Chemistry describes the connections (a.k.a. bonds) among atoms. At this level, it is the sharing or exchange of only the outermost electrons which facilitates an increase in complexity from smaller atoms to molecules, crystals or metals. In this sharing or exchange of electrons, properties are completely changed. As we will soon see, community at every level is transformative!
BONDING BY SHARING BETWEEN ADJACENT ATOMS (COVALENT BONDING)
To begin, we start with two hydrogen atoms in isolation from one another. Their energy in this state is defined as zero as shown above on the right side of the picture. As the two atoms approach one another they are attracted to one another and so their combined energy is gradually reduced to a lower value until the lowest energy is achieved at the bottom of the curve.
This distance where the two atoms have the most stable energy is called the bonding distance or bond length and signals the formation of the resulting molecule. If the two atoms continue past this point to come closer together, they repel one another increasing the molecule’s overall energy once again. In response to this repulsion, the atoms return to the “sweet spot” where the energy is the lowest.
A single bond distance defines the hydrogen molecule and a specific lowering or release of energy is associated with the formation of that bond, the bonding energy, for two hydrogen atoms. To break the bond and again separate the atoms requires an input equal to that energy. Hence, the molecule is more “stable” than two atoms existing separately. Coming together is about sacrifice yes, but also synergistic benefit.
A molecule is not the mere juxtaposition of more than one atom with others. It is about a new entity where the outer electrons are shared by the two nuclei making up the molecule. This sharing of electrons is so important, chemists use different terms to name the locations of the electrons. Before the two hydrogen atoms begin sharing the electrons, we say they exist in “atomic” orbitals, emphasizing their isolation from one another.
After the association of the two hydrogen atoms we say they now exist in “molecular” orbitals emphasizing instead their new identity as a hydrogen molecule instead of two hydrogen atoms. It is in this sharing of electrons that the two atoms are transformed into a single molecule. We call the sharing of the electrons, “covalent”, a co-operative sharing of the outer shell, or valence electrons from each of the atoms.
The combination of two atoms to form a hydrogen molecule is transformative for the atoms. The new hydrogen molecules have different physical and chemical properties than the originally separate hydrogen atoms. New possibilities for reactions now exist for the molecule that were not available before and other possibilities have been given up. The molecule is a trade-off with greater complexity.
Covalent bonds not only exist between atoms of the same type, but also between atoms of different types. These almost always share their bonding electrons somewhat unevenly, because the original atoms each have a different attraction for the electrons. Examples include water, H-O-H, better known as H2O, and carbon dioxide, CO2, or O=C=O. In both these bonds, the oxygen atom is known to attract the electrons more strongly than either the hydrogen or carbon atoms, resulting in uneven or polar bonds.
What is perhaps more amazing is that because water is a bent molecule it will interact with electric and magnetic fields, but carbon dioxide will not because it is linear. The details are not important here, except to say even the three-dimensional shape produced by the coming together of atoms makes the difference in the properties of the molecule.
Sharing of these outer shell electrons between atoms and the formation of a
3-dimensional molecule from atoms leads to an abundance of possibilities not open to just atoms alone. In fact, in the world of practical chemistry, it is the rearrangement of these very bonds that leads to the transformation of one compound to another, what we call chemical reactions.
Covalent compounds exist as molecules, literally “little lumps” of matter, very small communities where electrons are shared as needed among the member atoms. The elements called nonmetals such as oxygen (O), hydrogen (H), and carbon (C) (see above), participate in this type of sharing. They are known to hold onto these outer electrons quite tightly (have high ionization energies) and so although they are not likely to give them away, sharing is possible.
Metal atoms by contrast hold onto their electrons more loosely than nonmetals (have low ionization energies), so we will see a completely different type of bonding as atoms of metals and non-metals combine as discussed in the second section and as atoms of metals come together themselves (the third section)
In the periodic table notice the existence of “metalloids” which are intermediate in their properties between metals and nonmetals. These elements sometimes behave like metals and at other times like nonmetals, depending on what other elements are around them. (Sounds like some people whose behavior is dependent on their environment, eh?)
BONDING BY COMPLETE EXCHANGE (IONIC BONDING)
Another possibility for the combination of atoms involves the transfer of electrons rather than their equal or unequal sharing. A beautiful example of this is common table salt: NaCl, sodium chloride. Sodium (Na) is a highly reactive metal; indeed, it will violently react with water and form a solution of lye [drain cleaner], NaOH, in the process. Chlorine (Cl2), a yellow-green gas is poisonous, and was used in World War I as a chemical weapon before its ban by the Geneva Convention.
Yet, when sodium and chlorine are existent together as sodium chloride (NaCl), we obtain again an entirely new set of properties. Some of its most common uses include, but are not limited to: flavoring our food, melting snow and ice on our roads and sidewalks, softening our well water, and making paper and rubber.
