in the Newlands' law of octaves.

In the Indian music system there are seven main notes, namely, Sa, Re, Ga, Ma, Pa, Dha, Ni, and their collection is called 'Saptak'. The frequency of the notes goes on increasing from 'Sa' to 'Ni'. Then comes, the 'Sa' of the upper 'Saptak' at the double the frequency of the original 'Sa'. It means that notes repeat after completion of one 'Saptak'. The seven notes in the western music are Do, Re, Mi, Fa, So, La, Ti. The note 'Do' having double the original frequency comes again at the eighth place. This is the octave of western notes. Music is created by the variety in the use of these notes.

Teacher's Note

Newlands saw a pattern like music - every eighth element repeated. But this idea did not work for all elements, just like a song pattern does not work for all types of music!

Exam Trick

Remember: Octave = 8 elements repeating properties, like musical notes repeating. But it only worked up to calcium!

Points to Remember

Newlands arranged elements by atomic mass.
Every eighth element had similar properties to the first.
He compared this to musical octaves.
This law only worked up to calcium.
It could not explain all elements.

Mendeleev's Periodic Table

The Russian scientist Dmitri Mendeleev developed the periodic table of elements during the period 1869 to 1872 A.D. Mendeleev's periodic table is the most important step in the classification of elements. Mendeleev considered the fundamental property of elements, namely, the atomic mass, as standard and arranged 63 elements known at that time in an increasing order of their atomic masses. Then he transformed this into the periodic table of elements in accordance with the physical and chemical properties of these elements.

Dmitri Mendeleev (1834-1907) was a professor in the St. Petersburg University. He made separate card for every known element showing its atomic mass. He arranged the cards in accordance with the atomic masses and properties of the elements which resulted in the invention of the periodic table of elements.

Teacher's Note

Mendeleev created a table by writing each element on a card and arranging them. This is like how you arrange your study notes - by putting related topics together. His table was so good that scientists still use it today!

Exam Trick

Remember: Mendeleev used cards - like a card game! He arranged 63 elements in a smart way. Even left empty spaces for elements not yet found. This showed how smart his table was!

Points to Remember

Mendeleev arranged elements by atomic mass in a table.
He left empty spaces for undiscovered elements.
He predicted properties of missing elements.
His predictions were correct when elements were found later.
Vertical columns are called groups and horizontal rows are called periods.

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