(i) Group 1 elements are most reactive.
(ii) Group 18 elements are least reactive because they have stable electronic configuration.
(iii) It may cause high blood pressure and any other harm to the body. It disturbs the person physically and mentally also.
(iv) They are cool, calm, happy and help other people. They are more healthy than aggressive people.
(v) aggressive people pick up fight. They do not accept their fault. Sometimes, it leads to physical injury which put them in trouble. We should not involve in such act.

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

(i) Potassium, K (Z = 19)
(ii) Strontium, Sr (Z = 38)
(iii) Ruthenium, Ru (Z = 44)
(iv) Praseodymium, Pr (Z = 59 ) 
(v) Xenon, Xe(Z = 54 )

CI has smallest radius due to greater effective nuclear charge.
17 protons attract 17 electrons. In P, 15 protons attract 15 electrons. In Si, 14 protons attract 14 electrons.

B has highest ionisation enthalpy due to smallest atomic size and, therefore, highest effective nuclear charge, has two shells only. Al has 3 shells, Ga has 4 shells.

(i) Ne
(ii) F
(iii) Li (Covalent radii)
(iv) F
(v) Li
(vi) C.

(i) Most reactive non-metal has high Δ_i H₁ and Δ_i H₂ and most negative Δ _egH.Therefore, the element is B.
(ii) Most reactive metal has low Δ_i H₁ and high Δ_i H₂ (because the second electron has to be lost from noble gas configuration) and has small negative LlegH. Therefore, the element is A.
(iii) Noble gases are the least reactive elements. They have very high Δ_iH₁ and s, H₂ and have positive Δ egH values. Thus, the element is D.
(iv) Metal forming-binary halides are alkaline earth metals. They have Δ_i H₁ and Δ_i H₂ values little higher than those of most reactive metals (such as A) and have comparatively slightly less negative Δ_egH values. Thus, the element is C.

In a multielectron atom, the electrons present in the inner shells shield the electrons in the valence shell from the attraction of the nucleus or they act as a screen between the nucleus and these electrons. This is known as Sheil ding effect or screening effect. As the screening effect increases, the effective nuclear charge decreases. Consequently, the force of attraction by the nucleus for the valence shell electrons decreases and hence the ionization enthalpy decreases.

(i) Atomic size. With the increase in the atomic size, the number of electron shells increases. Therefore, the force that binds the electrons with the nucleus decreases. Thus, the ionization enthalpy decreases with increase in atomic size.
(ii) Nuclear charge. As the magnitude of the positive charge on the nucleus of an atom increases, the attraction with the electrons also increases. Therefore, the ionization enthalpy increases with the increase in the magnitude of the nuclear charge.
(iii) Screening or shielding effect. Greater the magnitude of the screening effect, less will be the value of ionization enthalpy or potential.

Electronic configuration of A (Z = 12)
1s² 2S² 2p⁶ 3s²
period = 3, Element's name = Mg
block = s, Group = II
Electronic configuration of B (Z = 19)
Element's name K (potassium)
1s² 2S² 2p⁶ 3s² 3p⁶ 4S¹
n = 4, period = 4
Block = s, Group = I
Electronic configuration of C (Z = 29)
1s² 2S² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4S¹
n = 4, period = 4
Block = d
Electronic configuration of D (Z = 36)
1s² 2S² 2p⁶ 3s² 3p⁶ 4S² 3d¹⁰ 4p⁶
period = 4
Block = p-Block
group = 18

Non-metallic character across a period (left to right) increase but on moving down the group it decreases. So, the increasing order of non0metalic character is P.

In general, on moving down the group (13 th group) from b to Al, the ionisation enthalpy decreases with increase in atomic size and screening effect as expected. But IE₁ of Ga is slightly higher (only 2 kJ mol^-1) than IE₁ of Al. It is due to imperfect shielding of the valence electrons by 3d-electrons. As a result of this, effective nuclear charge in Ga is sightly more than that of Al. That's why (IE₁)\((\Delta _{ i }H_{ i })\) of Ga is sightly more than that of Al. On moving from In to Tl,\((\Delta _{ i }H_{ i })\) of Tl is larger than that of In. It is due to the fact that effective nuclear charge outweighs the shielding effect of all the electrons present in 4f and 5d-electrons.

Cu is largest due to less effective nuclear charge. It has 29 elements,29 protons.Cu⁺ has 28 elements and 29 protons, Cu²⁺ has 27 electrons and 29 protons.

(a) Due to repulsions between the electrons on \(O^{ - }\)and the additional incoming electron, the electron gain enthalpy of \(O^{ - }\)is positive while that os S is negative.
(b) On similar grounds, the electron gain enthalpy of S⁻is positive while that of O is negative.
(c) Due to small size, repulsions between O⁻ and the incoming electron is much more than in S⁻. Therefore, electron gain enthalpy of O⁻ is more positive than that of S⁻.
(d) Due to repulsions between N⁻ and the incoming electron, electron gain enthalpy of N⁻ is positive while that of P is negative.

The outer electronic configuration of 1s²,2s²,2p⁵ is 2s²,2p⁵. Therefore, this element is a p-block element and belongs to the period and group 17. Thus, the element is fluorine, F.

(a) In general, as the size of atom or multiplicity of bond increases, bond dissociation energy increases but bond dissociation energy of F₂ is less than that of Cl₂ because of the small size of F. Thus, the correct order is
F₂  <  Cl₂   <  O₂   <   N₂
155   242   494   941 kJ mol
(b) Electropositive character means tendency to give an electron to form cation. It varies directly with atomic radii. Thus, the correct order is Mg > fe > Cu.

The first inert gas which contains d electrons is krypton. Its atomic number is 36 and its electronic configuration is 1s² , 2s² , 2p⁶, 3s², 3p⁶ , 3d¹⁰ , 4s² , 4p⁶
Total number of d-electrons = 10
Total number of p-electrons = 6 + 6 + 6 = 18
Total number of s-electrons = 2 + 2 + 2 + 2 = 8
\(\therefore \) Difference in total number of p and s electrons = 18 - 8 = 10
Thus, the inert gas is krypton.

(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).