6th February 1786. Chemical Bonding.
William Higgins was an early pioneer in chemistry and an early exponent of atomic theory which attempted to explain bonding of atoms into molecules.
Today in 1786 William matriculated at Magdalen Hall Oxford, but left one year later for Pembroke College, but never graduated: his precise birth and death dates are uncertain.(1)
In 1789 Higgins published views on the combination of ‘ultimate parts’, what we would know as atoms, and foreshadowed the concept of valency bonds. However Higgins’ contribution to Atomic Theory could have been exaggerated by his friendship with Humphry Davy, who promoted Higgins at the expense of his rival John Dalton.(2)
Chemical bonding as traditionally shown by stick and ball models, is an excellent way to make sense of an abstruse concept.
No one has seen the thing we call a bond, and there is no reason to think they exist except as a concept. It is better to think of bonding as an effect, causing certain certain atoms to form enduring molecular structures such as for example, table salt, carbon dioxide or water.
There are two main types of bonding: covalent when two atoms share valence (the outside atom shell) electrons, and ionic when opposite charged electrons are attracted to each other
Covalent bonds make strong connections between atoms so need a large amount of energy to force the molecules apart. However molecules of these bonds aren’t strongly attracted to each other, unlike ionic bonds so move freely about.(3)
Thus many of covalent bonds are non-metals, liquids and gases, as with water and carbon dioxide. Metals are exceptions in that though covalent bonded, are still solid, are flexible and easy to melt.(4)
For example bonds that hold carbon and hydrogen atoms (see Diagram), together, as in rubber are covalent. In this case each carbon atom shares its four outer electrons with its neighbour and under stress the bonds stretch and then snap back as each atom pulls on its shared electrons.
Substances with this valence bonding often form molecules with low melting and boiling point as with hydrogen (which is an element and molecule), and water.
Ionic bonds (see Diagram below) which happen in salts and other minerals. Here one atom gains a valence (outer shell) electron from a different atom.
Valence shells need to have a full outside ring and if not full there is an unbalanced charge on element, is ‘unhappy’, and will want to bond with something to make it ‘Happy’.
Thus looking at the diagram Chlorine now it has ‘pinched’ an electron from Sodium is ‘happy’, as is Sodium which is now in balance.
For two atoms to combine, by electrostatic attraction, there must be a total of eight electrons in the outer valence shells. In the diagram there is one Sodium and seven Chlorine electrons in the valence shells.
Neon can’t join with other atoms as it already has a full outer shell of eight electrons.
(1) 1763-June 1825.
(2a) Davy was one of Higgin’s proposers to the Royal Society in 1806. Higgins in turn arranged Davy’s highly lucrative Addresses to various Dublin Societies
(2b) The word atom comes from the Greek Atomos, meaning individual.
(3) Hydrogen is diatomic, having 2 atoms to form the gas, as it doesn’t exist in nature as a single atom. Other diatomic elements are Oxygen, Nitrogen and the Halogens.
(4a) Covalent bonding is also seen between an oxygen and each hydrogen atom in a water molecule (H2O) where each of the covalent bond contains two electrons: one from each hydrogen atom and one from the oxygen atom.
(4b) A Covalent Bond is a shared pair of electrons. An odd molecule (free radical) is when the electron is not shared. (Lewis Theory).
Ref: chem/com./acad/ bonding.
Ref: science.uwaterloo.ca. Chemical Bonds.
Ref: Electron sharing/covalent bonds Dept of Chem Oxford.