Periodic classification is a strategy for categorising various elements into distinct categories based on their characteristics. The periodic chart divides these elements into groups and eras. In the periodic table, the elements are listed in increasing order of their atomic number.
Chapter 3: Classification of Elements and Periodicity in Properties Revision Notes
**Dobereiner's Triads **
Dobereiner's Trids' Limitations
Newlands’ Law of Octaves
Newlands' Law of Octaves has certain limitations.
(i) Only up to the element calcium was this categorization effective. Following that, every eighth element did not have the same attributes as the member in the group above it.
(ii) When noble gas elements were found later, their inclusion in the table threw the entire system into disarray.
(i) The elements in the periodic table are organised into vertical rows called groups and horizontal rows called periods.
(ii) The Roman numerals I, II, III, IV, V, VI, VII, VIII, and zero designate nine groups. Group VIII is made up of nine components organised into three triads. The zero group comprises elements that are either inert or noble gases, and all of the elements in the group have zero valency.
(iii) In Mendeleev's periodic chart, there are seven periods (numbered 1–7) or horizontal rows.
Importance of Mendeleev’s Periodic Table
(i) This made element study more systematic in the sense that if the qualities of one element in a group are known, the properties of others can be predicted.
(ii) This aided in the eventual discovery of these elements to a large extent.
(iii) Mendeleev used anticipated locations and attributes to adjust the atomic weights of some elements.
Defects in Mendeleev’s Periodic Table
(i) Hydrogen, like alkali metals, is classified as a group IA element. However, it has several features with group VII A halogens. As a result, its place in Mendeleev's periodic table remains debatable.
(ii) Although the elements in Mendeleev's periodic table are ordered by atomic mass, in certain circumstances the element with the higher atomic mass comes before the element with the lower atomic mass.
(iii) We already know that an element's isotopes have distinct atomic weights but the same atomic number. Because the periodic table was created based on the rising atomic masses of the elements, all of the isotopes of a given element must have been assigned to distinct locations.
(iv) According to Mendeleev, elements in the same group must have characteristics that are similar. However, there is no resemblance between the components in two sub-groups of the same group.
**The Periodic Table in Its Current Form **
Physical Property Changes
Atomic Radii: The distance between the nucleus's centre and the outermost shell containing the electrons is measured in atomic radii. Three distinct types of atomic radii are employed depending on whether an element is a non-metal or a metal. These are the following:
(a) Covalent Radius: Half the distance between the nuclei of two atoms held together by a purely covalent single bond.
(b) Ionic Radius: This is the effective distance from an ion's nucleus to which it has an influence on the ionic bond.
(c) Van der Waals Radius: The weak van der Waals forces of attraction hold Noble gas atoms together. The van der Waals radius is equal to half the distance between the nuclei of noble gas atoms.
(d) Metallic Radius: In the metallic lattice, it is defined as half of the internuclear distance between two neighboring metal ions.
Periodic Trends in Chemical Properties along a Period
(i) Characteristics of metal: Decrease over a period maxima on the extreme left (alkali metals).
(ii) Non-metallic character: Becomes more non-metallic over time. (From the left to the right)
(iii) Basic nature of oxides: Decreases in a period from left to right.
(iv) Acidic character of oxides: Increases with time from left to right.
Differences in Moving Down a Group from Top to Bottom
(i) It has a metallic feel about it. From top to bottom, the ionizatiort energy of the elements in a group decreases due to an increase in atomic size and hence a drop in the ionizatiort energy.
(ii) Characteristics that aren't metallic. Reduces the size of a group in general. From top to bottom in a group, the electronegativity of elements diminishes.
(iii) The fundamental character of oxides. The fundamental nature of oxidise naturally rises as the metallic character or electropositivity of elements increases from top to bottom in a group.
(iv) The acidic nature of oxides. As the non-metallic property of elements drops from top to bottom in a group, it generally decreases.
(v) Metals' reactivity. In general, the number of people in a group grows. Because the potential to lose electrons is increasing.
(vi) Non-metal reactivity. The higher the electro-negativity of non-metals, the greater their reactivity. The reactivity of non-metals in a group reduces as their electronegativity falls from top to bottom.