What are isoelectronic ions? Give examples.

Important Points to Remember in Chapter -1 - Periodic Classification of Elements from Tamil Nadu Board Chemistry Standard 11 Vol I Solutions

1. Genesis of periodic classification:

(i) Dobereiner’s triads: When three elements of a triad were arranged in order of increasing atomic weights, the atomic weight of the middle element was either nearly the same or the atomic weight of the middle element was approximately the arithmetic mean of the other two

(ii) Newlands law of octaves: When lighter elements were arranged in order of their increasing atomic weights, the properties of every eighth element were similar to those of the first like the eighth note of a musical scale.

(iii) Lother-Meyer plotted a graph between atomic volume of the elements against their atomic weights. He observed that the elements with similar properties occupied similar positions on the curve.

(iv) Mendeleev’s period law states that physical and chemical properties of elements are a periodic function of their atomic weights.

2. Modern periodic law:

(i) Modern periodic law states that physical and chemical properties of the elements are a periodic function of their atomic numbers.

(ii) Modern periodic table is also called long form of the periodic table or Bohr’s table.

3. s, p, d, f-Blocks:

(i) s-Block elements: These elements contain $1$ or $2$ electrons in the s-orbital of their respective outermost shells. Their outer shell electronic configuration being $n s^{1 - 2}$ where $n = 1 - 7$ .

(ii) p-Block elements: These elements contain $1 - 6$ electrons in the p-orbitals of their respective outermost shells. Their general outer shell electronic configuration is $n s^{2} n p^{1 - 6}$ where $n = 2 - 7$ .

(iii) d-Block elements also called transition elements contain $1 - 10$ electrons in the d-orbitals of their respective penultimate shells. Their general outer shell electronic configuration is $(n - 1) d^{1 - 10} n s^{0 - 2}$ where $n = 4 - 7$ .

(iv) f-Block elements also called inner transition elements contain 1 to 14 electrons in the f-orbitals of their ante-penultimate shells. Their general outer shell electronic configuration is $(n - 2) f^{1 - 14} (n - 1) d^{0 - 1} n s^{2}$ where $n = 6 - 7$ . This group consists of lanthanoids and actinoids.

(v) All the elements of s- and p-block except noble gases are called representative elements.

4. Trends in Physical Properties:

(i) Atomic radius is the distance between the centre of the nucleus to the outermost shell containing electrons.

(ii) Covalent radius is one-half the distance between the nuclei of two covalently bonded atoms of the same element in a molecule.

(iii) van der Waals radius is one-half the distance between the nuclei of two adjacent identical atoms belonging to two neighbouring molecules of an element. It is also called inert gas radii.

(iv) Metallic radius is one-half the internuclear distance between the two adjacent metal ions in the metallic lattice.

(v) The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to convert into gaseous cation is called ionization enthalpy. It is denoted by $Δ_{i} H$ and is measured in terms of electron volts (eV) per atom or $k c a l m o l^{- 1}$ or $k J m o l^{- 1}$ .

(vi) Generally, $Δ_{i} H$ decreases from top to bottom in a group and increases from left to right in a period.

(vii) Electron gain enthalpy of an element is the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous anion. It is denoted by $Δ_{e g} H$ .

(viii) The $Δ_{e g} H_{1}$ for most of the elements is negative while their $Δ_{e g} H_{2}$ is always positive.

(ix) Electronegativity of an element is the tendency of its atom to attract the shared pair of electrons towards itself in a covalent bond. It is expressed by the symbol, χ. The electronegativity of an atom decreases regularly down a group from top to bottom and increases along a period from left to right.

5. Periodic Trends in Chemical Properties:

(i) Periodicity of Valence: The valence of representative elements is usually equal to the number of electrons in the outermost orbitals and / or equal to eight minus the number of outermost electrons.

(ii) Some elements of the second period show diagonal relationship with elements of third period and hence exhibit some similar properties.