Unit – 1

Atomic And Molecular Structure

Q1) Explain the causes for failure of Newtonian and Maxwell mechanics?

A1) The existence of an unphysical solution of Maxwell's equations for a plane-wave field exemplifies that electromagnetic potentials convey more physical information than the electric and magnetic fields derivable from them. Electromagnetic forces depend only on the fields, which means that Newtonian mechanics, which is based on forces, can produce improper results.

The only drawback of Newtonian mechanics, which can be ignored at small relative speeds. The drawback is that, it considers speed of light and gravitational effect to be infinite.

According to Maxwell's equations, speed of light is defined as

But velocity is relative. There is no such thing as absolute velocity. And the formula doesn't specify, speed of light is 299,795,638 m/s.

This is because, the speed of light is independent of velocity

This is possible only if two objects with different velocity will experience different time for each other (synchronous clocks will no longer remain synchronized).

Newtonian theory considers time to be constant for all frames of reference.

Black body Radiation: Classical physics predicted that hot objects would instantly radiate away all their heat into electromagnetic waves. The calculation, which was based on Maxwell's equations and Statistical Mechanics, showed that the radiation rate resulted to infinity as the EM wavelength to zero, Plank solved the problem by postulating that EM energy was emitted in quanta with E=hv.

The Photoelectric effect : When light was used to knock electrons out of solids, the results were completely different than expected from Maxwell's equations. The measurements were easy to explain (for Einstein) if light is made up of particles with the energies Plank postulated.

Q2) Briefly explain the dual nature of electron and electromagnetic radiation?

A2) An electromagnetic radiation is a particle as well as a wave. There are only two ways to transfer energy from one place to another place.

Through a particle, and through a wave, the photoelectric effect could be explained considering that radiations consist of small packets of energy called quanta. These packets of energy can be treated as particles. On the other hand, radiations exhibit a phenomenon of interference and diffraction which indicated that they possess wave nature. So, it may be concluded that electromagnetic radiations possess dual nature, (Particle nature and wave nature).

Q3) Write two properties of molecular orbitals?

A3) A molecular orbital (MO) can be used to represent the regions in a molecule where an electron occupying that orbital is found. Molecular orbitals are obtained from the combination of atomic orbitals, which predict the location of an electron in an atom. A molecular orbital can specify the electron configuration of a molecule: the spatial distribution and energy of one (or one pair of) electron(s). A molecular orbital describes the behaviour of one electron in the electric field generated by the nuclei and some average distribution of the other electrons. In the case of two electrons occupying the same orbital.

Q4) Define Planck’s theory with an equation.

A4) According to Planck’s quantum theory, the smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum. The energy of electromagnetic radiation absorbed or emitted is directly proportional to the frequency of the radiation.

Planck postulated that the energy of an   electromagnetic radiation is proportional to the frequency of the radiation-the higher the frequency the greater the energy. This is written as the formula below and called as Planck’s Equation:

h: Planck’s constant has a value of 6.626x10-34 J-s.

E=he

Planck’s quantum theory 

According to Planck’s quantum theory,

1.Different atoms and molecules can emit or absorb energy in discrete quantities only. The smallest or minute amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as its measure or quantum.

2.The energy of the radiation absorbed or emitted is directly proportional to the frequency of the radiation.

Meanwhile, the energy of radiation is expressed in terms of frequency as,

E=he

Where,

E= Energy of the radiation

h= Planck’s constant (6.626x10-34 J-s)

v= Frequency of radiation

Planck has also concluded that these were only an aspect of the processes of absorption and emission of radiation.

Q5) Mention the failure of Heisenberg uncertainty principle.

A5) Uncertainty principle, also called Heisenberg uncertainty principle or indeterminacy principle, statement, articulated (1927) by the German physicist Werner Heisenberg, that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. The very concepts of exact position and exact velocity together, in fact, have no meaning in nature. The complete rule stipulates that the product of the uncertainties in position and velocity is equal to or greater than a tiny physical quantity, or constant (h/(4π), where h is Planck’s constant, or about 6.6 × 10−34 joule-second). Only for the exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant.

Q6) Explain any one property of transition metal ions?

A6) Transition metals are defined as those elements that have (or readily form) partially filled d orbitals. They have large charge/radius ratio are hard and have high densities, have high melting and boiling points; form compounds which are often paramagnetic;

Q7) What are heteronuclear molecules?

A7) A heteronuclear molecule is a molecule composed of atoms of more than one chemical element. For example, a molecule of water (H 2 O) is heteronuclear because it has atoms of two different elements, hydrogen (H) and oxygen (O). Similarly, a heteronuclear ion is an ion that contains atoms of more than one chemical element.

Q8) Mention any one theory which failed to explain paramagnetic properties?

A8) Electrons in a molecule occupy atomic orbitals rather than molecular orbitals. The atomic orbitals overlap on the bond formation and the larger the overlap the stronger the bond, Valence bond theory predicts that all electrons in diatomic oxygen are spin paired, making O2 diamagnetic.  However, diatomic oxygen is paramagnetic.  This means that electrons are not spin paired.  

Q9) Explain the crystal field theory?

A9) The Crystal field theory (CFT) is a model for the bonding interaction between transition metals and ligands. It describes the effect of attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of a ligand. When the ligand approaches the central metal ion the dangerously of electronic orbital states, usually d or f orbitals, are broken due to the statistic electric field by a surrounding charge distribution. CFT successfully accounts for some magnetic properties, colours, hydration energies of transition metal complexes, but it does not help to describe bonding.

Q10) Write briefly about Geometrics?

A10) Different ligand structural arrangements result from coordination number. Most structures follow the pattern, central atom in the middle and the corners of that shape are the location of the ligands. These shapes are defined by orbital overlap between ligand and metal orbital and ligand-ligand repulsions, which tend to lead to certain regular geometrics. However, there are many cases that deviate from regular geometry. For example, ligands of different sizes and with different electronic effects often results in irregular bond lengths.