Classification of Elements and Periodicity in Prop

3.56. The elements of the s-block and p-block are collectively called as representative or main group elements. These include elements of group I (alkali metals), group 2 (alkaline earth metals).

3.55. The energy which is released by an atom in gaining an electron from outside the atom or ion to form a negative ion (or an anion) is called electron gain enthalpy.

The unit of electron gain enthalpy is kJ/ mol. In some cases, like in noble gases, atoms do not have any attraction to gain an electron. In that case, energy must be supplied.

3.54. 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 the s-block and the p-block.

3.53. They have filled s-orbitals and hence do not tend to accept an additional electron. That is why energy is needed if an extra electron is to be added. Therefore, the electron gain enthalpies of Be and Mg are positive.

3.52. Ionization enthalpy represents the energy required to remove an electron from an isolated gaseous atom in its ground state. Nitrogen has a greater ionization enthalpy than that of oxygen due to its exactly half-filled p-orbitals.

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

3.46. (b) Electronegativity refers to the tendency of an atom to share electrons with another atom.

3.45.  (b) It has only ones-electron and hence can be placed in group 1 (alkali metals). It can also gain an electron to achieve a noble gas arrangement and hence it can behave similarly to a group 17 (halogen family) element. Because it is a special case, we shall place hydrogen separately at the top of the Periodic Table

3.44.  (d) Assertion is a wrong statement. Non-metallic elements have a strong tendency to gain electrons. Therefore, electronegativity is directly related to those non-metallic properties of elements. It can be further extended to say that the electronegativity is inversely related to the metallic properties of elements.

Thus, the increase in electronegativities across a period is accompanied by an increase in non-metallic properties (or a decrease in metallic properties) of elements