What is the basic theme of organisation in the periodic table?

What is the most important cause of periodicity?

An element X belongs to the third period of p-block. It has 4 elements in the outermost shell. Name the element. 

Four elements have the following first ionisation enthalpies ( in kJ mol^-1); 869, 941, 1191 and 1142. The elements, in random order, are Se, Br, Te and I. Which element has an ionisation enthalphy of 869 kJ mol^-1 and 1142 kJmol^-1 respectively?

Give examples of three cations and three anions which are isoelectronic with neon.

Why are electron gain enthalpies of Be and Mg positive?

Electronegativity of F on Pauling scale is 4.0. What is the value on Mulliken's scale.

Arrange the following elements in the increasing order of non-metallic character. B, C, Si, N, F

What is the basic difference in approach between the Mendeleev’s Periodic Law and the Modern Periodic Law?

Explain why cations are smaller and anions are large in radii than their parent atoms?

What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?

Explain, why the electronegativity values if noble gases are zero while those of halogens are the highest in each period?

Assign the position of the element having outer electronic configuration
(i) ns² np⁴ for n = 3
(ii) (n-1) d² ns² for n = 4 and
(iii) (n-2) f⁷ (n-1) d¹ ns² for n = 6, in the periodic table.

Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However, oxygen has lower ionisation enthalpy than nitrogen. Explain.

Consider the following species \({ N }^{ 3- },{ O }^{ 2- },{ F }^{ - },{ Na }^{ + },M{ g }^{ 2+ },and \ { Al }^{ 3+ }\) Arrange them in the order of increasing ionic radii.

Arrange the elements N, P, O, and S in the order of Increasing non-metallic character. Give the reason for the arrangement assigned.

Write the name and the atomic number of the following elements.
(i) The third alkali metal
(ii) The fourth alkaline earth metal  
(iii) The sixth element of second transition series
(iv) The second inner transition element
(v) The fifth noble gas.

The first (∆_i H₁) and the second (∆_i H₂) ionization enthalpies (in kJ mol^–1) and the (∆_egH) electron gain enthalpy (in kJ mol^–1) of a few elements are given below:

Which of the above elements is likely to be :
(a) the least reactive element.
(b) the most reactive metal.
(c) the most reactive non-metal.
(d) the least reactive non-metal.
(e) the metal which can form a stable binary halide of the formula MX₂(X=halogen).
(f) the metal which can form a predominantly stable covalent halide of the formula MX (X=halogen)?

In p-block elements form acidic, basic and amphoteric oxides. Explain each property by giving two examples and also write the reactions of these oxides with water.