Classification of Elements and Periodicity in Properties Test - 9

All of them are isoelectronic species. They have same number of electrons (10). Their radii would be different because of their different nuclear charges. The cation with the greater positive charge will have a smaller radius because of the greater attraction of the electrons to the nucleus. Anion with the greater negative charge will have the larger radius. In this case, the net repulsion of the electrons will outweigh the nuclear charge and the ion will expand in size.

Actinoids are elements with Z = 90-103. They are characterised by the outer electronic configuration (n-2) \(f ^{-14}\) (n-1) \(dns ^2\). The last electron added to each element is filled in f-orbital. Hence Terbium (Z = 65) is not an actinoid. It is a lanthanoid.

Both the above elements contain vacant d-orbitals that is why they can extend their covalency.

Group I (alkali metals) and Group II elements (alkaline earth metals) are reactive metals with low ionization enthalpies.

The effective nuclear charge experienced by a valence electron in an atom will be less than the actual charge on the nucleus because of ‘‘shielding” or “screening” of the valence electron from the nucleus by the intervening core electrons. For example, the 2s electron in lithium is shielded from the nucleus by the inner core of 1s electrons.

Ionization enthalpy depends upon two factors:

(i) the attraction of electrons towards the nucleus, and

(ii) the repulsion of electrons from each other.

Across a period, increasing nuclear charge outweighs the shielding. Consequently, the outermost electrons are held more and more tightly and the ionization enthalpy increases across a period. In case of Mg the outermost electron is in s-orbital that is a stable gas configuration and penetration effect of s-subshell is more as compared to Al in which outermost electron is in p-subshell.

Gadolinium is a lanthanoid, it is a f-block element and the outer electronic configuration is (n - 2)\(f ^{-14}\) (n-1)\(d ^{0-1}\)\(ns ^2\).

The p-Block elements comprise those belonging to Group 13 to 18 and these together with the s-Block elements are called the Representative Elements or Main Group Elements.

According to Aufbau principle, 3d-orbitals are filled after 4s and before 4porbitals, as the energy of 4s is lesser than 3d.

Within a group, electron gain enthalpy becomes less negative down a group. However, adding an electron to the 2p-orbital leads to greater repulsion than adding an electron to the larger 3p-orbital. Hence, the element with most negative electron gain enthalpy is chlorine.

For many elements energy is released when an electron is added to the atom and the electron gain enthalpy is negative. For example, group 17 elements (the halogens) have very high negative electron gain enthalpies because they can attain stable noble gas electronic configurations by picking up an electron.

The period indicates the value of n for the outermost or valence shell. Hence, Period Number = maximum n of any element, where n refers to principal quantum number.