2. In 1789, Lavoisier first attempted to classify the elements into two divisions namely Metals and Non-metals.
3. Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group. On the basis of this, he named Gallium as Eka-Almunium, Germanium as Eka-Silicon and Scandium as Eka-Boron.
4. Mendeleev's periodic table helped in correcting the atomic masses of some of the elements, based on their positions in the periodic table. For example, atomic mass of beryllium was corrected from 13.5 to 9.0. Atomic masses of indium, gold and platinum were also corrected.
5. In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.
6. The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).
7. Lithium, Sodium, Potassium are called Alkalies metal as they react with metal and form alkalies (Caustic solution)
8. Calcium, Strontium, Bromine are called Alkalies Earth metal as they react with metal and form salt as their oxide are alkalies in nature and exist in the Earth
9. Chlorine, Bromine, Iodine are called Halogen as their oxide are alkalies in nature and exist in the Earth
10. Alkali metal and coinage metals (Cu, Ag and Au) which differ widely in properties are placed into the same group.
In 1913, an English physicist MOSELEY, as a result of his work on characteristics X-rays of elements discovered a fundamental and most accurate relation between chemical properties and atomic number of elements, known as "Modern periodic law".
ACCORDING TO THE MODERN PERIODIC LAW:
1. The chemical and physical properties of elements are the periodic function of their atomic numbers".
2. The properties of elements depend upon their electronic configuration which varies with increasing atomic number in a periodic way".
1. Atomic size decreases in a period.
2-Nuclear charge increases in a period.
3-Ionization potential increases in a period.
4-Electronegativity increases in a period.
5-Electropositivity and metallic character decreases in period.
6-Each period starts with Alkali metal and ends on a Noble gas.
2nd period : Li .................................. Ne
3rd period : Na ..................................Ar
7-First element of each period is most reactive (electropositive), and last element is chemically inert.
1-Group number of an element shows the number of electrons in the outermost shell of that element
2-Atomic size increases in group.
3-Ionization potential of elements decreases down the group.
4-Electronegativity decreases down the group.
5-Metallic character increases down the groups.
On the basis of valence shell electronic configuration, periodic table is divided into four distinct blocks of elements.
All the elements in which last electron enters "ns" orbital are known as s-block elements.
Elements of group I-A and II-A belong to s-block.
Their valence shell electronic is ns1 to ns2.
All the elements of s-block are very reactive and electropositive in nature.
S-block elements are good reducing agents.
They form ionic compounds.
They are all metals and solid. They are low density metals.
For example :
Li (z=3) 1s2, 2s1
Na (z=11) 1s2, 2s2, 2p6, 3s1
Be (z=4) 1s2, 2s2
The elements in which valence electrons enter p-orbital and this orbital is progressively filled are
called p-block elements.
Their valence shell electronic configuration is from ns2, np1 to ns2, np6.
P-block consists of the elements of group III-A to VIII-A (zero-group).
They include metals, nonmetals and metalloids.
There are 30 elements in six sub-groups of p-block.
The elements in which last electron enters (n-1) d-orbital are called d-block elements or outer transition elements.
Valence shell electronic configuration of these elements is ns2, (n-1) d1-10.
All these elements are metals.There are four series of outer transition elements.
Fourth series is incomplete.
The elements in which last electron enters (n-2) f-orbital are called f-block elements or inner transition elements. Their outer electronic configuration is ns2,(n-1)d1,(n-2) f1-14
f-block elements constitute 2 series of elements.
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