Chemistry MODERN PERIODIC LAW AND PRESENT FORM OF THE PERIODIC TABLE

Evaluation of Modern Periodic Table :

a) `text(Dobereiner's Triads, 1829)` : Although atomic weights were not available for all elements, John Wol fgang Dobereiner in 1829 tried to classify the elements with similar properties in the groups of three elements (Triads). He could succeed in making only a few triads. In the triads of elements the atomic weight of the middle element was the arithmetic mean of the atomic weights of the other two. Some of the triads are as under :

`Li` `Na` `K` `Sr` `Ba`

`7` `23` `40` `88` `137`

`P` `As` `Sb` `Cl` `Br` `I`

`31` `75` `120` `35.5` `80` `123`

In some of triads the at.wt. of all the three elements were approximately same.

b) `text(Telluric Helix)` : It was in `1852`, that a periodic classification of the elements was developed that approached the idea we have today. At that time A.E. de Chancourtois, a professor of Geology at the Ecole des Mines in Paris presented an account of his telluric helix in which he indicated the relative properties of elements and their atomic weights.

He used a vertical cylinder with `16` equidistant lines on its surface, the lines lying parallel to the axes. Then he drew a helix at `45` degree to the axis and arranged the elements on the spiral in the order of their increasing atomic weights. In this manner, elements that differed from each other in atomic weight by `15` or multiples of `15` fell very nearly on the same vertical line. In addition to the `16` vertical lines, de Chancourtois felt that other connecting lines could be drawn, and that all elements lying on such lines were related `10` some manner.

c) `text(Newland's Octet Law, 1864)` : Very shortly after the discovery of telluric helix, John Alexander Reina Newland in England made the first attempt to correlate the chemical properties of the elements with their atomic weights. If the elements are arranged in order of their increasing atomic weights, every eighth element had similar properties to first one like the first and eighth note in music. For example,

`sa` `re` `ga` `ma` `pa` `dha` ` ni`

`Li` `Be` `B` `C` `N` `O` `F`

`Na` `Mg` `Al` `Si` `P` `S` `Cl`

`K` `Ca` `Cr` `Ti` `Mn` `Fe`

The system worked quite well for the lighter elements. For example, `H`, `F` and `Cl` show similar properties and `Li`, `Na` and `K` exhibit same characteristics. The system, however, fails in the case of heavier elements as manganese has been placed along with nitrogen and phosphorus or iron has been placed along with oxygen and sulphur i.e. dissimilar elements have been grouped which is against the aim of classification. Also note that no space was left for the elements which were unknown at that time and many elements were provided wrong positions on account of wrong values of atomic masses in this classification.

`ast` All the elements could not be classified on this basis.

d) `text(Mendeleev's Periodic law)` : In spite of the importance of the earlier contributions the major portion of the credit for the development of the periodic table must go to the Russian Chemist, Dmitriilvanovich Mendeleev, who proposed

i) The physical and chemical properties of elements are periodic functions of their atomic weights.

ii) If the elements are arranged in the order of their increasing atomic weights, after a regular interval elements with similar properties are repeated.

Mendeleev's Periodic Table :

The table is divided into nine vertical columns called groups and seven horizontal rows called periods.

`text(Characteristics of Periods)`

i) First period is called shortest period and contains only two elements. Second and third periods are called short periods containing eight elements each. Fourth and fifth periods are long periods containing eighteen elements each. Sixth period is longest period with thirty-two elements. Seventh period is an incomplete period containing nineteen elements. Numbers `2`, `8`, `18`, `32` are called magic numbers.

ii) Lanthanide and actinide series containing `14` elements each are placed separately under the main periodic table. These are related to sixth and seventh periods of `text(group Ill)` respectively.

iii) Elements of third period from sodium `(Na)` to Chlorine `(Cl)` are called representative or typical elements.

iv) Valency of an element in a period increases from `1` to `7` with respect to oxygen.

