Atomic Structure and Quantum Mechanics
It is interesting to find that a large part of what seems as continuous matter is in reality, empty space. As is our common observation, in the external physical universe too, the largest constituent (taken as a whole) is space. Is it not surprising, then, that remarkable similarity exists between infinitesimal atom and this infinite universe?
The subject of atomic structure has been a field of gradual improvements. Bohr Model described it exactly on lines of solar system with nucleus as Sun and electrons as planets. This model was in accordance with Newtonian Laws of motion and Coulomb’s Law of electric force. However, it violated another fundamental pillar of Classical Physics, i.e. electromagnetic theory. This theory states that accelerated electric charge radiates energy in the form of electro-magnetic waves. Orbital motion of electron implies acceleration and therefore electron must necessarily emit energy. Therefore, it must collapse to the nucleus. However, this doesn’t happen in reality as the atom has been observed as a stable unit. In view of this, the Bohr model was discarded in understanding atomic structure.
It is now more or less established that electrons exist in a cloud form around the nucleus and their position can be expressed in terms of probabilities of their being at that position. Electrons are also understood in terms of their quantum numbers. They exist only in quantized energy levels (and not continuous ones) and any change in energy levels is accompanied by simultaneous emission of photon. The energy required to remove an electron is known as ionization energy.
With the advent of Schrodinger, Heisenberg etc, the concepts of Quantum Mechanics originated. In Classical Physics, the future history of particle is completely determined by its initial position, initial momentum and external forces. However, in Quantum Mechanics, though the cause and effect are still related, yet the certainty of future behavior is missing as the initial state of the particle can’t be measured or stated with sufficient accuracy. Therefore, the result in practical terms is as follows. Bohr would have said that the electron revolved at a radius of 5.3×10-11 m. In Quantum Mechanics, we would say that an electron is more likely to be found at that distance. It is obvious that the electron may actually be found anywhere near the nucleus.
It is found that Classical Mechanics is a mere approximation of Quantum Mechanics. The certainties of Classical Mechanics can be said to be illusory as ordinary objects consist of infinite atoms and their departure from average is unnoticeable.
Quantum Mechanics also establishes that the energy, angular momentum, direction of angular momentum and spin in atom are all quantized and not continuous. It also establishes that each electron in an atom has different set of quantum numbers.
Electron that have same principal quantum number n are found roughly at same distance from nucleus and therefore interact with similar electric field and consequently possess similar energies. They are said to be in same atomic shell which is denoted by K, L, M, N, O etc. Within the shell, those electrons which have same orbital quantum number (also known as angular quantum number and azimuthal quantum number) are said to form an atomic sub-shell. By atomic orbital, we mean that physical space in which the electron is most likely to be found. Similarly, there are magnetic quantum number and spin quantum number too.
It will suffice for our practical understanding of the atomic structure that atom, the basic unit, comprises a nucleus and the electrons, distinguishable by their quantum numbers, which are present in cloud form with differing probabilities of availability at different positions around the nucleus. Not to forget, space too is an important constituent of atom.
It is also found, as we know from periodic table, that number of protons (which is same as number of electrons) in an atom goes a long way to define the chemical properties of an element.
Now, as we understand the atomic structure, the question arises as to what makes these independent natural units, i.e. atoms, bind together and form solids, liquids, gases, elements, molecules etc. As we know from our initial Chemistry books, it is basically the electric forces which hold atoms together to form molecule by forming either covalent bond or ionic bonds etc. Let us also remind ourselves that molecules are formed as the energy of a molecule is lesser than the combined energy of the constituent atoms.