The modem periodic law states that, "the physical and chemical properties of the elements are periodic functions of their atomic numbers.

It is defined as the relative tendency of an element present in a covalently bonded molecule, to attract the shared pair of electrons towards itself.

The total number of electrons are less in the cation than the neutral atom while the nuclear charge remains the same. Therefore the effective nuclear charge of the cation is higher than the corresponding neutral atom. Thus the successive ionisation energies, always increase in the following order
IE₁ < IE₂ < IE₃ < .....

NOTE :
At any condition ice cube does not sink in water.
At 0°C, ice and liquid water will be in equilibrium and will coexist. Hence no freezing or melting will occur.

In the large-scale, hydrogen is produced currently by steam-reforming of hydrocarbons. That is, reaction of steam and hydrocarbons in the presence of a catalyst (nickel) at high pressure (35 atm.) and temperature (800°C).
CH₄(g)+H₂O(g) ➝ CO(g)+3H₂(g)

Unlike most liquids, water has a unique property of expanding when it freezes. This expansion allows life within water to survive the winter.

Variation down the group: On moving down a group both atomic and ionic radii increase with increasing atomic number. The increase in size is due to introduction of extra energy shells which outweigh the effect of increased nuclear charge.

Heavy water is toxic when taken in large quantities. Heavy water is not radioactive. The deuterium in it is stable; it does not decay. Nobody will be in danger at all from radiation. It is heavier than plain water. D₂O performs little different from H₂O in chemical reactions. One has to drink a lot of D₂O to kill him.

2H₂ + O₂ \(\rightarrow\) 2H₂O. This reaction is explosive and releases a lot of energy, which is used in fuel cells to generate electricity.

These are soft metals and possess low melting and boiling points with low ionisation enthalpies. They are highly reactive and form ionic compounds. They are highly electropositive in nature and most of the elements imparts colour to the flame.

These elements also show more than one oxidation state and form ionic, covalent and coordination compounds. They can form interstitial compounds and alloys which can also act as catalysts. These elements have high melting points and are good conductors of heat and electricity

The ionisation energy decreases down a group. Down a group, the valence electron occupies new shells, the distance between the nucleus and the valence electron increases. So, the nucleat forces of attraction on valence electron decreases and hence ionisation energy also decreases. down a group.

Metals have low ionisation energy and highly electropositive. As the ionisation energy of the metals decreases down the group, the electro positive character of elements increases. The basic character of hydroxides of alkaline earth metals increases down the group

Electronic Configuration: (n - 1)d² ns²
for n = 5, the electronic configuration is,
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 4d² 5s²
Atomic number: 40
4^th group 5^th period (d block element) = Zirconium

Cl(g) + e^- ⟶Cl^-(g)    AH = 348 kJ mol^-1
For one mole (35.5g) 348 kJ is released.
\(\therefore \text { For } 17.5 \mathrm{~g} \text { chlorine, } \frac{348 \mathrm{~kJ}}{35.5 \mathrm{~\not g}} \times 17.75 \mathrm{~\not g} \text { energy leased. }\)
∴ The amount of energy released =\(\frac { 348 }{ 2 } \) =174 kJ

The sun and other stars are composed mainly of 85-95% hydrogen which generates their energy by nuclear fusion of hydrogen nuclei into helium.

Gas hydrates are a kind of inclusion compounds, where gas molecules are trapped in the crystal lattice having voids of right size, without being chemically bonded. An interesting hydrate is that of the hydronium ion (H₃O⁺) in the gas-phase, similar to methane hydrate. Each water molecule is bonded to three others in the dodecahedron.

