CONTENT LIST
PERIODIC CLASSIFICATION OF ELEMENTS
Need for the classification of elements.
- All substances around us is present in the form of elements, compounds and mixtures
- As on date, 118 elements are known to us
- The classification of substance may further reveal relationship between the different elements.
- Out of 118 elements known today, only 94 are naturally occurring
- The classification may help to study them better and easily.
- Classification of elements correlate the properties of elements
During 17th Century, only few elements were known. Furthure many elements were discovered in nature, the number of elements in the list of known elements went on increasing. It was assumed that elements must be properly classified in order to make a systematic study of elements, otherwise it becomes impossible to understand the properties of elements.
Döbereiner’s Triads
- Dobernier was the first scientiest how correlated the properties of elements with their atomic masses.
- He observed some groups having three elements each.
- He called these groups ‘triads’.
- Döbereiner highlighted 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 in group.
Newlands’ Law of Octaves
- In 1863, J.A. Newland found that when elements are arranged in the increasing order of their atomic masses
- The chemical properties of elements repeated at every eighth element, like repetition of musical notes in an Octave. Newland termed this as Law of Octaves
- It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
- 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. Furthure several new elements were found, whose properties couldn’t be defined as per the Law of Octaves.
- In order to fit elements into law of octaves Newlands . Newland adjusted two elements in the same slot which were unlike elements .
- Nickel and cobalt are in the same slot and are positioned in the same column with fluorine, chlorine and bromine possessing different properties than these elements.
- When Mendeléev begin his work, only 63 elements were known. He establised the relationship between the atomic masses of the elements and their physical and chemical properties.
- He took 63 cards and on each card he wrote down the properties of one element. He sorted out the elements with similar properties and pinned the cards together on a wall. He found that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses.
- He observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.
- Mendeléev’s Periodic Table contains vertical columns known as ‘groups’and horizontal rows known as ‘periods’
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Achievements of Mendeléev’s Periodic Table
- 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 slightly less atomic mass.
- The sequence was inverted at few place so that elements with similar properties could be grouped together. For example, cobalt (atomic mass 58.9) came 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 given name of these future elements by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.
- Later on, scandium, gallium and germanium, discovered, they have properties as 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
- Mendeléev could classify all the elements discovered at that time.
- He helped in the discovery of new elements.
- He helped in the correction of the atomic mass of some elements.
Limitation of Newlands’ Law of Octaves
MENDELÉEV’S PERIODIC TABLE
On the basis of above observations, Mendeléev gave a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.
Achievements:
Limitations of Mendeléev’s Classification
Mendeleev’s periodic table was a great success,but it had some defects :
- Position of Hydrogen
- Position of Isotopes
- Grouping of chemically dissimilar elements
- Separation of chemically similar elements
- Anomalous* Pairs of Elements
- Henry Moseley in 1913, discovered that the atomic number and not the atomic mass is the most fundamental property of an element.
- The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
- All these elements contain the same number of valence electronS are placed in a group.
- Group 1 is called alkali metals and group 17 elements are called halogens.
- The first period is the shortest period. It have only two elements; H and He.
- The second and third periods are called short periods containing 8 elements each.
- The fourth and fifth periods are long periods. They have 18 elements each.
- The sixth and seventh periods are very long periods. They have 32 elements each.
- Metals: All metals are present in the left side of the periodic table. The strong metallic elements called alkali metals (Li, Na, K, Rb, Cs, Fr) and alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) placed in group 1 and 2 respectively.
- Non-metals: All non-metals are present in the right side of the periodic table. The Strong non-metallic elements called halogens (F, Cl, Br, I, At) and chalkogens (O, S, Se, Te, Po) placed in groups 17 and 16 respectively.
- Metalloids: All Metalloids are the elements that are mixture of 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).
The position of hydrogen which is placed in group IA along with alkali metals is ambiguous because it resembles alkali metals as well as halogens (group VII A).
All the isotopes of an element have different atomic masses therefore, each isotopes should have been assigned a separate position. On the other hand, All isotopes are chemically similar; hence they should be kept at the same position.
In fact, Mendeleev’s periodic table did not provide any place for different isotopes. For example, two isotopes of carbon are represented as 6C12, 6C14 but placed at the same position.
Elements such as copper and silver have been grouped together in the first group however they have no resemblance with alkali metals (lithium, sodium etc..
Elements which are chemically similar have been placed in separate groups such as gold and platinum.
At some places in Mendeleev’s periodic table, elements with greater 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
Atomic number (Z) of an element is the number of protons in the nucleus of its atom.Since atom is as electrically neutral entity, the number of electrons is also equal to its atomic number i.e.the number of protons.
Mendeléev’s Periodic Law was modified and atomic number was taken 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.’
The Modern Periodic Law states that the chemical and physical properties of elements are periodic functions of their atomic numbers i.e. 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 which is based on the modern periodic law is called the Modern Periodic Table. At present long form of periodic table is accepted as modern periodic table.
Element's position in the Modern Periodic Table
Groups in the Periodic Table refer an identical outershell electronic configuration. On the other hand, the number of shells increases as we go down the group.The groups have been numbered from 1 to 18 (in Arabic numerals).
Periods: Seven horizontal rows in the periodic table is called period. The elements in a period have consecutive atomic numbers.
The periods have been numbered from 1 to 7 (in Arabic numerals). The period number is also representes the number of the shell which starts filling up as we move from left to right across that particular period.