CLASS XII SINDH BOARD
CHAPTER 1
NEWLAND'S LAW OF OCTAVES
In 1864 an English chemist Newland arranged the elements in the ascending order of their atomic weights . He observed that :
" The properties of every eighth element is similar to that of the first ".
He called this regularity of elements as the law of octaves.
Li BE B C N O F
Na Ma Al Si P S Cl
He called this regularity of elements as the law of octaves.
Li BE B C N O F
Na Ma Al Si P S Cl
ADVANTAGES OF LAW OF OCTAVES :
1- This law provided a basis for the classification of elements into groups having similar properties.
2- The law provided a scope to arrange all the elements into Tabular form.
DISADVANTAGES OF LAW OF OCTAVES :
1- Newland's law was only applicable for the first seventeen elements and broken down afterward e.g first two rows and three columns obey the law.
2- This arrangement did not include noble gases and heavy elements.
LOATHER MEYER'S CONCEPT
In 1869 Loather Meyer , a German Chemist plotted the atomic volumes of elements against their atomic weights and found different curves . He observed that :
" Elements with similar physical properties occupied similar position in the curve " .
Loather Meyer concluded from this curve that :
" Physical properties of elements are periodic function of their atomic weights ".
DISADVANTAGES OF LOATHER MEYER'S CLASSIFICATION :
1 -The periodic classification is incomplete as compared to Mendeleev periodic table which shows the remarkable prediction of discoveries of certain elements.
2 -Meyer's classification based on the physical properties related with atomic weights only.There is no empirical or logical basis for classification as found as that in Mendeleev periodic classification.
" The physical and chemical properties of elements are the periodic functions of their atomic weighs."
The Mendeleev periodic law includes 63 elements which are arranged in the order of their increased atomic weights in seven horizontal lines called Periods and eight vertical columns known as Groups.
MENDELEEV'S PERIODIC LAW
In 1869 , D.I Mendeleev , a Russian chemist discovered an empirical law of classification of elements known as periods of Newland Law of Octaves.This is known as periodic law and stated as :
" The physical and chemical properties of elements are the periodic functions of their atomic weighs."
The Mendeleev periodic law includes 63 elements which are arranged in the order of their increased atomic weights in seven horizontal lines called Periods and eight vertical columns known as Groups.
ADVANTAGES OF MENDELEEV'S PERIODIC LAW :
1.Prediction of new elements : While arranging the elements in order of increasing atomic masses in the periodic table , Mendeleev came across with certain positions where no known element was seen fit to him.He left those spaces vacant and predicted that new elements would be discovered which could be fit in those spaces.
2.Correction of atomic masses : At the time of Mendeleev the atomic masses of some elements were not correctly known . Mendeleev's classification of elements help to correct the atomic masses of the elements.
3. Prediction about properties : Mendeleev also made prediction about the properties of the unknown elements.Many of those predictions were later found correct.He was able to make these predictions because because the elements falling in the same group have same properties.
4. Unification of elements : Mendeleev's classification co-relates all the elements.The discovery of periodic classification showed that all the elements despite their apparent diversity are co-related with one another and make one systematic whole . Thus all the elements are unified through a classification.
3. Prediction about properties : Mendeleev also made prediction about the properties of the unknown elements.Many of those predictions were later found correct.He was able to make these predictions because because the elements falling in the same group have same properties.
4. Unification of elements : Mendeleev's classification co-relates all the elements.The discovery of periodic classification showed that all the elements despite their apparent diversity are co-related with one another and make one systematic whole . Thus all the elements are unified through a classification.
DISADVANTAGES OF MENDELEEV'S PERIODIC LAW
In spite the above advantages Mendeleev's periodic table suffered defects as follows :
1. The position of Hydrogen was not correctly defined.it was placed in group one I although it resembles with both the group I elements - the Alkali metals and the group VII elements - the Halogens , in their properties.
2. In some cases Mendeleev placed elements according to the similarities in the properties and not in the increasing order of their atomic masses.Thus position of these elements was not justified. e.g cobalt (atomic mass 58.9) was placed before Nickle (atomic mass 58.6).
3. Isotopes were not given in separate places in the periodic table although Mendeleev's classification is based on the atomic masses.
4. Some similar elements were grouped separately while some dissimilar elements were grouped together.For example copper and mercury are similar in their properties but were placed separately . Copper was placed in group I although it did not resemble the elements of this group.
5. Mendeleev could not explain the cause of periodicity in the elements.
6. The position of lanthanides and actinides were not included in this periodic table.
MODERN PERIODIC LAW
In 1913 , British physicist Henry Mosley succeeded to reveal a relationship between properties and atomic numbers of elements and introduced modern periodic law which states as :
" The physical and chemical properties of elements are the periodic function of their atomic numbers. "
The modern periodic law provided a logical and scientific ground for the classification of elements.This law also provided justification for correcting the position of some elements wrongly placed in Mendeleev's periodic table.Periodic law was originally an empirical approach to express periodicity of elements with increasing atomic number , but with further development in atomic structure periodic law was related as :
" The physical and chemical properties of elements are a function of electronic configuration which vary with increasing atomic number in a periodic manner."
PERIODS
Horizontal rows in the periodic table are called Periods.It is given by the quantum nuber of the outermost shell.
1st Period:
* It is the shortest period containing H and He.
2nd & 3rd Periods:
* Each contains eight elements and called the short period.
4th & 5th Periods:
* Each contains eighteen elements and called the long period.
6th Period:
* It is the longest period that contains 32 elements.
7th Period:
* This is incomplete period.
GROUPS
The vertical columns in the periodic table are called Groups.Groups have been divided into subgroups A and B.
Subgroup A (IA-VIIIA) expresses the representative elements while B shows Transaction elements.
S - BLOCK ELEMENTS :
The elements of groups IA and IIA are called s-block elements.These have a valence shell configuration.These elements are lighter and soft metals and act as good reducing agents.
P-BLOCK ELEMENTS:
Those elements in which the last electron occupies "p" orbital are called p-block elements of the periodic table.The elements of IIIA to VIIIA (six groups) are p-block elements.
ALKALI METALS:
Elements of group IA are called Alkali metals because they are highly soluble in water.
PROPERTIES OF ALKALI METALS
1.All the elements in group I are metals.
2.They are electropositive elements and form mono positive ion such as Na+ ,K+ etc.
3.They are monovalent.
4.They are reducing agent as they lose their single valence electron.
5.Oxides and Hydroxides of these elements are alkaline.ADVANTAGES OF MODERN PERIODIC TABLE
- All elements in a group have same chemical properties and have same valencies.
- A gradual change in the properties occurred along a period.
- Molecular formula of elements can be predicted.
- Chemical properties of elements can be predicted.
- The exact location of an element in the periodic table can be predicted.
DISADVANTAGES OF MODERN PERIODIC TABLE
- The exact position oh Hydrogen in a group is undecided.
- Anomalous changes in thermodynamic values.
- Certain chemically similar elements are placed in different groups.
- In certain groups metals and non metals are produced.
No comments:
Post a Comment