Hydrogen has electronic configuration 1s¹. So it resembles ns¹ general valence shell configuration of alkali metals.
Its similarity with alkali metals are:
1. Form uni positive (H⁺) ions like Na⁺, K⁺, Cs⁺
2. Form halides, (HX), oxides (H₂O) peroxides (H₂O₂) and sulphides (H₂S) like (Nax, Na2O, Na₂O₂, Na₂S)
3. Acts as reducing agent. 
4. Has +1 oxidation state.
5. If it is placed in the halogen family it should be the most electro negative element which is not true.

a) HCl is a gas. It is an electron rich covalent hydride. It is a covalent molecule with strong covalent bond. However the forces between the molecules are not as strong.
b) NaH is a solid. It is an ionic hydride of group I sodium metal. It is an insoluble hydride

(i) WO₃ + 3H₂ ➝ W + 3H₂O
(ii) H₂ + Cl₂ ➝ 2HCl.

Interstitial hydrides have metallic bonding. These are non - stoichiometric; their composition varies with temperature and pressure. The crystal lattice expands due to the inclusion of dihydrogen and there is distortion of crystal lattice. So they have lower density than the parent metal. Recent studies have shown that except the hydrides of Ni, Pd, Ce & Ac, other hydride are different from the parent metals.

Ionisation energy is the amount of energy required to remove the electron from the ground state (EI) to excited state (E^∞)
E₁=-2.18 x 10^-18 J; E_∞=0
ΔE=E_∞-E₁
=0-(-2.18 x 10^-18 J)=2.18 x 10^-18 J
I.E per hydrogen atom = 2.18 x 10^-18 J
I.E per mole of H-atom =2.18 x 10^-18 J x 6.023 x 10²³
=13.13 x 10⁵ J mol^-1

(i) Electronic configuration is the arrangement of electrons in an atom. The outermost electron shell is often referred to as the "valence shell"determines the chemical properties.
(ii) Ionization energy and electron affinity is the amount of energy released or required in pulling out or adding an electron to a neutral atom. So both depend on electronic configuration of the element.

Z = 118; [₈₆Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶
In the periodic table the element with Z = 118 is located in p-block,
Period no = 7 (as n = 7 for valence shell)
Group no. = 18 (group no = 10+ ns electrons + np electrons) (n - outer most shell)

(i) According to aufbau's principle 5th period has nine orbital (one 5s, five 4d and three 6p) to be filled.
(ii) Nine orbitals can accommodate a maximum of 18 electrons. Hence fifth period of the periodic table should has 18 elements from rubidium (2 = 37) to Xenon (Z = 54).

In the periodic table vertical columns are called groups.
Elements in the same vertical column possess similar number of electrons in the outer orbitals.
∴ Elements a and c belongs to group 18
Elements b and d belongs to group 13

(i) 2KMnO₄ + 3H₂O₂ ➝ 2MnO₂ + 2KOH + 2H₂O + 3O₂ (Redox reaction)
(ii) CrCl₃ + 6H₂O ➝ [Cr Cl₂ (H₂O)CI. 2H₂O [Hydration reaction]
(iii) CaO + H₂O ➝ Ca(OH)₂ (Hydration reaction)

When compounds containing hydrogen are treated with D₂O, hydrogen undergoes an exchange for deuterium
\( 2 \mathrm{NaOH}+\mathrm{D}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOD}+\mathrm{HOD} \)
\(\mathrm{HCl}+\mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{DCl}+\mathrm{HOD} \)
\(\mathrm{NH}_{4} \mathrm{Cl}+4 \mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{ND}_{4} \mathrm{Cl}+4 \mathrm{HOD}\)
These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound.
For example, when D₂O is treated with of hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that, it is a monobasic acid.
\(\mathrm{H}_{3} \mathrm{PO}_{2}+\mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{DPO}_{2}+\mathrm{HDO}\)

1. Deuterium is used in heavy water moderated fission reactors, usually as liquid D₂O to slow neutrons
2. To prepare D₂O (heavy water)
3. To prepare deuterated halides
4. To prepare compounds like deutero methane, deutero ammonia etc..
5. To manufacture deuterium lamps with quartz or UV glass bulb.

(i) Sodium (Z = 11)
(ii) krypton (Z = 36)
(iii) Bromine (Z = 35)
(iv) Scandium (Z = 21)

Water (H₂O) is polar because of the bent shape of the molecule.

The shape means most of the negative charge from the oxygen is on one side of the molecule and the positive charge of the hydrogen atoms is on the other side of the molecule. This is an example of polar covalent chemical bonding.

