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Grade: 11


how an oxygen satisfies their valency in superoxide by sharing of ½ – ½ electrons ?

4 years ago

Answers : (1)

dolly bhatia
200 Points
Oxidative stress is an imbalance, in biological cells in particular, between the state of two chemical processes:
formation of ‘reactive oxygen species’(ROS), more generally ‘reactive species), some of which qualify as ‘oxygen free radicals’ –potent ‘oxidizing’ (electron-capturing) atoms, ions or molecules and
elimination or ‘reduction’ of those oxidizing agents by ‘antioxidants’ – ‘reducing’ (electron-donating’) atoms, ions or molecules;
imbalance characterized by an excess of the former or a deficiency of the latter, or both, leading to alteration of a cell’s ‘redox’ state towards ‘oxidized’ state.
The term ‘oxidation stress’ refers to a serious imbalance between RS [reactive chemical species] production and antioxidant defenses.
Core of the definition of oxidation stress: imbalance between ‘oxidant’ and ‘antioxidant’ activity, favouring ‘oxidants’, potentially causing cellular injury.
Experimental evidence suggests that, for a living system adapted to uptake and utilization of oxygen, beneficial effects separate themselves narrowly from potentially injurious effects produced by oxygen-linked ‘oxidation reactions’.
Thus, normal physiological processes which involve oxidation reactions like anti-microbial defense, acute inflammation and normal signaling processes, share underlying mechanisms with pathophysiological phenomena like chronic inflammation, aging, carcinogenesis, atherosclerosis and drug toxicity. For organisms living an aerobic (living only in presence of oxygen) existence entails between Scylla and Charybdis.
‘Oxidative challenge’ does not necessarily lead to oxidative damage. When it does, treatment involves efforts to increase antioxidant reactions, by countering culprits with increased antioxidant defenses or both.
Free radicals are a major cause of biological oxidative stress:
A free radical is any chemical species (e.g., atoms, ions, molecules), capable of independent existence – hence the term ‘free’ – that contains one or more unpaired electrons in its outermost or ‘valence’ electron shell.
An unpaired electron is one which occupies an atomic or molecular orbital without an accompanying electron – orbitals have maximal capacity for holding two electrons.
A superscript dot is used to denote unpaired electron.
Radicals can form in many ways, in both industrial and biological systems and in the latter may form because of substances in food, water, pesticides, pollutants including tobacco smoke, drugs and radiation; they may form in biological cells because of normal responses to microbial infection and as a byproduct of mitochondrial function.
Because of their unpaired ‘valence’ electrons, radicals possess high degree of chemical reactivity and potential for disruption structure and activity of biologically important molecules.
Full understanding of free radical chemistry requires understanding many concepts from quantum mechanics and quantum chemistry, among which include:
Atomic orbitals
Pauli principle
Spin quantum numbering
Covalent bonding
Molecular orbitals
Hund’s rules
Chemists define a ‘free radical’ as an atom, ion or molecule which has one or more unpaired electrons whereas all its other electrons, if it has others, exist in pairs that have ‘opposite sign’, technically ‘intrinsic angular momentum’ or differing spin quantum numbers (+1/2 and -1/2). Qualifier ‘free’ denotes independent existence of radical species and many biological chemists drop the term as they consider only those radicals which can exist independently.
Physicist Ernest Rutherford discovered positively charged nucleus and conceived of an atom as low-mass negatively charged electrons in orbits around a more massive nucleus in the center, like planets orbiting the sun. Danish physicist Niels Bohr developed a model of the atom which proposed that electron orbits could occupy only certain positions around nucleus. Those positions were conceived as ‘shells’ successively more distant form nucleus and having successively greater energy. French physicist Louis de Broglie, thinking about how one could view light as either particles (photons) or waves (electromagnetic waves), proposed that electrons may have wave-like properties. Austrian physicist Erwin Schrödinger showed in 1926 that one could treat an electron as a wave occupying a so-called ‘orbital’, strictly a mathematical ‘wave-function’ which gave probability of electron’s location everywhere in space around the nucleus.  Modern description of the atom takes as fundamental that electrons occupy orbitals within their shells that have distinctive shapes indicating likely volumes of electron occupancy. They are referred to as spherical harmonic functions – surfaces of the shapes enclose region of space that likely contains the electron. Physicists and chemists designate differently shaped orbital’s with lowercase alphabets s, p, d, f and others and successive shells by uppercase letters K, L, M N or successive numerals: 1, 2, 3, 4….
One can imagine an atom’s orbitals as imaginary enclosures of particular shapes depicting where electrons likely spend most of their life ‘surrounding’ their nucleus. ‘Shapes’ of enclosures reflect three-dimensional mathematical wave functions. Wave functions have algebraic sign analogous to peaks and troughs of a water wave, often represented in visual depictions as colours. Two different ‘colours’ of electron distribution in an orbital have significance in understanding molecular orbitals, types of orbitals electrons occupy in molecules.
4 years ago
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