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
| Elements | Symbols | Atomic masses |
|---|---|---|
| Chlorine | Cl | 35.5 |
| Bromine | Br | 80 |
| Iodine | I | 127 |
Limitations
Dobereiner failed to place all the known elements in the form of traids of elements having similar properties.
Newland’s Law of octaves
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.
Limitations
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.
What is a Periodic table?
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.
Mendeleev’s Periodic Table
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.
Merits
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.
Limitations
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
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.
What is periodicity?
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.
Characteristics of periods
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.
Characteristics of groups
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.
Merits of Modern periodic table
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
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