On moving down the group, electronegativity decreases because atomic size increases. Iodine is the largest, therefore it is least electronegative.

Lithium aluminium hydride (Li AlH₄) and sodium borohydride (NaBH₄) are used as a reducing agent in organic chemistry. They are prepared as follows:
\(4LiH+AlCl_3 \rightarrow Li[AlH_4]+3LiCl \)
\(4NaH+B(OCH_3)_3\rightarrow Na[BH_4]+3CH_3ONa\)

These are hydrides composed of an electropositive metal, generally, an alkali or alkalineearthmetal, except beryllium and magnesium, formed by transfer of electrons from metal to hydrogen atoms. They can be prepared by the reaction of elements at about 400⁰ C. These are salt-like, high-melting white crystalline solids having hydride ions (H^-) and metal cations Mⁿ⁺ where 'n' is the valency of the metal.
\(2Li+H_2\rightarrow2LiH\)

(i) Fluorine is expected to have higher first IE than chlorine because, it is smaller in size than chlorine and IE decreases down the group.
(ii) Chlorine is expected to have higher IE than sulphur, because it has a smaller size than sulphur. IE decreases along the period.
(iii) Neon is expected to have higher IE because it has completely filled orbitals from which removal of electron is difficult.
(iv) Argan is expected to have higher IE, because it has completely filled orbitals.

In sixth period, 32 elements are present. This period starts with the filling of 6^th energy shell, n = 6.There are sixteen orbitals (one 6s, seven 4f, five 4d and three 6p) to be filled.These sixteen orbitals can accommodate 32 (16\(\times\)2 = 32) electrons. Hence, 32 elements are present in sixth period.

Variation in period: The number of valence electrons increases from 1 to 8 on moving across a period. The valency of the elements with respect to hydrogen and chlorine increases from 1 to 4 and then decreases from 4 to zero.
Variation in group: On moving down a group, the number of valence electrons remains same. All the elements in a group exhibit same valency. For example, all the elements of group 1 have valency equal to 1.
Hence, valency is a periodic property.

(i) Metallic hydrides are obtained by hydrogenation of metals and alloys in which hydrogen occupies the interstitial sites (voids). Hence, they are called interstitial hydrides.
(ii) The hydrides show properties similar to parent metals and hence they are also known as metallic hydrides.
(iii) They are mostly non-stoichiometric with variable composition (TiH_1.5-1.8 and PdH_0.6-0.8)
(iv) Some are relatively light, inexpensive and thermally unstable which makes them useful for hydrogen storage applications. Example, TiH₂, ZrH₂, ZnH₂.

(i) Ionic radius of uni-univalent crystal can be calculated using Pauling's method from the inter ionic distance between the nuclei of the cation and anion.
(ii) Pauling assumed that ions present in a crystal lattice are perfect spheres, and they are in contact with each other therefore,
d=rC⁺ + r_A^-   ...(1)
Where d is the distance between the centre of the nucleus of cation C+ and anion A-and rC⁺, rA^- are the radius of the cation and anion respectively.
(iii) Pauling also assumed that the radius of the ion having noble gas electronic configuration is inversely proportional to. the effective nuclear charge.
\({ r }_{ C }^{ + }\alpha \frac { 1 }{ ({ Z }_{ eff }){ C }^{ + } } \) ....(2) and
\({ r }_{ A }^{ - }\alpha \frac { 1 }{ ({ Z }_{ eff }){ A }^{ - } } \)...(3)
Where Z_eff is the effective nuclear charge and Z_eff= Z - S
Dividing the equation 1 by 3
\(\frac { { r }_{ C }^{ + } }{ { r }_{ A }^{ - } } =\frac { ({ Z }_{ eff }){ A }^{ - } }{ ({ Z }_{ eff }){ C }^{ + } } \) ...(4)
On solving equation and (1) and (4) the values of r_C+ and r_A^- can be obtained.

On moving diagonally across the periodic table, the second and third period elements show certain similarities. It is quite pronounced in the following pair of elements.

