Scientific Reasons for XII

Reason

4s orbital is filled prior to 3d orbital but on ionization electrons are first lost from 4s orbital instead of 3d orbital.

Answer

In writing electronic configuration of transition metals, the ns orbital is filled before the (n–1)d orbital. But on ionization ‘ns’ electrons are lost prior to (n–1)d electron. In fact, it is generally assumed that when a positive ion is formed from an atom, electrons are always lost first from the sub-shell having the largest value of n. The reason is that the electron-electron and electron-nucleus interactions in a neutral atom can be quite different from those in ions. In transition metal ions, (n–1)d orbital is more stable than the ns orbital. Moreover, according to crystal field theory the order of filling of empty orbitals is different from the order of filling of filled orbitals. Thus when (n–1)d orbitals start filling with electrons, it repels ns orbital away from nucleus thereby increasing its energy. Since electrons are first lost from high energy orbital, thus electrons are first removed from ns orbital and after that from (n–1) orbital.

 Reason

The elements of same group are chemically similar but their physical properties gradually change down the group.

Answer

The chemical properties of elements depend upon the number and arrangement of electrons in valence shells. Atoms with same number of valence electrons would be expected to be chemically similar. Since elements of same group have same valence shell electronic configuration, therefore, they have same chemical properties.

The regular variation in physical properties such as atomic size, I.P., E.N., etc down a group is called Group Trend. Group trend in physical properties such as atomic size, ionic size, and metallic character is that they increase down the group whereas E.N., I.P. and E.A. decrease down the group. The physical properties of elements depend upon atomic mass and the total number of electrons in inner shells. As going down a group, a new shell is added in each period, hence there is a regular change in physical properties.

For Example

In group IIIA elements, the number of total valence electrons are 3 (ns² np¹) but number of inner electrons are different. e.g. B, Al and Ga have 2, 10 and 28 inner electrons respectively, due to this reason, regular change in physical properties is observed down a group.

₅B      =  5e    =  1s², 2s² 2p¹

₁₃Al    =  13e  =  1s², 2s² 2p⁶, 3s² 3p¹

₃₁Ga   =  31e  =  1s², 2s² 2p⁶, 3s² 3p⁶, 3d¹⁰, 4s², 4p¹

Reason

Li and Be differ from other members of their group.

Answer

Lithium and Beryllium, the first members of groups IA and IIA differ markedly from other members of their respective groups. This is because of their small atomic sizes, which results in high charge densities on Li⁺ and Be²⁺ ions, which produces high heat of hydration. Thus Li and Be have more tendency to form covalent compounds.

Reason

Lithium resembles with Mg (Beryllium resembles with aluminium) in properties.

Answer

Li and Be has a number of common properties, such as their carbonates, fluorides, sulphates and phosphates are only slightly soluble in water. Lithium resembles more with Mg while Beryllium matches more with Al. This is called Diagonal Relationship. The element of the second period belonging to the IA or IIA group is similar in behaviour to the element of IIA or IIIA group in the third period. This close resemblance between the properties of an element of 2^nd period with those of the element of 3^rd period belonging to next group is called Diagonal Relationship. This is due to approximately equal sizes of Li⁺ (0.060 nm) and Mg²⁺ (0.065 nm). Similarly Be²⁺ (0.031 nm) and Al³⁺ (0.05 nm) ions are of equal size. Even their atoms are of nearly equal size.