`Na_2O` `MgO` `Al_2O_3` `SiO_2` `P_2O_5` `SO_3` `Cl_2O_7`

`1` `2` `3` `4` `5 ` `6` `7`

v) From left to right in a period generally

a) Atomic weight, effective nuclear charge, ionization potential, electronegativity and electron affinity of an element increases.

b) Atomic radius, electropositive character and metallic character of an element decreases.

`text(Characteristics of groups)` :

i) Mendeleev's periodic table contains nine groups. These are represented by Roman numerals `I`, `II`, `Ill`, `IV`, `V`, `VI`, `VII`, `VIII` and zero. Groups `I` to `VII` are divided into two subgroups `A` and `B`. Group `VIII` consists of three sets, each one containing three elements.

ii) Inert gases are present in zero group. These were not discovered till that time.

iii) The valency of an element in a group is equal to the group number.

iv) There is no resemblance in the elements of subgroups `A` and `B` of same group, except valency.

v) The elements of the groups, which resemble with typical elements are called normal elements. For example `IA`, `IIA`, `IIIA`, `IVA`, `VA`, `VIA`, `VIIA` group elements are normal elements

vi) Those elements of the groups, which do not resemble with typical elements are called transition elements. For example- `IB`, `IIB`, `IIIB`, `IVB`, `VB`, `VIB`, `VIIB`, and `VIII` group elements are transition elements.

vii) Hydrogen can be placed in both `IA` and `VIIA` groups.

viii) In a group, from top to bottom in general

a) Atomic weight, atomic size, electropositive character and metallic character of an element increases.

b) Ionization potential, electron affinity and electronegativity of an element decreases.

`text(Defects of Mendeleev's periodic table)`

i) Position of hydrogen is uncertain. It has been placed in `IA` and `VIIA` groups because of its resemblance with both the groups.

ii) No separate positions were given to isotopes.

iii) It is not clear whether the lanthanides and actinides are related to `IIA` or `III B` group.

iv) Although there is no resemblance except valency of subgroups `A` and `B`, they have been put in the same group.

v) Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g. -`Co` (At. Wt. = `58.9`) is placed before `Ni` (At. Wt. = `58`) and `Ar` (At. Wt. = `39.9`) before `K` (At. Wt. = `39`).

Long form of Periodic Table or Moseley's Periodic Table :

i) Moseley (1909) studied the frequency of `X`-rays produced by the bombardment of a strong beam of electrons on a metal target. He found that the square root of the frequency of `X`-rays (`sqrt v`) is directly proportional to the total nuclear charge (`Z`) of metal. The relation between them was found to be

`sqrt v=a(Z-b)`

where `a` and `b` are constants. Nuclear charge of metal is equal to the atomic number. So Moseley related the properties of elements with their atomic number and gave the new periodic law.

ii) Moseley's Periodic Law or Modern Periodic Law: Physical and chemical properties of elements are the periodic functions of their atomic number. If the elements are arranged in order of their increasing atomic number, after a regular interval, element with similar properties are repeated.

Long form of the Periodic Table and Electronic Configuration of Elements :

Many different forms of a periodic classification of the elements have appeared since the `1871` table by Mendeleev. Each table was designed to point up the various trends and relationship, which its author considered most significant. From the literally hundreds of tables which have been proposed, perhaps the most popular and easily reproduced periodic table is the conventional extended or long form, which is shown in table.

i) Each period starts with an alkali metal whose outermost electronic configuration is `ns^ 1`.

ii) Each period ends with a noble gas of outermost electronic configuration `ns^2np^6` except `He`. The electronic configuration of `He` is `1s^2`.

iii) The number of elements in a period is equal to the number of necessary electrons to acquire `ns^2np^6` configuration in the outermost shell of first element (alkali metal) of the period. First period contains two elements.

iv) The number of elements in each period may be determined by the number of electrons in a stable configuration as under