The similarity in properties existing between the diagonally placed elements is called diagonal relationship.

The electronic configuration of Sodium (Z = 11) Is²2s²2p⁶3s¹.
Magnesium (Z = 12) 1s²2s²2p⁶3s²
Magnesium atom has a smaller radius and higher nuclear charge than a sodium atom, thus more energy will be required to remove the electron from the same orbital (3s), making the first ionisation energy of magnesium higher than that of sodium.
However, the second ionization enthalpy of sodium is higher than that of magnesium. This is because after losing 1 electron, sodium attains the stable noble gas configuration of neon (1s²2s²2p⁶). On the other hand, magnesium, after losing 1 electron still has one electron in the 3s-orbital(1s²2s²2p⁶3s¹). In order to attain the stable noble gas configuration, Thus, the energy required to remove the second electron in case of sodium is much higher than that required in case of magnesium. Hence, the second ionization enthalpy of sodium is higher than that of magnesium.

(i) By treatment with catalyst like Pt or Fe.
(ii) By passing an electric discharge
(iii) By heating to 800°C or more.
(iv) By mixing with paramagnetic molecules like O₂,NO,NO₂·
(v) By mixing with nascent hydrogen or atomic hydrogen.

The element which occupies group number (16) and period number (2) is oxygen. (B) is D₂O which is used as a moderator in nuclear reactions.
So (A) must be deuterium, which is an isotope of hydrogen
    \(2\underset { (A) }{ { D }_{ 2 } } +{ O }_{ 2 }\rightarrow 2\underset { (B) }{ { D }_{ 2 }O } \)
So (B) is D₂O
(A) adds to (C) as follows :
\(3 \mathrm{D}_{2}+\mathrm{C}_{3} \mathrm{H}_{6} \rightarrow \mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2}\)
So (D) is 1,2 - dideutero propane. 

Screening effect: The repulsive force between the inner shell electrons and the valence electrons leads to a decrease in the electrostatic attractive forces acting on the valence electrons by the nucleus. Thus, the inner shell electrons act as a shield between the nucleus and the valence electrons. This effect is called shielding effect.
Pauling's scale: Pauling, he assigned arbitrary value of electronegativities for hydrogen and fluorine as 2.2 and 4.0 respectively. Based on this the electronegativity values for other elements can be calculated using the following expression.
\(({ X }_{ A }-{ X }_{ B })=0.182\sqrt { E_{ AB } } -({ E }_{ AA }*{ E }_{ BB })^{ 1/2 }\)
Where E_AB' EAA and E_BB are the bond dissociation energies of AB, A₂ and B₂ molecules respectively. The electronegativity of any given element is not a constant and its value depends on the element to which it is covalently bound. The electronegativity values play an important role in predicting the nature of the bond.

(i) Variation of Electronegativity in a period: The electronegativity generally increases across a period from left to right. The atomic radius decreases in a period, as the attraction between the valence electron and the nucleus increases. Hence the tendency to attract shared pair of electrons increases. Therefore, electronegativity also increases in a period.
(ii) Variation of Electronegativity in a group: The electronegativity generally decreases down a group. As we move down a group the atomic radius increases and the nuclear attractive force on the valence electron decreases. Hence, the electronegativity decreases.

(i) According to the recommendation of IUPAC, the groups are numbered from 1 to 181A.
(ii) There are 18 vertical columns which constitute 18 groups or families.
(iii) There are 7 horizontal rows of the periodic table known as periods.
(iv) The first period contains two elements. One present in first group and the other in 18th group.
(v) Second and third periods contain 8 elements in each.
(vi) Fourth and fifth periods are completely filled as they contain 18 elements in each.
(vii) The sixth period contains 32 elements. The seventh is incomplete. Fourteen elements of both sixth and seventh periods are placed in separate panels at the bottom of the table.
(viii) This periodic table is important and useful because we can predict the properties of any element using periodic trend.