How is it that this transfer of electrons comes about? Sodium metal, Na, begins with 11 electrons, in what we call “shells” of 2, 8, and 1. Each non-metal chlorine atom, Cl, begins with 17 electrons, with 2, 8, and 7 electrons in its shells. Shells listed first are nearest the nucleus and not likely to be exchanged, but those listed last, the valence, or outer-shell electrons, are furthest from the nucleus and are more loosely held.
As shown in the diagram, by the simple transfer of the one valence electron in the last shell from sodium to the last shell of 7 electrons in chlorine we are left with the sodium ion, Na+, with 10 electrons in shells of 2 and 8 and the chloride ion, Cl–, with 18 electrons in shells of 2, 8, and 8.
For reasons we won’t explore here these octets, or sets of 8 electrons, in the last shell are much more stable than the electron arrangement in the neutral atoms, not containing these octets in the last shells. The resulting Na+ and Cl– ions, being opposite in charge, are strongly attracted to one another.
In fact, we call sodium chloride, NaCl, for simplicity’s sake. This one-to-one ratio is simply the lowest ratio (1:1) of atoms in sodium chloride. The actual number of pairs is overwhelmingly great at room temperature. Atoms come together in such great numbers that we can see a single crystal of sodium chloride at room temperature. Think of that the next time you hold a crystal of salt in your hand!
Ionic compounds are almost always composed of a metallic element with a nonmetallic clement. The loosely held electrons are donated from the metal to the nonmetals, which hold on tightly to them. Examples include
- calcium chloride, CaCl2 (used to melt snow from our sidewalks in winter)
- sodium nitrate, NaNO3 (used to preserve some lunch meats and bacon), and
- aluminum sulfate, Al2(SO4)3 (used in the purification of drinking water).
So, what difference does a transfer of electrons cause compared to sharing? Check out the contrasting properties of covalent and ionic compounds:
|Properties||Covalent Compounds||Ionic Compounds|
|representative unit||isolated molecule||ions in crystals|
|types of elements||nonmetals with nonmetals||metals with nonmetals|
|melting/boiling point||usually low||usually high|
|physical state (250C)||usually gases, also liquid & solid||usually solids|
|solubility in water||usually lower, except for acids||usually higher|
|electrical conducitivity of solid||does not conduct electricity||does not conduct electricity|
|electrical conductivity of solution with water||does not conduct electricity, except for acids||does conduct electricity|
BONDING BY OPEN AND WIDESPREAD SHARING (METALLIC BONDING)
Metals have a third way of coming together. Individual atoms of metals have loosely held electrons in their outermost shells as we know from their easy donation of electrons to nonmetals in ionic bonding. Yet when metals come together with other metals the sharing is remarkably different: they now share the electrons over a vast community. The outermost electrons are free to move in a matrix of nuclei with only their inner shells of electrons. We call these freely wandering valence electrons, the “sea” of electrons.
This property is what makes metals such wonderful conductors of electricity. The electrons are free to move throughout the metal from one place to another, when moved by an electric field. This sea of electrons is also the reason that metals are malleable and can be pounded in sheets, like aluminum or gold foil. Metals are also ductile and can be drawn in wires for the same reason. We use copper, for instance, to wire our homes and businesses.
|representative unit||widespread connection among metal atoms|
|types of elements||metal with metal|
|melting/boiling point||usually very high|
|physical state (250C)||solid, except for mercury (Hg)|
|solubility in water||does not dissolve in water|
|electrical conductivity of solid||conducts electricity easily|
|electrical conductivity of solution with water||no electrical conductivity due to dissolution|
The way electrons are either shared or exchanged among atoms and how that leads to the creation of molecules, metals, or crystals, can remind us of the benefit for us as human beings to freely exchange or share our resources. For us too, the possibility of greater complexity and new possibilities awaits.
I don’t know about you, but I’m good at donating that to which I’m not particularly attached to someone in need. And, I have become better at receiving what seems to be “extra” from other people, especially if I really need it. I do the transfer of resources to and from myself (an ionic process) fairly well.
And, I suppose I’m even OK at sharing what little I have with others if they put in a similar share and none of us is particularly attached to what we’re sharing. My instinct to follow the wisdom of the metallic atoms seems intact too.
But, when I look at what I’m attached to, especially when I think a resource is scarce, I have grown accustomed to keeping “extra” for myself. My first inclination is the opposite of sharing, hoarding. So it makes me wonder: what new possibilities I’ve been missing because I haven’t participated in as much sharing (a covalent process) as the atoms do naturally? It’s a good question!
There’s a lot to learn from atoms, besides academic chemistry!
Bless the Lord, all you molecules that share electrons,
All you atoms that transfer electrons, bless the Lord.
All you metals with your seas of electrons, bless the Lord,
Praise and exult God forever!