NATIONAL INSTITUTE OF OPEN SCHOOLING

Atomic number (Z) is the number of protons in the nucleus. Mass number (A) is the total number of protons and neutrons. Relation: A = Z + n, where n is the number of neutrons.

Isotopes have the same atomic number but different mass numbers, for example ³⁵Cl and ³⁷Cl. Isobars have the same mass number but different atomic numbers, for example ⁴⁰Ar and ⁴⁰Ca.

Any two observations: most alpha particles passed straight through the foil; some were deflected through small angles; a very few were deflected through large angles or rebounded.

Any two postulates: electrons revolve only in certain permitted stationary orbits; electrons do not radiate energy while in these orbits; radiation is emitted or absorbed only during transition between energy levels; angular momentum is quantised.

Wavelength (λ) is the distance between two successive crests or troughs. Frequency (ν) is the number of wave cycles per second. Relation: c = νλ.

Using E = hν, E = 6.626 × 10^-34 × 6.0 × 10¹⁴ = 3.98 × 10^-19 J approximately.

An ionic bond is the electrostatic force of attraction between oppositely charged ions. Na → Na⁺ + e^-; Cl + e^- → Cl^-; then Na⁺ + Cl^- → NaCl.

Any two characteristics: ionic compounds are crystalline solids; they have high melting and boiling points; they conduct electricity in molten or aqueous state; they are generally soluble in water.

A covalent bond is formed by sharing of electron pairs between atoms. A bond pair is a shared pair of electrons, whereas a lone pair is a pair of electrons not involved in bonding.

Dipole moment is the product of the magnitude of charge and the distance between the centres of positive and negative charges. Expression: μ = Qr.

VSEPR theory states that electron pairs around the central atom arrange themselves to minimize repulsion. NH₃ has a trigonal pyramidal shape.

Hybridisation is the mixing of atomic orbitals of comparable energy to form equivalent hybrid orbitals. BeCl₂: sp, linear. CH₄: sp³, tetrahedral.

Valence electrons are the electrons in the outermost shell which take part in bond formation. Oxygen has 6 valence electrons.

The three isotopes of hydrogen are protium (¹H), deuterium (²H) and tritium (³H).

E = hν, where E is energy, h is Planck’s constant, and ν is frequency.

Wave number is the number of waves per unit length, or the reciprocal of wavelength. Relation: ṽ = 1/λ.

Na → Na⁺ + e^- and Cl + e^- → Cl^-.

Example of ionic compound: NaCl. Example of covalent compound: H₂ or Cl₂.

A lone pair is a pair of valence electrons not shared in bond formation. Example: nitrogen in NH₃ has one lone pair.

BF₃ is trigonal planar and CH₄ is tetrahedral.

A coordinate covalent bond is a bond in which both shared electrons are donated by one atom. Example: NH₄⁺.

In Rutherford’s experiment, alpha particles were bombarded on a thin gold foil. Most particles passed straight through, some were slightly deflected, and very few rebounded. Hence most of the atom is empty space and nearly all the positive charge and mass are concentrated in a small dense nucleus.

Rutherford’s model could not explain stability because a revolving electron is an accelerating charged particle and should continuously lose energy, finally collapsing into the nucleus. Bohr improved this by proposing stationary permitted orbits in which electrons do not radiate energy, and radiation is emitted or absorbed only during transitions between orbits.

Any three differences: ionic compounds are formed by electron transfer, while covalent compounds are formed by sharing of electrons; ionic compounds usually have high melting and boiling points, covalent compounds generally have lower melting and boiling points; ionic compounds conduct electricity in molten or aqueous state, covalent compounds are generally poor conductors.

In H₂O, oxygen has four electron pairs around it, two bond pairs and two lone pairs. The electron-pair arrangement is tetrahedral, but lone pair repulsion is stronger than bond pair repulsion. Therefore the bond pairs are pushed closer together and the molecular shape becomes bent.

sp hybridisation involves mixing of one s and one p orbital to form two equivalent sp orbitals with linear geometry, 180°, for example BeCl₂. sp² hybridisation involves one s and two p orbitals to form three equivalent orbitals with trigonal planar geometry, 120°, for example BF₃. sp³ hybridisation involves one s and three p orbitals to form four equivalent orbitals with tetrahedral geometry, 109.5°, for example CH₄.

Sodium has electronic configuration 2,8,1 and chlorine has 2,8,7. Sodium loses one electron: Na → Na⁺ + e^-. Chlorine gains one electron: Cl + e^- → Cl^-. Thus Na⁺ and Cl^- are formed and attract each other electrostatically to form NaCl. High lattice energy means a large amount of energy is released when gaseous ions form the ionic crystal, so the crystal becomes more stable.