NCERT chapter 5 class10 :Periodic Classification of Elements

Need for the classification of elements.

• In our surroundings, substance is found in the form of elements, compounds and mixtures
• 118 elements are known to us at present, .
• The classification may further unfold relationship between the different elements.
• Only 94 are naturally occurring out of 118 known till date.

During 17th Century, only few elements were known. Further many elements were discovered in the nature, the number of elements in the list of known elements went on increasing. It was belived that elements must be properly classified in order to make a systematic study of elements, otherwise it becomes difficult to understand the properties of elements.

• The classification may make to study them better and easily.
• Classification of elements correlate the properties of elements

Döbereiner’s Triads

• Dobernier was the first person, Who correlated the properties of elements with their atomic masses.
• Dobernier identified some groups of three elements .
• He named these groups ‘triads’.
• Döbereiner discovred that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Newlands’ Law of Octaves

• In 1863, J.A. Newland discovered that if elements are arranged in the increasing order of their atomic masses
• The properties of elements reoccur same after every eighth element, just like repetition of musical notes in an Octave. Newland called this as Law of Octaves

Limitation of Newlands’ Law of Octaves

• It was noticed that the Law of Octaves was applicable only upto Ca, as after Ca every eighth element did not possess properties as similar to that of first element in octave.
• At the time Newlands , only 56 elements existed in nature and he assumed that no more elements would be further discovered in the nearer future. But later on several new elements were discovered, whose properties couldn’t be defined as per the Law of Octaves.
• Newland adjusted two elements in the same slot in order to fit into law of octaves . However element placed in same slot were unlike elements .
• Cobalt and nickel positioned in same slot and with fluorine, chlorine and bromine group,Which are possessing different properties than these elements.

MENDELÉEV’S PERIODIC TABLE

MENDELÉEV’S PERIODIC TABLE

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63 elements only were known,When Mendeléev commenced his work, . He observed the relationship between the atomic masses of the elements and their physical and chemical properties.

He took 63 cards representing each one as element and on each card he wrote down the properties of one element. He segregated the elements with similar properties and pinned the cards together on a wall. He noticed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses.

He noticed that there occurs a periodic recurrence of elements with similar physical and chemical properties.

On this basis, Mendeléev derived a Periodic Law, which says that ‘the properties of elements are the periodic function of their atomic masses’.

Mendeléev’s Periodic Table contains some vertical columns called ‘groups’and horizontal rows called ‘periods’

Achievements of Mendeléev’s Periodic Table

Achievements of Mendeléev’s Periodic Table

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During the course of development the Periodic Table by Mendeléev , there were a few instances faced by him where Mendeleev had to place an element with a slightly greater atomic mass before an element with a lower atomic mass.

The sequence was altered at few place so that elements with similar properties could be grouped together. example, cobalt (atomic mass 58.9) come before nickel (atomic mass 58.7) .

Further, Mendeléev left some gaps in his Periodic Table & boldly predicted the existence of some elements that had not been discovered yet will fulfill this gap in future.

Mendeléev called them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.

Example, scandium, gallium and germanium, found later & have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively

The properties of Eka–Aluminium predicted by Mendeléev and those of the element, gallium which was discovered later and replaced Ekaaluminium

Achievements:

Mendeléev’s Periodic law could classify all the elements discovered at that time.

Mendeléev’s Periodic helped in the discovery of new elements.

Mendeléev’s Periodic helped in the correction of the atomic mass of some elements.

Limitations of Mendeléev’s Classification

Mendeleev’s periodic table was a great success,but it had some defects :

1. Position of Hydrogen

The position of hydrogen is ambiguous ,which is placed in group IA along with alkali metals. because it behaves alkali metals as well as halogens (group VII A).

2. Position of Isotopes

Isotopes of an element have different atomic masses so each one of them should have been assigned a separate position. On the other hand, they all have similar chemical properties; hence they should all be placed at the same position.

Mendeleev’s periodic table did not provide suitable place for different isotopes. For example, two isotopes of carbon are represented as 6C12, 6C14 are placed at the same position.

3. Grouping of chemically dissimilar elements

Elements like copper and silver have no resemblance with alkali metals (lithium, sodium etc.), but they have been grouped together in the first group.

4. Separation of chemically similar elements

Elements those are chemically similar such as gold and platinum have been placed in separate groups.

5. Anomalous* Pairs of Elements

At few places, an element with higher atomic mass had been placed before an element with lower atomic mass due to their properties. For example, cobalt with higher atomic mass (58.9) was placed before nickel with lower atomic mass (58.7).Such pairs of elements (called anomalous pairs) violated the periodic law.

Modern Periodic Table

Henry Moseley in 1913 discovered that the atomic number and not the atomic mass is the most fundamental property of an element.

Atomic number (Z) of an element is equal to the number of protons in the nucleus of its atom.Since atom is as electrically neutral entity,so number of electrons is also equal to its atomic number or number of protons.

Mendeléev’s Periodic Law was redesigned and atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law can be stated as follows: ‘Properties of elements are a periodic function of their atomic number insteed atomic mass.’

According to Modern Periodic Law " the physical and chemical properties of elements are periodic functions of their atomic numbers " means if elements are arranged in the order of their increasing atomic number, the elements with similar properties are repeated after certain regular intervals.

The periodic table based on the modern periodic law is named as the Modern Periodic Table. Presently, the accepted modern periodic table is called the Long Form of Periodic Table.

Position of Elements in the Modern Periodic Table

Element's Position in the Modern Periodic Table

• The Modern Periodic Table contains 18 vertical columns known as ‘groups’ and 7 horizontal rows called as ‘periods’.
• All these elements contain the same number of valence electronS are placed in a group.
• Group 1-contains alkali metals and group 17 have elements named halogens

Groups in the Periodic Table represents an identical outershell electronic configuration. On the other hand, the number of shells increases as we proceed down the group.The groups have been numbered from 1 to 18 .

Periods: The periodic table have seven horizontal rows . Each row is named a period. The elements in a period have consecutive (increasing/decreasing) atomic numbers. The periods have been numbered from 1 to 7 .

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The period number is also repsresents the number of the shell which starts filling up as we move from left to right across that particular period.

• First period is the shortest period of all. It have only two elements; H and He.
• The second as well as third periods are short periods containing 8 elements each.
• The fourth as well as fifth periods are called long periods containing 18 elements each.
• The sixth as well as seventh periods are very long periods containing 32 elements each.

Metallic and Non-metallic Properties

Metals: Metals are placed in the left hand portion of the periodic table. The strong metallic elements; alkali metals (Li, Na, K, Rb, Cs, Fr) and alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) arranged in groups 1 and 2 respectively.

Non-metals: Non-metals are placed towards the RHS portion of the periodic table. Strong non-metallic elements i.e., halogens (F, Cl, Br, I, At) and chalkogens (O, S, Se, Te, Po) placed in groups 17 and 16 respectively.

Metalloids: Metalloids are the elements that have mixed properties of both metals and non-metals. They are placed along the diagonal line starting from group 13 (Boron) and going down to group 16 (Polonium).