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. Their relation is \(A = Z + n\), where \(n\) is the number of neutrons.

Isotopes are atoms of the same element having the same atomic number but different mass numbers. Two isotopes of uranium are \(^{235}\mathrm{U}\) and \(^{238}\mathrm{U}\).

According to Thomson's plum pudding model, the atom is a positively charged sphere in which negatively charged electrons are embedded like plums in a pudding. The total positive and negative charges balance, so the atom is electrically neutral.

Rutherford's model could not explain atomic stability because an electron revolving around the nucleus is an accelerating charged particle and should continuously radiate energy. It would lose energy and spiral into the nucleus, so the atom should collapse.

Wavelength \(\lambda\) is the distance between two successive crests or troughs of a wave. Frequency \(\nu\) is the number of wave cycles passing a point in one second. They are related by \(c = \nu \lambda\).

Using \(E = h\nu\): \(E = 6.626 \times 10^{-34} \times 1.2 \times 10^{10} = 7.95 \times 10^{-24}\) J approximately.

A line spectrum consists of distinct bright lines of specific wavelengths. A continuous spectrum contains all wavelengths in a region without gaps. Thus a line spectrum is discontinuous, while a continuous spectrum is unbroken.

The five hydrogen emission series are Lyman, Balmer, Paschen, Brackett, and Pfund.

Any two postulates: electrons move only in certain permitted stationary circular orbits; electrons do not radiate energy while moving in these orbits; energy is emitted or absorbed only when an electron jumps from one orbit to another; angular momentum is quantised, \(mvr = nh/2\pi\).

A chemical bond is the force of attraction that holds atoms together in a molecule or compound. Atoms combine to attain lower energy and a stable noble gas-like electronic configuration.

Sodium atom loses one electron to form \(Na^+\): \(Na \rightarrow Na^+ + e^-\). Chlorine atom gains one electron to form \(Cl^-\): \(Cl + e^- \rightarrow Cl^-\). These oppositely charged ions then attract each other.

Lattice energy is the energy released when one mole of an ionic solid is formed from its gaseous ions, or equivalently the energy required to separate the solid into gaseous ions. Higher lattice energy increases the stability of ionic compounds.

An alpha particle is the nucleus of helium containing 2 protons and 2 neutrons. Its charge is \(+2e\).

Wave number \(\overline{\nu}\) is the number of waves per unit length, or the reciprocal of wavelength. Relation: \(\overline{\nu} = 1/\lambda\).

A quantum is the smallest discrete amount of energy that can be emitted or absorbed. A photon is the quantum of electromagnetic radiation.

Valence electrons are the electrons present in the outermost shell and they take part in bond formation. Nitrogen has 5 valence electrons.

Lewis dot symbols: Lithium is \(\mathrm{Li}\cdot\); Oxygen is written with six dots around \(\mathrm{O}\).

A covalent bond is formed by sharing of one or more pairs of electrons between atoms. Example: \(H_2\) or \(Cl_2\).

A lone pair is a pair of valence electrons not shared in bonding. In ammonia, nitrogen has one lone pair.

Two examples are \(NH_4^+\) and the adduct formed by \(BF_3\) with \(NH_3\).

Dipole moment is the product of magnitude of charge and separation between the centres of positive and negative charges: \(\mu = Qr\). Its SI unit is coulomb-metre.

In Rutherford's gold foil experiment, a beam of alpha particles was directed at a very thin gold foil and their scattering was observed on a fluorescent screen. Most particles passed straight through, some were deflected by small angles, and a very few rebounded. Conclusions: most of the atom is empty space; nearly all the mass and positive charge are concentrated in a very small central nucleus; electrons occupy the space around the nucleus.

The Born-Haber cycle explains the stepwise formation of NaCl using Hess's law. Sodium is sublimed, chlorine molecule is dissociated, sodium is ionized, chlorine gains an electron, and finally gaseous ions combine to form solid NaCl releasing lattice energy. It shows that large lattice energy helps make the ionic crystal stable.

Ionic compounds generally have high melting and boiling points, conduct electricity in molten or aqueous state, and are usually soluble in water. Covalent compounds generally have lower melting and boiling points, are poor conductors, and are usually insoluble in water but soluble in non-polar solvents.

VSEPR theory states that electron pairs around the central atom arrange themselves to minimize repulsion. In \(H_2O\), oxygen has two bond pairs and two lone pairs. The basic electron-pair arrangement is tetrahedral, but because lone pair repulsion is greater, the molecular shape becomes bent or V-shaped.

Bohr proposed that electrons revolve around the nucleus only in certain permitted stationary orbits of fixed energy and do not radiate energy while in those orbits. Radiation is emitted or absorbed only when an electron jumps between energy levels, with \(\Delta E = h\nu\). Angular momentum is quantised: \(mvr = nh/2\pi\). Because electrons can fall from higher to lower levels in different ways, different spectral series are produced. Transitions to \(n=1\) give the Lyman series, to \(n=2\) the Balmer series, to \(n=3\) the Paschen series, to \(n=4\) the Brackett series, and to \(n=5\) the Pfund series.

Hybridization is the mixing of atomic orbitals of comparable energy to form equivalent hybrid orbitals. In \(sp\) hybridization, one \(s\) and one \(p\) orbital mix to form two \(sp\) orbitals arranged linearly at \(180^\circ\), as in BeCl\(_2\) or acetylene carbon. In \(sp^2\), one \(s\) and two \(p\) orbitals mix to form three coplanar orbitals with trigonal planar geometry at \(120^\circ\), as in BF\(_3\) or ethene carbon. In \(sp^3\), one \(s\) and three \(p\) orbitals mix to form four equivalent orbitals directed tetrahedrally at \(109.5^\circ\), as in CH\(_4\).