Haryana State Board HBSE 10th Class Science Notes Chapter 5 Periodic Classification of Elements Notes. BANKBARODA Pivot Point Calculator
Haryana Board 10th Class Science Notes Chapter 5 Periodic Classification of Elements
Making Order Out of Chaos – Early Attempts at the Classification of Elements
Need of classifying elements:
Classifying the elements help us to understand their properties and produce various products.
Dobereiner’s classification of elements
(Dobereiner’s triads):
In 1817, German chemist Johann Wolfgang Dobereiner started classifying elements on the basis of their chemical properties.
Law of Triads:
If three elements are arranged in the increasing order of their atomic masses, the atomic mass of the intermediate (i.e. second) element would be almost equal to the average of atomic masses of first and third elements. This is known as Law of Triads. It was given by Dobereiner.
Limitation of Dobereiner’s classification :
- Certain elements could not be classified by Dobereiner’s method.
- Only a limited number of elements could be classified into triads.
The three triads identified by Doberiner:
- Lithium, Sodium and Potassium
- Chlorine, Bromine and Iodine
- Calcium, Strontium and Barium.
Newlands’ Law of Octaves:
- When elements are arranged in the increasing
order of their atomic masses, properties of every 8th element are found to be similar to the properties of the first element. - In 1866, a scientist named John Newlands arranged the elements in the increasing order of their atomic masses.
- During this arrangement, he found properties of every 8th element to be similar i.e. property of 1st and then 8th element would be similar. This periodicity pattern was known as the Law of Octaves (octaves = eight).
Limitation of Newlands’ Law of Octaves:
1. The law was applicable only upto calcium.
2. Newlands thought that there were only 56 elements in nature. He also thought that no more elements would be discovered in the future.
3. In order to fit elements any how into his table, Newlands adjusted two elements in the slot even if the properties of elements did not match with other elements.
Making Order Out of Chaos Mendeleev \ Periodic Table
Mendeleev’s Periodic Table:
1. In 1869, Russian chemist Dmitri Ivanovich Mendeleev started his work of classifying 63 elements known to man.
2. Mendeleev started by examining the relationship of
- ‘Atomic mass of an element’ with
- ‘Physical property of the element’ and
- Chemical property of the element.
3. Through this classification, he concluded that “The properties of elements are periodic function (i.e. periodic in nature) of their atomic mass.” This law came to be known as Mendeleev’s Periodic Law
Criteria used by Mendeleev for developing periodic table:
- The properties of elements are the periodic function of their atomic masses. Hence, arranging elements in the increasing order of their atomic masses.
- Elements with similar properties are arranged in the same group.
- The formula of oxides and hydrides formed by an element.
Anomalies (irregularities) of Mendeleev’s Periodic Table:
- Sequence of few elements was inverted,
- Gaps were kept at few places
Limitations of Mendeleev’s Periodic Table:
- The position of hydrog en could not be correctly assigned,
- The table could not assign proper position for the isotopes of various elements
- Some elements were not arranged on the basis of their increasing atomic mass. This posed a question as to how many elements could still take the place between two elements.
Making Order out of Chaos – The Modern Periodic Table:
Periodic table:
The Periodic Table is a chart in which all the elements known to us are arranged in a systematic manner.
Modern Periodic Law:
Properties of elements are a periodic function of their atomic numbers.
Arrangement of elements in groups and periods in Modern Periodic Table:
(a) Placement of an element in a group:
Elements are placed in a particular group on the basis of the ‘valence electrons’ i.e. elements having same number of valence electrons will be placed in same group.
(b) Placement of an element in a period:
Within a horizontal period, as one moves from left to right, the ‘elements have same number of shells’ but, ‘different valence electrons’. Moreover, on moving left to right, the number of electrons of valence shell increase by 1 unit.
Thus, elements having same number of shells are placed in the same period. For example, Na, Mg, Al, Si, etc. have 3 shells and hence are placed in the 3rd period.
Trends in the Modern Periodic Table Periodic properties:
The properties which are determined by the electronic configuration of elements or which depend on the electroninc configuration of elements are known as periodic properties.
Trends of change in (1) Valency, (2) Atomic size and (3) Metallic and non-metallic properties in groups and periods:
| Characteristic | Groups | Periods |
| 1. Valency | All the elements of a group have same valency. | On moving from left to right, the valency of elements increase from 1 to 4 and then goes on decreasing to O (zero). |
| 2. Atomic size (Radius of atom) | As we move down in a group, the size of atoms i.e. atomic size increase | On moving from left to right in a period, the size of the atoms decrease. |
| 3. Metallic and Non-metallic properties | Going down in the group, the metallic property of elements increase | On moving left to right in a period, the metallic property of elements decreases while non-metallic property increases. |
Metalloids or semi-metallic elements:
1. Elements which possess properties of both metals and non-metals are known as metalloids or semi- metallic elements.
2. In the Modern Periodic Table, a zig-zag line separates metals and non-metals. The border line elements such as boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po) on this zig-zag line are known as metalloids or semi-metals.