Periodic Classification Of Elements Flashcards
In 1817, _______ a German chemist, suggested a method of grouping elements based on their relative atomic masses
Johann Wolfgang Dobereiner,
Dobereiner arranged the elements into groups containing three elements each. He called these groups as_____ ‘
triads’ (tri - three)
Dobereiner law
when the three elements in a triad are arranged in the ascending order of their atomic masses, the atomic mass of the middle element is nearly the same as average of atomic masses of other two elements
Dobereiner limitation
■Dobereiner could identify only three triads from the elements known at that time and all elements could not be classified in the form of triads.
The law was not applicable to elements having very low and very high atomic mass.
In 1866, ________ arranged 56 known elements in the increasing order of their atomic mass.
John Newlands
Newlands law of octaves
He observed that every eighth element had properties similar to those of the first element like the eighth note in an octave of music is similar to the first. This arrangement was known as law of octaves
Limitation of Newlands law of octaves
There are instances of two elements being fitted into the same slot, e.g. cobalt and nickel
Some elements, totally dissimilar in their properties, were fitted into the same group. (Arrangement of Co, Ni, Pd, Pt and Ir in the row of halogens)
The law of octaves was not valid for elements that had atomic masses higher than that of calcium.
Newlands’ table was restricted to only 56 elements and did not leave any room for new elements.
Discovery of inert gases (Neon. Argon….) at later stage made the 9th element similar to the first one. Eg: Neon between Fluorine and Sodium.
Find the pair of elements having similar properties by applying Newlands’ law of Octaves (Example: Mg & Ca):
Set I: F, Mg, C, O, B
Set II: Al, Si, S, Cl, Ca
F&Cl
C&Si
O& S
B&Al
In 1869, Russian chemist, _______ observed that the elements of similar properties repeat at regular intervals when the elements are arranged in the order of their atomic masses
Dmitri Mendeleev
Who proposed the law of periodicity
Dmitri mendeleev
the law of periodicity
the physical and chemical properties of elements are the periodic functions of their atomic masses”
Mendeleev arranged 56 elements known at that time according to his law of periodicity. This was best known as .
the short form of periodic table
Features of Mendeleev’s Periodic Table:
It has eight vertical columns called ‘groups’ and seven horizontal rows called ‘period’
Each group has two subgroups ‘A’ and ‘B’ All the elements appearing in a group were found to have similar properties.
For the first time, elements were comprehensively classified in such a way that elements of similar properties were placed in the same group.
It was noticed that certain elements could not be placed in their proper groups in this manner. The reason for this was wrongly determined atomic masses. Consequently those wrong atomic masses were corrected.
Eg: The atomic mass of beryllium was known to be 14. Mendeleev reassessed it as 9 and assigned beryllium a proper place.
Columns were left vacant for elements which were not known at that time and their properties also were predicted. This gave motivation to experiment in Chemistry
Limitation of mendeleev’s
Elements with large difference in properties were included in the same group. Eg: Hard metals like copper (Cu) and silver (Ag) were included along with soft metals like sodium (Na) and potassium (K).
No proper position could be given to the element hydrogen. Non-metallic hydrogen was placed along with metals like lithium (Li), sodium (Na) and potassium (K).
The increasing order of atomic mass was not strictly followed throughout. Eg. Co & Ni, Te & I.
No place for isotopes in the periodic table
In 1913, the English Physicist ________ through his X-ray diffraction experiments, proved that the properties of elements depend on the atomic number and not on the atomic mass.
Henry Moseley
who prepared the modern periodic table by arranging elements in the increasing order of their atomic number.
Henry Moseley
This modern periodic table is the extension of the original Mendeleev’s periodic table and known as
long form of periodic table.
The physical and chemical properties of elements depend not only on the number of protons but also on _______ and their arrangements in atom
the number of electrons
the modern periodic law can be stated as follows:
The chemical and physical properties of elements are periodic functions of their atomic numbers”.
Based on the modern periodic law, the modern periodic table is derived.
Features of Modern Periodic Table
■All the elements are arranged in the increasing order of their atomic number.
The horizontal rows are called periods. There are seven periods in the periodic table.
The elements are placed in periods based on the number of shells in their atoms.
■Vertical columns in the periodic table starting from top to bottom are called groups. There are 18 groups in the periodic table.
Based on the physical and chemical properties of elements, they are grouped into various families
The elements are placed in periods based on the _______ in their atoms.
number of shells
1st group
Alkali metal
Hydrogen lithium sodium potassium rubidium caesium francium
