Students must go through these JAC Class 10 Science Notes Chapter 5 Periodic Classification of Elements to get a clear insight into all the important concepts.
JAC Board Class 10 Science Notes Chapter 5 Periodic Classification of Elements
→ There are 118 elements which are known at present and out of these, 98 elements are naturally occurring.
→ The classification of elements is made on the basis of similarity of their properties.
→ Dobereiner, Newlands, Mandeleev, Lothar Meyer and Henry Moseley made attempt of the classification of elements.
→ Dobereiner’s triads When he observed that three elements were arranged in the order of their increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. These groups of three elements are called triads.
→ Newlands’ law of octaves When elements are arranged in the order of their increasing atomic masses, the every eighth element (starting from a given element) had properties similar to that of the first element.
→ Mendeleev’s periodic law: The properties of elements are the periodic function of their atomic masses.
→ Mendeleev classified the elements in increasing order of their masses in –
- vertical columns called groups and,
- horizontal rows called periods.
→ Mandeleev named scandium, gallium and germanium as Eka-boron, Eka-aluminium and Eka-silicon respectively.
→ Isotopes : Atoms of the same element having same atomic number but different atomic masses are known as isotopes of each other.
→ Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
→ The modem periodic law: The properties of elements are a periodic function of their atomic number.
→ Elements in the modern periodic table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
| Period number | Number of elements |
| 1 | 2 |
| 2 | 8 |
| 3 | 8 |
| 4 | 18 |
| 5 | 18 |
| 6 | 32 |
| 7 | Incomplete period |
→ The position of an element in the periodic table provides information to predict its chemical properties and reactivity.
→ Periodic properties : Properties of the elements which are periodic function of their electronic configuration and are repeated after definite interval of atomic numbers.
→ Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
OR
The valency is the combining capacity of an atom of an element to acquire noble gas configuration.
→ In a period, on moving from left to right, the valency of the elements first increases from 1 to 4 and then decreases from 4 to 0, while the valency of all elements in a group remain the same.
→ Atomic size (Atomic radius) : The distance between the centre of the nucleus and the outermost shell of an isolated atom is called atomic size (atomic radius).
→ Trend of atomic radii in a period and in a group The atomic radius decreases on moving from left to right in a period while on moving down in a group, the atomic radius of elements increases.
→ In a periodic table, metallic elements are arranged on left while non-metals are arranged on right and semi-metals or metalloids are arranged in the middle.
→ In the modern periodic table, boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po) have intermediate properties between metals and non-metals and hence called metalloids or semi-metals.
→ Metallic elements are electropositive in nature, while non-metals are electronegative in nature.
→ Generally, oxides of non-metals are acidic, while oxides of metals are basic in nature.
→ Atomic number (Z) : The number of protons in the nucleus of an atom of an element is known as the atomic number.