Cbse Class 10 Chemistry Chapter 5 Notes

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Cbse Class 10 Chemistry Chapter 5 Notes Pdf

.Classification is identification of similar species and group them with each other.There are 115 elements on earth which have been discovered till today.A systematic classification of the elements has made their study possible and easy.

Early Attempts

The earliest classification was by Lavoisier ,He categorized elements into metals and non-metals. It was difficult to classify the elements, such as boron, which exhibited the properties of both metals as well as non-metals.

A German scientist,Johann Wolfgang Dobereiner arrived at a hypothesis in the year 1829.At that time 30 plus elements were known.

According to Dobereiner, all elements occurred in groups of three, when arranged in increasing order of atomic masses. He referred to these groups as triads. In a traid the elements had similar chemical properties.

Key characteristics

Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.

Elements in Traid have similar properties.

Traids of the Dobereiners classification:

Traid1:

Element Atomic mass

Lithium (Li) 7

Sodium (Na) 23

Potassium (K) 39

In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23.

Traid2:

Element Atomic mass

Chlorine (Cl) 35.5

Bromine (Br) 80

Iodine (I) 127

Arithmetic mean of atomic mass of Chlorine(Cl),Iodine(I) = (35.5+127)/ 2

=81.5

Actual atomic mass of Bromine (Br) = 80

Traid3:

Element Atomic mass

Calcium (Ca) 40

Strontium (Sr) 88

Barium (Ba) 137

mean of atomic mass of Calcium(Ca),Barium(Ba) is almost equal to the atomic mass of Strontium(Sr).

Arithmetic mean of atomic mass of Calcium(Ca),Barium(Ba) = (40+137) / 2 =88.5

Actual atomic mass of Strontium(Sr) = 87.6

Drawbacks:

(i)All the known elements could not be arranged in the form of triads.

(ii)This law did not hold good for elements with very low or very high atomic mass.

Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5.

(iii) As the technique improved for measuring atomic masses accurately, the law was unable to remain strictly valid.

Since Dobereiner’s law could not successfully group elements, the attempts at classification continued. The next attempt came in 1864, when an English chemist, John Newlands, stated his observations in the form of Newlands Law of Octaves.

Newlands Law of Octaves:

When Newlands arranged elements in according to their atomic weights then there was similarity of every eighth element(starting from first). Newland described it as "law of octaves".

According to this law every eighth element is similar to that of the first element, similar to the first and the eighth notes in the musical scale. It was due to its similarity with musical notes that it was called the law of octaves where, after seven different notes the eighth note is repetition(harmonic) of the first one .

Newlands classification of elements:

Li Be B C N O F

Na Mg Al Si P S Cl

K Ca

Drawbacks:

Not valid for elements having atomic masses higher than calcium.

Newly discovered elements like noble gases could not find a place in Newland's table.

Placing of Iron (Fe) far away from Cobalt (Co) and nickel,Which have similar properties as Iron,could also not be explained

Mendeleev's periodic table:

Working on the previous researches of the scientists, in 1869,a Russian chemist, Dimitri Mendeleev presented a much bolder and scientifically useful classification of elements.Till then 63 elements were known. He felt that effective grouping of elements and prediction of properties could be based on two parameters:

Atomic mass

Chemical reactivity

Mendeleev’s periodic law states that "the physical and chemical properties of all elements are the periodic function of their atomic masses".

The main features of Mendeleev’s periodic table:

The known 63 elements were classified into groups and periods.

The table had 8 vertical columns called groups and 12 horizontal rows called periods.

In every group, a gradation of physical and chemical properties of elements was observed.

The table provided gaps for undiscovered elements.

The table helped to predict the properties of three unknown elements of that time. These elements were named eka-boron, eka-aluminium and eka-silicon. When these elements were discovered, they were named scandium, gallium and germanium. The properties of these elements were very close to those predicted by Mendeleev.

A periodic function is the one which repeats itself after a fix interval. So, according to the periodic law, the chemical and physical properties of elements repeat themselves after certain intervals when they are arranged in the increasing order of their atomic masses.

Mendeleev believed that atomic mass of elements was the most fundamental property and arranged them in its increasing order till he encountered an element which had properties similar to the first element. He placed this element below the first element and thus started the second row elements. Proceeding in this manner he could arrange all the known elements according to their properties and thus created the first periodic table.

Merits:

The table helped in the correction of atomic mass for many elements. It predicted the existence of some elements that have not been discovered at the time the table was created.

Mendeleev’s was the first classification which included all the elements known at that time and even the elements discovered later.

Demerits:

The atomic weights of two pairs of elements were reversed.

Alkali metals and coinage metals were placed in the same group.

Lanthanides and actinides were not given proper place in the periodic table.

Isotopes were not placed in the periodic table.

The position of hydrogen was not clearly discussed

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Cbse Class 10 Chemistry Chapter 5 Notes