Chapter 1 Notes | Chemistry 2nd Year
“Periodic Classification Of Elements And Periodicity”
- Although a number of chemists attempted to classify the elements but Dmitri Mendeleev gave the most the most useful and comprehensive classification.
- In Mendeleev’s periodic table the elements were arranged according to ascending order of their atomic masses
- The modern periodic law states “if the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner”.
- In modern periodic table elements similar properties are placed in eight vertical columns called groups. Each group is divided into two subgroups A and B. Normal or typical elements are placed in subgroups A and transition elements ae placed in subgroups B.
- The seven horizontal rows of the periodic table are called “periods”.
- Metals of sub-groups IA and IIA are called alkali metals and alkaline-earth metals, respectively. Members of subgroups VIIA are called halogens.
- Due to their less reactivity the elements shown in subgroups VIIIA are called noble gases.
- Elements of periodic table can also be classified into s-block, p-block, d-block and f-block elements depending upon the valence orbital which is in the process of completion.
- Elements of periodic table can also be divided into metals, non-metals and metalloids depending upon their properties.
- Atomic radii increase from top to bottom in a group and decrease along a period.
- Positive ions are always smaller than their parent’s atoms while the negative ions are usually larger than the atoms from which they formed.
- Electron affinities generally increase with increasing atomic number within period and decrease from lighter to heavier elements in a given group.
- Metallic character of elements increase down the group decreases along a period.
- The oxidation state of a typical element is directly or indirectly related to the group number of which the element belongs in the periodic table.
- The electrical conductance of an element depends upon the number of free movable electrons.
- There are three types of halides: ionic, polymeric and covalent. Halides of group IA are ionic in nature, have three dimensional lattices with high melting boiling points.
- There are three types of hydrides formed by the elements of periodic table: ionic, intermediate and covalent.
- Highly polar hydrides show hydrogen bonding in them
- Oxides may be divided on the basis of their acidic, basic or amphoteric character.
- Metallic oxides are basic in character, nonmetallic oxides are acidic in character and oxides of less electropositive elements like Zn Pb are amphoteric.
- Hydrogen is unique element of the periodic table. Due to similarities in properties it can be placed at the top of group IA or IVA or VIIA
See Also: 2nd Year Chemistry Guess Papers 2021
See Also: Chemistry 2nd Year Paper Scheme 2021