CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS
EARLY CLASSIFICATION OF ELEMENTS:
1. Dӧbereiner’s Triads
Mendeleev’s Periodic Table
Mendeleev formulated the Periodic Law which states that the properties of elements are the periodic function of their atomic masses. He arranged the elements in a table called the periodic table consisting of vertical columns called ‘groups’ and horizontal rows called ‘periods’.
THE MODERN PERIODIC TABLE
Henry Moseley showed that that the atomic number of an element is a more fundamental property. The modern periodic law states that properties of elements are a periodic function of their atomic numbers.
Trends in the modern periodic table:
1. Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. Valency increases in a period on going from left to right. Valency remains constant on going down the group.
2. Atomic size: Atomic size refers to the radius of an atom i.e. the distance between the centre of the nucleus and the outermost shell of the isolated atom. The atomic size decreases in moving from left to right along a period but it increases as we go down the group.
3. Metallic and non-metallic character: Metallic character decreases across a period and increases down a group. Non-metallic character increases across a period and decreases down a group.
CHAPTER 4 CARBON AND ITS COMPOUNDS
Carbon atom forms covalent bonds. Covalent bonds are formed by sharing of an electron pair between two atoms.
VERSATILE NATURE OF CARBON ATOMS
The ability of carbon to form chains gives rise to a homologous series of compounds in which the same functional group is attached to carbon chains at different lengths.
The functional groups such as alcohols, aldehydes, ketones and carboxylic acids lead to the carbon compound having characteristic properties.
CHAPTER 3 METALS AND NON-METALS
The reactivity series of metals is as follows. Reactivity decreases as the series progresses:
REACTION OF METALS AND NON-METALS
Metals and non-metals react to form compounds like ionic compounds.
Ionic compounds: The compounds formed by transfer of electrons from a metal to a non-metal are known as ionic compounds or electrovalent compounds. The properties of ionic compounds are:
OCCURRENCE OF METALS
Extraction of metals: Metals are extracted from their ore. To extract metals which are low in the reactivity series, their oxides are reduced to metals by heating alone. The metals which are in the middle of the reactivity series are usually found in the form of sulphides and carbonates. These are converted into oxides first and then are extracted from the oxides. The highly reactive metals that are on the top of the reactivity series are obtained by electrolytic reduction.
Enrichment of ores: Ores usually contain large amount of impurities. They have to be enriched first before extracting the metal.
Refining of metals: The metals obtained by various reduction processes are not very pure and have to be further refined to remove the fine impurities. The most widely used method for refining impure metals is electrolytic refining.
Electrolytic refining: in this process, the impure metal is made the anode and pure metal is made the cathode. A solution of the metal salt id used as an electrolyte. When current is passed through the electrolyte, the pure metal from the anode dissolves into the electrolyte and an equivalent amount of pure metal from the electrolyte is deposited on the cathode.
Corrosion is the deterioration of a metal due to its reaction with its surroundings. For e.g. Rusting of iron when it comes in contact with the water vapour in the air.
Prevention of corrosion: There are a number of methods to prevent corrosion. Some of them are: