Classification of Elements and Periodicity in Prop

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Mendeleev's arranged the elements as the periodicity of their atomic weights.

The drawbacks of Mendeleev's periodic table are as follows:-

(a) The position of hydrogen in the periodic table is not specified

(b) Isotopes are not included in the periodic table

(c) Elements with higher atomic mass are placed before the elements with lower atomic mass. For e.g- Co & Ni

(d) Gaps are left in his table considering the fact that more elements are yet to be discovered

(e) Inappropriate position of group VII.

This is a Long Answer Type Questions as classified in NCERT Exemplar

The group 1 elements are called alkali metals which possess outermost electronic configuration as ns1 .All the elements that have same outermost electron configuration possess similar properties and are placed in the same group.

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The chemical properties and physical properties of the elements depends upon its outer electron configuration which ultimately is the periodic function of the atomic number. The elements which possess the same outer electron configuration belong to the same group or family showing similar properties.

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The factors that affects electron gain enthalpy are as follows:

(a) Effective nuclear charge with its increase the electron gain enthalpy also increases

(b) Atomic size with its increase the electron gain enthalpy decreases

(c) The e-e - repulsion with its increase the electron gain enthalpy decreases

(d) Whenever there exists half filled or completely filled orbital than it lead to decrease the electron gain enthalpy because it leads to give extra stability to the atom due to symmetry

The electron gain enthalpy decreases down the group and increases across the period.

3.60. In a multi-electron atom, the electrons present in the inner shells shield the electrons in the valence shell from the attraction of the nucleus. They act as a screen between the nucleus and these electrons. This is known as the shedding effect or the screening effect.

As the screening effect increases, the effective nuclear charge decreases. Consequently, the force of attraction by the nucleus for the valence shell electrons decreases, and hence the ionization enthalpy decreases.

3.59. The d-block elements are known as transition elements.

They have electronic configuration= (n – 1) d^1-10 ns^1-2

Characteristics of d-block elements are:

They show variable oxidation states.

Their compounds are generally paramagnetic.

Most of the transition elements form coloured compounds.

They are all metals with high melting and boiling points.

These elements are used in the formation of alloys.

3.58. Elements gallium and germanium were unknown at the time Mendeleev published his Periodic Table. He left the gap under aluminium and a gap under silicon, and called these elements Eka-Aluminium and Eka-Silicon. Mendeleev predicted not only the existence of gallium and germanium but also described some of their general physical properties.