Class 11 Periodic Properties DOBEREINER’S Triads
Periodic Properties
Dobereiner’s Triads
According to Dobereiner when elements of the same properties are kept in the increasing order of atomic weights, the atomic weight of the middle element is equal to the mean atomic weight of the remaining two elements. Such a group of elements is called Dobereiner’s triad.
Dobereiner could arrange only a few elements as triads and there are some such elements present in a triad, whose atomic weights are approximately equal, e.g.
Therefore, this hypothesis was not acceptable for all elements.
Newland’s Rules of Octave
(a) As in music, the eighth node is the same as the first node. If the elements are arranged in the increasing order of atomic weights, on starting with an element, the first element will exhibit similarities with the eighth element e.g.
(b) It is clear from the above table that sodium is the eighth element from lithium, whose properties resemble that of lithium.
(c) This type of classification was limited to only 20 elements.
LOTHAR MAYER’S VOLUME CURVES
(a) The graphs drawn the plotting the aforesaid atomic volumes against atomic weights are known as Lothar Mayer volume curves.
(b) The alkali metals have the highest atomic volumes.
(c) alkaline earth metals (Be, Mg Ca, Sr, Ba, etc.) which are relatively a little less electropositive. Occupy positions on the descending part of the curve.
(d) Halogens and the noble gases (except helium) occupy positions on the ascending part of the curve.
(e) Transition elements have very small volumes and therefore these are present at the bottoms of the curve
MENDELEEF’S PERIODIC LAW
According to Mendeleev’s periodic law, the physical and chemical properties of elements are a periodic function of their atomic weights.
Mendeleev’s Periodic Table
(a) The periodic table is based on atomic weight.
(b) In the periodic table, the horizontal lines are called periods and the vertical lines are called groups.
(c) The periodic table consists of seven periods and nine groups (The earlier periodic table had only 8 groups. The noble gases were added later in the zero group because these were not discovered when Mendeleef put forward his periodic table.
(d) All the groups (except VIII and Zero groups) are divided into subgroups A and B.
(e) 2, 8, 18 and 32 are called magic numbers.
Merits of Mendeleev’s Periodic Table
(a) Classification of elements then known was done for the first time and the elements having similar properties were kept in the same group.
(b) It encouraged to research and led to the discovery of newer elements.
(c) Mendeleev had even predicted the properties of many elements not discovered at that time. This helped in the discovery of these elements. For example, Mendeleev predicted the properties of the following elements.
(1) Eka-boron-This was later called scandium (Sc)
(2) Eka-aluminium-This was later called gallium (Ga)
(3) Eka-silicon-This was later called germanium (Ge)
Defects of Mendeleev’s Periodic Table
(a) Position of Hydrogen: Like alkali metals hydrogen has one electron in its outermost level and like halogens, it has one electron less than the next inert gas. Therefore, placing hydrogen with alkali metals is equally appropriate as placing it with halogens.
(b) Position of Isotopes: The isotopes have different atomic weights and the periodic table is based on atomic weights. Therefore, isotopes should get different places in the periodic table on the basis of atomic weights.
(c) The periodic table is not fully based on increasing order of atomic weights.
(d) It is not proper to place together the elements having differing properties, such as coinage metals (Cu, Ag and Au) with alkali metals; Zn, Cd and Hg with alkaline earth metals and metal-like Mn with halogens. Similarly. Pt and Au having similar properties have been placed in different groups.
(e) There is no indication of whether lanthanides and actinides are associated with group III-A or group III B.
(f) Position of Isobars: These elements have different groups when mass remains the same.