(i) Al₄C₃ + 12D₂O ➝ 4Al(OD)₃ + 3CD₄
(ii) CaC₂ + 2D₂O ➝ Ca(OD)₂ + C₂H₂
(iii)Mg₃N₂ + 6D₂O ➝ 3Mg(OD)₂ + 2ND₃
(iv) Ca₃P₂ + 6D₂O ➝ 3Ca(OD)₂ + 2PD₃

(i) Hard water is harmful for boilers, because of deposition of salts in the form of scale. This reduces the efficiency of the boilers.
(ii) Hard water forms scum/precipitate with soap. Soap containing sodium stearate
(C₁₇H₃₅COONa) reacts with hard water to precipitate out Ca/Mg stearate.
2C₁₇H₃₅COONa_(aq)+M²⁺_(aq)➝ (C₁₇H₃₅COO)2M↓2Na⁺_(aq); M is Ca/Mg
It is, therefore, unsuitable for laundry.

(a) Treatment with sodium carbonate: It can be removed by adding washing soda, which reacts with these metal (M = Ca or Mg) chlorides and sulphates in hard water to form insoluble carbonates.
MCl_2(aq) + Na₂CO_3(aq) ➝ MCO_3(s) + 2NaCl_(aq)
MSO_4(aq) + Na2CO_3(aq)➝MCO_3(s) + Na2SO_4(s)
(b) Ion exchange method : Another way to soften water is through a process called ion-exchange. That is, hardness can be removed by passing through an ion-exchange bed like zeolites or resin (polymer) containing column.
(i) Eg : Zeolites are hydrated sodium alumino-silicates with a general formula
NaO.Al₂O₃. xSiO₂.yH₂O (x = 2 - 10, y = 2 - 6).
(ii) Zeolites have porous structure in which the monovalent sodium ions are loosely 
held and can be exchanged with hardness producing divalent metal ions (M = Ca or
Mg) in water.
(iii) Na₂-Z_(s) + M²⁺_(aq) ➝ M-Z_(s) + 2Na⁺_(aq)
(iv) The complex structure can conveniently be represented as Na₂Z with sodium as exchangeable cations.
(v) When exhausted, the materials can be regenerated by treating with aqueous
sodium chloride. Metal ions Ca²⁺ and Mg²⁺ in the zeolite (or resin) are released
and they get replenished with sodium ions.
(vi) M-Z_(s) + 2NaCl_(aq) ➝ Na₂ - Z_(s) + MCl_2(aq)

Characteristics of p-block elements:
(i) General electronic configuration is ns²np^1-6 (Group 13 to 18).
(ii) They are generally non-metals but some metals and metalloids are present.

• Non-metallic character increases from left to right along period.
• Metallic character increases on moving down a group.

(iii) They have high electron affinity and ionization enthalpy.

• Both the properties increase along the period.
• Both the properties decrease on moving down the group.

(iv) Oxidising character.

• Increases on moving from left to right along a period.
• Reducing character increases from top to bottom in a group.

(v) s-block and p-block elements are collectively called representative elements.

(i) Element of atomic number 15 belong to p-block and forms covalent hydride (PH₃).
(ii) Element of atomic number 19 belong to s-block and forms saline hydride (KH).
(iii) Element of atomic no. 23 belong to d-block and forms interstitial hydride VH_1.6 which is a nonstoichiometric hydride.
(iv) Element of atomic number 44 belong to d-block it is ruthenium. It does not form any hydride.
(v) Only ionic hydride KH reacts violently with water and involved in hydrogen gas production.
KH + H₂O ➝ KOH + H₂ ↑

(i) An element (A) belonging to group number 1 and period number 3 is sodium (A).
(ii) Sodium reacts with hydrogen to form sodium hydride (B)
2Na + H2 ➝ 2NaH
   (A)  (B)       (C)
(iii) Sodium reacts with water to form sodium hydroxide (C).
2Na + 2H₂O ➝ 2NaOH + H₂
  (A)                    (C)

2H₂ + O₂ ➝ 2H₂O
The isotope of hydrogen which has one proton and one neutron in its nucleus is Deuterium.
2D₂ + O₂ ➝ 2D₂O
The product is heavy water (Deuterium oxide). H₂O and D₂O have same chemical properties but the
reaction velocity of D2O is slightly less due to the difference in the mass number of the isotopes known as isotopic effect. Deuterium is heavier than protium so reacts slowly.

