Notes on Periodic Table, Chemical Reactions & Classification of Elements
Evolution of Periodic Table:
- After the discovery of a large number of elements and their compounds, the study of these elements and compounds individually became difficult, so it is felt that these elements should be classified into a few groups to make their study more systematic and easier.
- Various attempts were made in this direction by several scientists such as Dobereiner’s triads law, Newland’ law of octave, but the first impressive attempt in this direction was given by Mendeleev.
Mendeleev's Periodic Table:
- Mendeleev's Periodic table states that the physical and chemical properties of an element are a periodic function of their atomic masses.
- It contains 7 periods and 8 groups, but O group i.e. inert gases is absent in this table.
- It has no fixed position for hydrogen, no spaces for isotopes and not a regular arrangement of atomic masses.
Modern Periodic Table:
- Moseley gives the modern periodic table and states that the physical and chemical properties are a periodic function of their atomic number.
The long form of Periodic Table:
- This table is based on the Bohr-Burry concept of electronic configuration.
- It consists of 7 periods (horizontal rows) and 18 groups (vertical columns).
- The first element of each period except I period is an alkali metal and the last element is an inert gas.
- The first period has two elements.
- Second and thirds periods have 8 elements.
- Fourth and fifth periods have 18 elements.
- The sixth period has 32 elements.
- Seventh is a very long period and incomplete.
- The number of valence electrons in elements increases from 1 to 8 on moving from left to right in a period.
- The valence of the element increases from 1 to 4 and then decreases to zero on moving from left to right in a period with respect to hydrogen.
- All the elements of a group have the same number of valence electron and hence, have almost similar chemical properties.
Blocks in Periodic Table:
The elements can be classified into the following four blocks:
- It contains groups 1 and 2 i.e. Hydrogen and alkali metal group and alkaline earth metal.
- These elements are soft metal, electropositive.
- This block consists of groups from 13 to 18.
- This only blocks which consist of both metals, non-metal and metalloids.
- It consists of the group from 3 to 12.
- Elements of this group are known as transition elements and generally used as a catalyst.
- Elements of this block contain unpaired electrons and are paramagnetic.
Elements of this block is called inner-transition elements.
It consists of Lanthanides and Actinides.
Some properties that are based on the Periodic Table:
- The ability of an atom to attract the shared pair of electrons towards it is known as electronegativity and it increases along a period and decreases down the group.
- Valency with respect to hydrogen increases from periods 1 to 7 but with respect to oxygen first increases from periods 1 to 4 and then decreases to one.
- Valency is a group for all the elements is the same.
Electron gain Enthalpy:
- It is the energy released by an element when an extra electron is added to its neutral gaseous atom.
- It increases through the periods and decreases down the group with some exception.
- It is the tendency of an element to form cation by the loss of electrons.
- It decreases along a period and increases down the group.
- A chemical reaction involves a chemical change in which substances react to produce a new substance.
- Formation of curd from milk.
- Rusting of iron.
Types of Chemical reactions:
- In combination reactions, compounds are formed as a result of the chemical combination of two or more elements.
C + O2 = CO2
- In decomposition reactions, a single compound splits up into two or more simpler substances.
- These reactions need energy in the form of heat, light or electricity.
- A reaction in which reactants combine to form products and again products recombine to form reactants is called reversible reaction.
2HI ↔ H2 + I2
- A reaction which proceeds in only one direction is called an irreversible reaction.
CaCO3 → CaO + CO2
- In these reactions, an atom/ ion present in a compound gets replaced by an atom/ ion of other elements.
Fe + CuSO4 → FeSO4 + Cu
- The reactions which involve oxidation and reduction both are called redox reactions.
- When an acid reacts with a base, salts and water is formed.
- This reaction is called a neutralisation reaction.
HCl + NaOH → NaCl + H2O
- A catalyst is a substance which alters the rate of reaction but the catalyst itself does not alter during the reaction.
- The main function of a catalyst in a reaction is to decrease the activation energy.
- Iron is used as a catalyst in the Haber process for manufacturing of ammonia and molybdenum is used as the prompter of catalyst iron
- In the contact process for the manufacturing of Sulphuric acid, Vanadium pentaoxide is used as a catalyst.
- In the Ostwald process for the manufacturing of nitric acid, Platinum gauze is used as a catalyst.
- In the synthesis of petrol, nickel, iron, cobalt and alumina is used as a catalyst.
- In the manufacturing of Vanaspati, Ghee nickel is used as a catalyst.