(i) As the principal quantum number for the valence shell is 4, the element is present in the 4th period.
(ii) Since the last electron has been filled in 4s sub-shell (or orbital), the element belongs to s-block.
(iii) As there is only one electron in the valence s-subshell, the element is present in group 1.

General electronic configuration of f-block elements = (n - 2)^{f -14} (n - 1) d^{0 -1} ns².

The elements of group 1 (alkali metals), group 2 (alkaline earth metals) and group 13 to 17 constitute the representative elements. They are elements of s-block and p-block.

Al and Si.

Ar, K⁺, Cl^- S^2-, or P^3- are isoelectronic with ca²⁺.

Modem Periodic Law states that physical and chemical properties of the elements are a periodic function of their atomic numbers.

(i) s-Block elements: ns^1-2 where n = 2 - 7.
(ii) p-Block elements: ns² np^1-6 where n = 2 - 6.
(iii) d-Block elements: (n -1) d^1-10ns^O-2 where n = 4 - 7.
(iv) f-Block elements: (n - 2)f^0-14 (n - 1) d^0-1 ni where n = 6 - 7

The elements of Group I have only one electron in their respective valence shells and thus have a strong tendency to lose this electron. The tendency to lose electrons in turn, depends upon the ionization enthalpy. Since the ionization enthalpy decreases down the group therefore, the reactivity of group 1 elements increases in the same order Li < Na < K < Rb < Cs. In contrast, the elements of group 17 have seven electrons in their respective valence shells and thus have strong tendency to accept one more electron to make stable configuration. It is linked with electron gain enthalpy and electronegativity. Since both of them decreases down the group, the reactivity therefore decreases.

The decrease in \(\triangle \)_i H₁ value from B to Al is due to the bigger size of AI. In Ga, there is 10 3d electrons which do not screen as is done by S and P electrons. Therefore, there is an unexpected increase in the magnitude of effective nuclear charge resulting in increased \(\triangle \)₁ H₁ values. The same is with into TI. The later has fourteen \(\triangle \)f electrons with very poor shielding effect. This also increases, the effective nuclear charge thus the value of \(\triangle\) _i H₁ increases.

Na₂O with water forms a strong base whereas Cl₂O₇ forms strong acid.
\({ Na }_{ 2 }O+{ H }_{ 2 }O\rightarrow 2NaOH\)
\({ Cl }_{ 2 }O_{ 7 }+{ H }_{ 2 }{ O }\rightarrow { 2 }HClO_{ 4 }\)
Their basic or acidic nature can be qualitatively tested with litmus paper.

Ionic radius is affected by following factors
(i) Effective nuclear charge As the effective nuclear charge increases, ionic radius decreases
(ii) Shielding effect As the shielding effect increases, ionic radius increases.

Elements in which the last electron enters in the d-orbitals, are called d-block elements or transition elements. These elements have the general outer electronic configuration (n-1)d^1-10 ns^0-2 . Zn, Cd and H having the electronic configuration, (n-1)d¹⁰ ns² do not show most of the properties of transition elements. The d-orbitals in these elements are completely filled in the ground state as well as in their common oxidation states. Therefore, they are not regarded as transition elements. Thus, on the basis of properties, all transition elements are d-block elements are not transition elements.

Mendeleev used atomic weight as the basis of classification of elements in the periodic table. He arranged 63 elements known at that time in the periodic table on the basis of the order of their increasing atomic weight. At some places he ignored the increasing order of atomic weights where the elements having similar properties are placed together.