The first element of the periodic table is hydrogen.
2H₂ + O₂ ➝ 2H₂O
The compound formed is water.
Auto - ionization or auto-protolysis of water proceeds as follows
\(\underset { Acid }{ { H }_{ 2 }O(l) } +\underset { Base }{ { H }_{ 2 }O(l) } \longleftrightarrow \underset { Conjugate\quad Acid }{ { H }_{ 3 }O } +(aq)+\underset { Conjugate\quad Acid }{ OH-(aq) } \)

d = r_Na +r_F-
i.e r_Na + r_F- = 231 pm
We know that
\(\frac { { r }_{ Na }+ }{ { r }_{ F- } } =\frac { (Zeff)F- }{ (Zeff){ Na }^{ + } } \)
(Z_eff)_F- = Z -Z
= 9 - 4.15
=4.85
(Z_eff)Na⁺ = 11 - 4.15
=6.85
\(\frac { r{ Na }^{ + } }{ r{ F }^{ - } } =\frac { 4.85 }{ 6.85 } \)
\(\Rightarrow\) r_Na+ = 0.71 x r_F-
substituting (2) in (1)
(1) \(\Rightarrow\) 0.71 r_F- + r_F- = 231 pm
1.71 r_F- = 231 pm
\(r{ F }^{ - }=\frac { 231 }{ 1.71 } =135.1\) pm
Substituting the value of r_F- in equation (1)
r_Na+ 135.1 =231
r_Na+ = 95.9 pm

(a) Mendeleev's periodic law:
Mendeleev's periodic law states that the physical and chemical properties of elements are a periodic function of their atomic weights.
(b) Merits of Mendeleev's periodic table:
(i) The comparative studies of elements were made easier.
(ii) The table shows the relationship'in properties of elements in a group.
(iii) The table helped to correct the atomic masses of some elements later on. At the time of Mendeleev, the atomic weight of Au and Pt were known as 196.2 and 196.7 respectively. However, Mendeleev placed Au (196.2) after Pt (196.7) saying that atomic weight of Au is incorrect, which was later on found to be 197.
(iv) At the time of Mendeleev, about 70 elements were known and thus blank spaces were left for unknown elements which helped further discoveries.
(v) Both Gallium (Ga) in III group and Germanium. (Ge) in IV group, were unknown at that time by Mendeleev predicted their existence and properties. He referred the predicted elements as eka-aluminium and eka-silicon. After discovery of the actual elements, their properties were found to match closely to those predicted by Mendeleev.

(i) The long form of periodic table of the elements is constructed on the basis of modem periodic law. The arrangement resulted in repeating electronic configurations of atoms at regular intervals.
(ii) The elements placed in horizontal rows are called periods and in vertical columns are called groups.
(iii) According to IUPAC, the groups are numbered from 1 to 18.
(iv) There are 18 vertical columns which constitute 18 groups or families. All the members of a particular group have similar outer shell electronic configuration.
(v) There are 7 horizontal rows called periods.

The elements are shown in the above table along with its atomic number.
(vi) The atomic number also indicates the number of electrons in the atoms of an element.
(vii) This periodic table is important and useful because we can predict the properties of any element using periodic trend, even though that element may be unfamiliar to us.

Hydrogen has a tendency to react with reactive metals like lithium, sodium to give corresponding hydrides.
2 Li + H₂ ⟶ 2 LiH
2 Na + H₂ ⟶  2 NaH
These hydrides are used as reducing agents in synthetic organic chemistry. It is also used to prepare important hydrides such as lithium aluminium hydride and sodium boro hydride (organic reducing agents).
4 LiH + AICl₃   ⟶ Li[AlH₄] + 3 LiCI
4 NaH + B(OCH₃)₃ ⟶ Na[BH₄] + 3CH₃ONa