10th Class- Periodic Classification of Elements Xth Class Chemistry Notes Part-1

Periodic Classification of Elements

In year of 1800 about 30 elements were known but at present we know about 114 elements. All these elements have different properties. So to study about these elements easily, scientists start searching some patterns in the properties to arrange these elements.

Early Attempts in the Classification of Elements

                                                                   This is practice to arrange elements in order out of chaos, means arranging elements in group of metals and non-metals. Chaos means complete disorder or confusing. Furthermore attempts were made to achieve best classification of the elements.

Dobereiner’s Triads

                             A German chemist, Johann Wolfgang Dobereiner in 1817 tried to arrange elements in the group of 3-elements in each group with similar properties and he called these groups as ‘Triads’. Dobereiner shows that when we take any triad and arrange its elements in the order of increasing atomic masses then the atomic mass of the middle element in the triad is roughly equal to the average of the 1^st and 3^rd element of the triad.

Dobereiner Triads

Li Na K

Ca Sr Ba

Cl Br I

In first triad Li, Na, K atomic mass of   Na (23) = [ Li(7) + K(39)]/2

Newlands Law of Octaves

                                      An English scientist, John Newlands in 1866 arranges known elements in order of their increasing atomic masses. At that time he started with Hydrogen as 1^st element with lowest atomic mass and ended at Thorium as 56^th element.  John Newlands observe that the property of every eighth element  is similar to that of first element and compare this to octaves of music so he called it ‘Law of Octaves’ and this is known as ‘Newlands Law of Octaves’.

Newlands Octaves

H	Li	Be	B	C	N	O
F	Na	Mg	Al	Si	P	S
Cl	K	Ca	Cr	Ti	Mn	Fe
Co and Ni	Cu	Zn	Y	In	As	Se
Br	Rb	Sr	Ce and La	Zr	-	-

1.     Newlands law of octaves is applicable only up to Calcium and after Calcium it is not applicable because after Calcium every eighth element is do not similar to that of first element. 
2.     Newlands assumed only 56 elements exists in nature but later several elements discovered whose properties are very different to get fit in Newlands law of octaves. 
3.     With the discovery of new elements, Newlands try to fit these elements in octaves so he put two elements in same slot and Newland also put elements with different properties in same slot for example Co and Ni placed in F, Cl column.

Mendeleev’s Periodic Table

                                      A Russian chemist, Dmitri Ivonovich Mendeleev in 1872 published his ‘Mendeleev Periodic Table’ in a German journal.  He arranges elements in the form of table on the basis of fundamental property of elements i.e. atomic mass and also on the basis of similarity of chemical properties of elements, means elements with similar chemical properties are placed together in table.

Mendeleev Periodic Law

                             According to this law “the properties of elements are the periodic function of their atomic masses”.

In Mendeleev periodic table horizontal rows are called as ‘Periods’ and vertical columns are called as ‘Groups’.

Mendeléev’s Periodic Table

Achievements of Mendeléev’s Periodic Table

                                                          Mendeléev’s Periodic Table contains some gaps but Mendeléev predicted that these gaps are filled by elements discovered in future. And named these undiscovered elements by placing eka (one) as a prefix to the name of preceding element of the same group. For example Gallium discovered later but Mandeleev predict it as Eka-Aluminium.

Properties of Eka-Aluminium and Gallium

Property	Eka-Aluminium	Gallium
Atomic Mass	65	69.7
Formula of Oxide	E2O3	Ga2O3
Formula of Chloride	ECl3	GaCl3

This prediction of Mandeleev proves correctness and usefulness of Mendeléev’s Periodic Table. Another achievement of Mandeleev is that many scientists now recognize him as originator of the concept on which periodic table is based and also when inert gases (means Nobel gases like He, Ne, Ar) are discovered, they are placed in separate column without disturbing existing order of elements.

Limitation of Mandeleev Classification

                                                These given below are the limitation of Mandeleev Classification. 
1. Position of Hydrogen- No fixed position given to Hydrogen as it behaves like both alkali metals and halogens. Like alkali, Hydrogen react with halogen oxygen and sulphur and also like halogen, Hydrogen exist in diatomic form and react with metals and non-metals. 
2. Isotopes- Isotopes have similar chemical properties but different atomic masses, so Isotopes are challenge to Mandeleev Periodic Law. 
3. Prediction of New Elements- Atomic masses of elements do not increase in regular manner so we cannot predict how many elements can be discovered between two elements.

Periodic Classification of Elements Part 2