what are chemical bonds?

Answer 1

A chemical bond is the physical phenomenon of chemical species being held together by attraction to each other through sharing, as well as exchanging, of electrons and is a phenomenon that is fully described by the laws of quantum electrodynamics. In general, strong chemical bonds are found in molecules, crystals or in solid metal and they organize the atoms in ordered structures.

Answer 2

A chemical bond is the force that hold two atoms together in a molecule. There are two main types: ionic and covalent. Both types involve electrons and whether an atom has too many or too few. In an ionic bond, one atom gets rid of an electron entirely and gives it to the other atom. This give the first atom a positive charge and the second a negative charge, so they're attracted to each other. So for instance, an electron might travel from sodium (Na) to chlorine (Cl), giving Na+ and Cl-, which stick to each other to form sodium chloride or table salt.

In a covalent bond, two atoms share electrons because neither has enough by itself. In each bond, one electron from each atom is shared, and two atoms can form multiple bonds if they have enough electrons. So for instance, oxygen wants two more electrons and hydrogen wants one more. So the oxygen forms one bond with each of two hydrogens. It gets two shared electrons, one from each hydrogen, and they each get on, so they're all satisfied. That's how water is formed.

Answer 3

Chemical bonds form to lower the energy of the system, the components of the system become more stable through the formation of bonds. Everything wants to be more stable - its easy to lie down than stand up, bonding is Nature's way of allowing the elements to lie down.

There are several types of chemical bond. Initially we will concern ourselves with three basic types, ionic bonding, which involves the transfer of electron(s), covalent bonding, which involves sharing of electrons and metallic bonding, which in some ways can be considered as a combination of both.

If we examine the periodic table, we find that the elements in Group VIII (or 18), helium, neon, argon and so on, are particularly stable, so much so that they were once labeled the "inert gases". We now know that these elements are not inert, indeed xenon forms a range of compounds, but, nevertheless, they are very stable (although now we refer to these elements as the noble gases). This stability is the result of their electronic configuration, they have a full valence shell of electrons (ns2, np6) and this imparts stability. G. N. Lewis (1916) suggested that bonds (covalent) formed to enable elements to attain this "noble gas configuration". We can extend this idea to ionic compounds, in a compound such as sodium chloride, one element loses electron(s) to gain this stable electronic configuration whilst the other gains electron(s) to achieve the same result.

We can see that in each case, sharing or transfer of electrons, results in a more stable system.

So can we predict the type of bonds which will form between different elements in the periodic table ? The answer is yes (usually). The elements to the left of the periodic table (Groups I and II) can achieve the noble gas electronic configuration by losing electron(s). Ionization enthalpies show that lose of these is relatively easy (ionization enthalpies are fairly low for the elements in these groups). These elements are termed "electropositive". The opposite is true of the elements to the right of the periodic table. Elements in groups VII and VIII (17, 18) can most easily attain the noble gas configuration by gaining electron(s). These are the electronegative elements. (A measure of their ability to accept electrons can be seen in the electron affinity values for these elements).

This information allows us to predict that compounds formed by the combination of an electropositive element with an electronegative element will involve the transfer of electron(s) from the electropositive element to the electronegative element and hence ionic bonding will occur. The degree of ionic bonding will depend upon how extreme the differences are (in terms of electropositive / electronegative character) between the particular elements. Electropositive character increases down the group whilst electronegative character increases up a group. Combination of the most highly electropositive elements (e.g. Cs, Fr) with the most electronegative element (F) will result in the "most ionic" compounds, other combinations (such as Li with Br) will be much less ionic. In other words, transfer of electron(s) is never total, for example CsF is approximately 97% ionic, there is always some degree of covalency (however small).

When we get combination between two electronegative elements, the result is a sharing of electrons and so covalent bonds form. This is best illustrated by the combination of two atoms of the same element (a homonuclear diatomic). Combination of two fluorine atoms to produce F2, for example is achieved by sharing electrons, each fluorine atom donating one electron to form a two electron (two centre, two atoms) bond. The covalent bond is formed by overlap (or combination) of atomic orbitals of each element to form a molecular orbital. The formation of molecular orbitals lies at the heart of Molecular Orbital Theory and is the subject of another set of notes. In simplistic terms, we can visualise the sharing of two electrons between atoms as forming a single bond between the two atoms, joining them together. If the atoms share two electrons each (i.e. 4 in total) we have a double bond which will be stronger (and shorter) than a single bond. Likewise, sharing six electrons gives us a triple bond.

For combination of two electropositive elements we have metallic bonding. Perhaps the best way to model this is with "band theory" but that is the basis of other notes (when they are written). For now, we can simply visualise this as each metal donating electron(s) to a common "sea" of electrons which are shared by all the ions within the solid.