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 0⁰C, ice and liquid water will be in equilibrium and will coexist. Hence no freezing or melting will occur.

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.

a) Electron-precise hydrides:
These have required number of electron to represent their conventional d lewis structure. All the elements of carbon group (14) form such hydrides.
Eg CH₄, C₂H₆, SiH₄, GeH₄
b) Electron - deficient hydrides:
There act as Lewis acids (ie) electron acceptors. The elements from group (13) form such hydrides.
Eg: B₂H₆
c) Electron - rich hydrides:
There have excess electron, which are present as lone pairs. Elements of group 15-17 form such hydrides. They behave as Lewis bases (ie) electron donors.
Eg: NH₃ ,H₂O, HF

Lanthanides: [₅₄Xe] 4f^1-14 5d¹ 6s²
Actinides : [₈₆Rn] 5f^0-146d^0-2 7s²

(i) Used in emergency exit signs.
(ii) Used in illumination of wrist watches in place of radium.

Halogens react with water to give an acidic solution. For example, chlorine forms hydrochloric acid and hypochlorous acid.
Cl₂(g) + H₂O(l) ➝ HCl (aq) + HOCl(aq)

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.

Prolonged electrolysis of heavy water produces heavy hydrogen. The electrolysis is continueduntil the resulting solution becomes enriched in heavy water. Further electrolysis of the heavy water gives deuterium.
\(2D_2O\xrightarrow{electrolysis}2D_2+O_2\)

It is artificially prepared by bombarding lithium with slow neutrons in a nuclear fission reactor. The nuclear transmutation reaction for this process is as follows.
\(_{ 3 }^{ 6 }{ Li+ }_{ 0 }^{ 1 }{ n }\rightarrow _{ 2 }^{ 4 }{ He }+_{ 1 }^{ 3 }{ T }\)

Atomic radius tends to decrease in a period. Along a period, the valence electrons are added to the same shell. The simultaneous addition of protons to the nucleus, increases the nuclear charge, as well as the electrostatic attractive force between the valence electrons and the nucleus. Therefore atomic radius decreases along a period.

The valence of an atom primarily depends on the number of electrons in the valence shell. As the number of valence electrons remains same for the elements in same group, the maximum valence also remains the same. However, in a period the number of valence electrons increases, hence the valence also increases.

 The noble gases having completely filled electronic configuration neither accept nor lose their electron readily and hence they are chemically inert in nature

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

The nuclear charge in Mg⁺² is more than Na+ and therefore electrons are drawn more closely, and hence size decreases

Noble gases: Ioniation energy ranging from 2372 KJmol^-1 to 1037 kJ mol^-1.
For element X, the IE₁ value is in the range of noble gas, moreover for this element both IE₁ and IE₂ are higher and hence X is the noble gas.
For Y, the first ionisation energy is low and second ionisation energy is very high and hence Y is most reactive metal. For Z, both IE₁ and IE₂ are higher and hence it is least reactive.

Molecular hydrogen have ortho and para form in which the nuclear spins are aligned or opposed, respectively

The most reactive alkali metals decompose water in the cold with the evolution of hydrogen and leaving an alkali solution
2Na_(s) + 2H₂O₍₁₎ ⟶ 2NaOH_(aq) + H_2(g)

The oxidising property of hydrogen peroxide and harmless nature of its products such as water and oxygen, leads to oxidation of pollutants in water and act as a mild antiseptic.

The Law of octaves states that, "when elements are arranged in the order of increasing atomic weights, the properties of the eighth element are a repetition of the properties of the first element".

In, hydrogen fluoride (HF), for example, one molecule is strongly attracted to the fluorine on its neighboring hydrogen. In both liquid and solid, hydrogen fluoride forms long hydrogen bonded zig-zag chains as a consequence of the orientation of the lone pairs on the fluorine atoms.

The ionic radius of an ion is the distance between the centre of the ion and the outermost point of its electron cloud.

The energy required to remove the most loosely held electron from an isolated gaseous atom is called ionization energy.
M_(g)+ energy ⟶ M⁺_(g) + electron
The unit of ionization energy is KJ mole^-1

Be (Z = 4) 1s² 2s². It has completely filled valence electrons, which requires high IE₁.
B (Z = 5) 1s² 2s².2p¹. It has incompletely filled valence electrons, which requires comparatively less IE₁.Hence I.E₁ Be > I.E₁B.

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.

Water is amphoteric in nature and it behaves both as an acid as well as base. With acids stronger than itself (e.g., H₂S) it behaves as a base and with bases stronger than itself (e.g., NH₃) it acts as an acid.
(i) As a base: H₂O₍₁₎ + H₂S_(aq)⟶H₃O_(aq) + HS^-_(aq)
(ii) As an acid: H₂O₍₁₎ + NH_3(aq)⟶OH^-_(aq) + NH₄⁺_(aq)

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