• Dobereiner arranged elements having similar properties in the increasing order of their atomic weight in a group of three called triads.

• He found that in these group of triads the atomic mass of the middle element was approximately the mean of the atomic masses of the other two elements.

• Eg. Li Na K

6.9 23 39

Ca Sr Ba

40.1 87.6 137.3

DOBEREINER’S LAW OF TRIADS

• Newland’s law of octaves states that when the elements are arranged in the increasing order of their atomic masses, the properties of the eight element are similar to the first element.

• Limitations:-i) Newland could arrange elements only upto calcium out

of total 56 elements known.ii) After calcium every eight element did not posses

properties similar to that of the first.iii)The periodic table did not include inert gases as they

were not discovered.

• DEF:Mandeleev’s periodic law states that the physical and the chemical properties of the elements are the periodic function of their atomic masses.

• MERITS : Mandeleev was the first person who successfully classified all the known elements.

• He kept blank spaces in the periodic table for undiscovered elements which were discovered later and the properties of these elements were found to be correct as predicated by him.

PREDICTED ELEMENT ACTUAL ELEMENT Eka-boron Scandium Eka- aluminium Gallium Eka-Silicon Germanium • Noble gases which were discovered later were placed in his periodic table

without disturbing the places of the other elements.

DEMERITS:• No fixed position could be given to hydrogen as it resembles

both alkali metals as well as halogens.• Isotopes of the same element have different atomic masses

hence they should be given different position but since they exhibit the same chemical properties they should be given the same position.

• At certain places elements having higher atomic mass are placed before those having lower atomic mass. Eg. Cobalt(Co=58.93) is placed before Nickel(Ni=58.71).

• Some elements having different properties were placed in the same sub - group. Eg. Manganese(Mn) is placed with halogens which totally differ in properties.

MANDELEEV’S PERIODIC TABLE

PERIODIC TABLE

MODERN PERIODIC TABLE

• The modern periodic law states that the physical and chemical properties of elements are a periodic function of their atomic number.

SALIENT FEATURES OF MODERN PERIODIC TABLE

• In modern periodic table elements are placed in seven horizontal rows called periods and eighteen vertical columns called groups.

• The elements belonging to the same period have the same number of shells which is equal to the period number.

• Each period starts with one electron in its outermost orbit and ends with a zero group element.

• PERIOD NO. OF ELEMENT 1 2 Shortest periods 2 & 3 8 Short period 4 & 5 18 Long period 6 32 Longest period 7 32 Incomplete period• The group number indicate the number of electrons in the outermost orbit of an

atom of an element . Thus the elements belonging to the same group have the same outer electronic configuration.

• Elements belonging to the same group show same chemical properties.

• Metals present on the left hand side of the periodic table while non-metals are placed on the right hand side of the periodic table.

Group 1 ------- Alkali metals Group 2 ------- Alkaline earth metals Group 3 ------- Halogens Group 4 ------- Inert gases• Group 1 and 2 together with group 13 to 17 are called normal elements

which has the outermost orbit incompletely filled.• Elements present in the middle of the periodic table in groups 3 to 12

are called transition elements which has their outer two electronic shells incomplete.

• Elements placed at the bottom of the periodic table in two separate series are called inner transition elements.

The first series contains 14 elements (Ce-Lu) are called as Lanthanides(3rd Group & 6th period).The second series contains 14 elements (Th-Lr) are called as Actinides placed along with actinium (Ac=89) in the same group and 7th period due to close resemblance in properties.

TRENDS IN THE PERIODIC PROPERTIES OF ELEMENTS IN THE MODERN PERIODIC TABLE

• VALENCY:1) The valency of elements is determined by the number of valence electrons present in the outermost shell of an atom of an element.2) Elements belonging to the same group have the same outer electronic

configuration hence they have the same valency.3) The valency increases from 1 to 4 for the second and third period and then

decreases from 4 to 0 when we move from left to right of the periodic table.• ATOMIC SIZE:1) The atomic size of an atom of an element depends on the atomic radius i.e. the

distance between the centre of an atom and the outermost shell.2) In a period the atomic size decreases from left to right of the periodic table

because the electrons are added to the same shell and the protons are added in the nucleus and hence the atomic size decreases.

3) As we move down the group i.e. from top to bottom of the periodic table the atomic size increases as new shells are added added taking the outermost electrons farther away from the nucleus.

• METALLIC AND NON-METALLIC PROPERTIES:-• Metals have the tendency to loose electrons and are

said to be electropositive while non-metals have the tendency to gain the electrons and are said to be electronegative.

• As we move from left to right across the period the atomic size decreases and hence the outermost electrons are not easily released therefore the metallic character decreases and the non-metallic character increases from left to right of the periodic table while the non-metallic character decreases.

FOUR TYPES OF ELEMENTS BASED ON THE ELECTRONIC CONFIGURATION IN MODERN PERIODIC TABLE

• s-block :- Elements of groups 1 & 2 are called the s-block elements.They have one or two valence electrons.All elements that belong to this block are metals.

• p-block :- Elements of groups 13 to 17 together with the zero group elements are called p-block elements.The s-block and p-block elements are together known as normal elements.They have their outermost shell incompletely filled except zero group elements which have completed the outermost shell and are called inert gases or noble gases.

• d-block :- Elements of groups 3 to 12 are called d-block elements.They have the outer two elements incompletely filled.They are known as transition elements.All the elements are metals.

• f-block :- The elements placed in two separate series at the bottom of the periodic table are called f-block elements.They have the outer three electronic shell incompletely filled.These elements are called inner transition elements.All these are metals.

MODERN PERIODIC TABLE DIVIDED IN BLOCKS

ZIG-ZAG LINE IN THE PERIODIC TABLE

• In the modern periodic table,a zig zag line seperates the metals and the non-metals.

• Metals are placed to the left of the zig-zag line while the non-metals of the right hand side.

• The borderline elements show intermediate properties of metals and non-metals and are called as metalloids or semi-metals.

Eg. Boron(B),Silicon(Si),Germanium(Ge),Arsenic(As), Tellurium(Te)and Polonium(Po) are the borderline elements.

ZIG-ZAG LINE