Döbereiner’s Triads
- when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements
- Döbereiner could identify only three triads from the elements known at that time
Newlands’ Law of Octaves
When the elements are arranged according to
increasing atomic masses, the properties of every eighth element are similar to that of the first
element
Newlands compared this to the octaves found in music( sa, re, ga, ma, pa,
da, ni) , he called it the ‘Law of Octaves’.
Limitation:
- It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first
- When new elements were discovered, whose properties did not fit into the Law of Octaves
- Iron element which resembles cobalt and nickel in properties, has been placed far away from these elements.
MENDELÉEV’S PERIODIC TABLE<:
- The chemical and physical properties of element are periodic function of their atomic masses
- Mendeléev’s Periodic Table contains eight vertical columns called ‘groups’ and seven horizontal rows called ‘periods’
Achievements of Mendeléev’s Periodic Table:
- Mendeléev left some gaps in his Periodic Table: some elements that had not been discovered at that time.Mendeléev named them Eka–boron, Eka–aluminium and Eka–silicon, respectively when scandium, gallium and germanium, discovered later, have found properties are similer to Eka–boron, Eka–aluminium and Eka–silicon,
- Placement of noble gases: When new element called noble gases discovered it got place in priodic table in a seperate group without disturb the Mendeléev’s Periodic Table
- Placement of elements in Mendeleev’s periodic table helped in correcting the atomic masses of certain elements
Limitations of Mendeléev’s Classification:
- no fixed position can be given to hydrogen in the Periodic Table
- Could not explain the position of Isotopes
- Wron order of atomic mass of some element could not explained
- Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.
THE MODERN PERIODIC TABLE:
Modern
Periodic Law :‘Properties of elements are a periodic function of their atomic
number.’
The Modern Periodic Table has 18 vertical columns known as ‘groups’
and 7 horizontal rows known as ‘periods’.
Periods in Modern Periodic Table:
- Elements present in the same period have the same number of shells and atomic number increases by one unit on moving from left to right in a period.
- 1st period contains only 2 elements it is known as the shortest period
- The second and third period contain 8 elements each it known as short periods.
- The fourth and the fifth period contain 18 elements each it are called long periods
- The 6th period and 7th period contains 32 elements it are known as the longest period.
Characteristics of Periods:
- On moving left to right in period the number of velence electron increase from one to eight
- On moving left to right in period in each short period velency of element first increase from 1 to 4 then decreses 4 to 0
- On moving left to right in period metellic character of element decrease
- On moving left to right chemical reactivity first increase then decreases
- On moving left to right in period the basic nature decreases and acidic nature of oxide increases
Groups in Modern Periodic Table:
- 1st Group of elements are known as alkali metals.
- 2nd Group of elements are known as alkaline earth metals.
- 15th Group of elements are known as pnicogens.
- 16th Group of elements are known as chalcogens.
- 17th Group of elements are known as halogens.
- 18th Group of elements are known as noble gases.
- Group 3 to group 12 of elements are known as transition element
Characteristics of group:
- All element of group have the same number of velence electron
- On going down in group of the periodic table the size of atoms increase
- On going down in group of the periodic table the metallic character increase
- On going down in group of the periodic table the chemical reactivity of metal increase and non metal decreases
- No change in the nature of oxide of element
Valency : the number of valence electrons present in the outermost shell of its atom.On moving left to right in period the number of velence electron increase while velence in group remain same
Atomic size: the distance between the centre of the nucleus and the
outermost shell of an isolated atom
In period the atomic radius decreases in moving from left to
right along a period. This is due to an increase in nuclear charge which
tends to pull the electrons closer to the nucleus and reduces the size of
the atom
In group, the atomic size increases down the group. This is
because new shells are being added as we go down the group. This
increases the distance between the outermost electrons and the nucleus
so that the atomic size increases in spite of the increase in nuclear charge.
Metallic: metals tend to lose electrons while
forming bonds, they are electropositive in nature
Down the group, the effective nuclear charge
experienced by valence electrons is decreasing because the outermost electrons are farther away
from the nucleus. so they lost electron easily. metallic
character decreases across a period and increases down a group
Non-metals:Non-metals are electronegative. They tend to form
bonds by gaining electrons. so non-metals are found on
the right-hand side of the Periodic Table towards the top.
These trends also help us to predict
the nature of oxides formed by
the elements because it is known to you that the oxides of metals are
basic and that of non-metals are acidic in general.