Dobereiner’s traids
When elements are arranged in the. Order of increasing atomic masses, group of three elements known as traids, having similar properties are obtained. The middle element of the traid being equal to the arithmetic mean of the atomic masses of the other two elements.
Following are the examples.
1. The alkali Metal group
| Elements | Symbols | Atomic masses |
|---|---|---|
| Lithium | Li | 7 |
| Sodium | Na | 23 |
| Potassium | K | 39 |
2. Alkaline Earth metal group
| Elements | Symbols | Atomic masses |
|---|---|---|
| Calcium | Ca | 40 |
| Strontium | Sr | 88 |
| Barium | Ba | 137 |
3. The halogen group Elements
Dobereiner failed to place all the known elements in the form of traids of
elements having similar properties.
When elements are arranged in the order of increasing atomic masses, the
properties of the eighth element (starting from a given element) are a
repetition of the properties of the first element.
Following are the limitations of Newland’s law of octaves.
1. Newland’s law of octaves was
applicable till calcium only.
2. Newland assumed that only 56
elements existed in nature and no more elements will discovered in the future.
But later on, several new elements are discovered which doesn’t fit in
Newland’s law of octaves.
3. In order to fit elements into his
table, Newlands put two elements together in one slot which have different
chemical properties.
The periodic table is a chart in which elements having similar chemical
properties lie in the same vertical column. A periodic table contains of
horizontal rows, periods and vertical column, groups.
A Russian scientist, Mendeleev found that when elements are arranged in the
order of atomic masses, the elements with similar properties occur at regular
intervals.
Following are the two factors on which Mendeleev’s Periodic Table work:
1. Increasing atomic masses
2. Grouping together of elements having
similar properties.
Following are the merits:-
1. Mendeleev’s periodic table predicted
the existence of some elements which was not discovered that time (Gallium,
Scandium and Germanium)
2. Mendeleev’s periodic table could
predict the properties of several elements on the basis of their
properties.
3. When can place noble gases group in
Mendeleev’s periodic table.
Following are the limitations:-
1. The positions of isotopes could not
explained.
2. Cobalt and Nickel are not arranged
on the basis of increasing atomic masses.
3. Position of hydrogen remain
unsolved.
Modern periodic table or long form of periodic table was prepared by Neils
Bohr. In modern periodic table, elements are arranged on the basis of
increasing atomic masses.
Explanation of Modern periodic table When the elements are arranged on the
basis of increasing atomic mass, then the elements having same number of
valence electrons occur at regular interval.
The repetition of elements having similar properties at regular intervals on
regular intervals. Periods and groups
The horizontal rows in periodic table are called periods. There are 7 periods
in modern periodic table.
The vertical columns in a periodic table are called groups. There are 18
groups.
1. Valence Electrons On moving left to right, the number of valence electrons
increases from 1 to 8.
2. Valency On moving left to right, in each period the valency of elements
increases from 1 to 4 then decreases from 4 to 0.
3. Size of atoms On moving from left to right in a periodic table, the
size of atoms decreases.
4. Metallic Character On moving from left to right in a period, the
metallic character of elements decreases.
5. hemical reactivity On moving from left to right in a period, the
chemical reactivity of elements first decreases and then increases.
6.Nature of oxides On moving left to right, the basic nature of oxides
decreases and acidic nature of oxides increases.
1.Valence electrons All the elements of a group of the periodic table have the
same number of valence electrons.
2.Size of atoms On going down in a group of the periodic table, the size of
the atoms increases.
3.Valency All the elements in a group have same valency.
4.Metallic character On going down in a group of the periodic table, the
metallic character of elements increases.
5.Chemical Reactivity All the elements in a group have similar chemical
properties.
6. Nature of oxides On going down, there is no change in the nature of oxides
of elements.
Following are the merits of the modern periodic table:-
1. The modern periodic table is based
on the atomic numbers of elements which is the most fundamental property of
elements.
2. The position of hydrogen assigned.
3. It tells us why
properties of elements are rrpeated after 2, 8, 18 and 32 elements.
4. There are no anomalies in the
arrangement of elements in the modern periodic table
Elements
Symbols
Atomic masses
Chlorine
Cl
35.5
Bromine
Br
80
Iodine
I
127
Limitations
Newland’s Law of octaves
Limitations
What is a Periodic table?
Mendeleev’s Periodic Table
Merits
Limitations
Modern periodic table
What is periodicity?
Characteristics of periods
Characteristics of groups
Merits of Modern periodic table
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