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.

(i) Ca
(ii) S.

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.

The roots 2,7,5,9 and 0 are referred as bi, sept, pent,enn and nil respectively. Therefore, their names and symbol are

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.

Similarity in outer electronic configuration and gradual addition of an electron into the successive elements,are the most important cause of periodicity.

Mendeleev's periodic law : It states that the properties of the elements are a periodic function of their atomic weights
Modern periodic law : It states that the properties of the elements are a periodic function of their atomic numbers. Thus, change in the base of classification of elements from atomic weight to atomic number is the basic difference between Mendeleev's periodic law and the modern periodic law.

The number designation of a group that has 2 electrons beyond a noble gas configuration will be 2 which means it will belong to group 2 of the periodic table.

Chemical and many physical properties of the elements depend on the electronic configuration of the atoms, whereas the nuclear properties do not.

Atomic mass of Y \(=\frac{\text{atomic mass of X+atomic mass of Z}}{2}\)
\(=\frac{7+39}{2}=\frac{46}{2}=23\)

The basic theme of organisation in the periodic table is to simplify and systematise the study of physical and chemical properties of all the elements and their innumerable compounds.

Ionization Enthalpy. The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom so as to convert it into a gaseous cation is called its ionization enthalpy or energy. It is represented by\(\triangle \)_i H. This process may be represented as
M(g) +\(\triangle \)_i H⟶M⁺(g) + e^- (g)
where M (g) is isolated gaseous atom. M⁺ (g) is the resultant cation (a position ion) Variation along a period. Moving from left to right in a period, the ionization enthalpy increases with atomic number.

Exothermic reactions Reactions which are accompanied by evolution of heat are called exothermic reactions. The quantity of heat produced is shown either along with the products with a'+' sign or in terms if \(\Delta H\)with a '-' sign.
 \(C(s)+{ O }_{ 2 }(g)\rightarrow CO_{ 2 }(g)+393.5kJ\)
\(H_{ 2 }(g)+\frac { 1 }{ 2 } O_{ 2 }(g)\rightarrow { H }_{ 2 }O(l);\Delta H=-285.8kJmol^{ -1 }\)
Endothermic reactions Reactions which proceed with absorption of heat are caleed endothermic reactions. The quanity of heat absorbed is shown either along with the products with a '-' sign or in terms of \(\Delta H\) with a'+' sign.
e.g
\({ C }(s)+{ H }_{ 2 }O(g)\rightarrow CO(g)+{ H }_{ 2 }(g)-131.4kJ\)
\( { N }_{ 2 }(g)+3H_{ 2 }(g)\rightarrow 2NH_{ 3 }(g);\Delta H=+92.4kJmol^{ -1 }\)

(i) In third period, 3s and 3p orbitals are filled.
(ii) Promethium (Pm) with atomic number 61 is a man-made lanthanoid.
(iii) Actinoid series (f-block elements).

Metallic character increases down a group and decreases along a period as we move from left to right. Hence the order of increasing metallic character is: P < Si < Be < Mg < Na.

There are numerous physical properties of elements such as melting points, boiling points, heats of fusion and vaporisation, energy of atomisation, etc., which show periodic variations. The cause of periodicity in properties is the repetition of similar outer electronic configuration after certain regular intervals. e.g. all the elements of 1s group (alkali metals) have similar outer electronic configuration, i.e. ns¹.
₃Li = 1s², 2s¹
₁₁Na = 1s², 2s², 2p⁶ , 3s¹
₁₉K = 1s², 2s², 2p⁶ , 3s², 3p⁶, 4s¹
Therefore, due to similar outermost shell electronic configuration all alkali metals have similar properties. e.g., sodium and potassium both are soft and reactive metals. They all form basic oxides and their basic character increases down the group. They all form unipositive ion by the loss of one electron. Similarly, all the elements of 17th group (halogens) have similar outermost shell electronic configuration, i.e. ns²  np⁵ and thus possess similar properties.
₉F = 1s², 2s¹ , 2p⁵
₁₇Cl = 1s², 2s², 2p⁶ , 3s² , 3p⁵
₃₅Br = 1s², 2s², 2p⁶ , 3s², 3p⁶, 3d¹⁰, 4s² , 4p⁵