How do you think of chemicals? I view them as arrangements of matter particles that are themselves composed of energy; they are arrangements of energy nodes if you like. Particle energy nodes have energy desires and consequently absorb and emit photon energies. All structures from the smallest atom to the largest object in our universe are composed of energy exchanging particle energies. The particles in structures are always seeking to get their energy needs in the most energy efficient way. If there are energy savings to be made and if the circumstances are right they will chemically combine with other structures to that end.
In chemistry we learn about electrons, protons and neutrons. We learn of their arrangements in atomic elements and in molecules, mixtures and compounds. We discover states of matter, ions, isotopes and covalent, ionic and metallic bonds. We are introduced to chemical equations and learn how to balance them in terms of atoms, volumes and masses. Rarely do we hear that atomic particles seek to satisfy their energy desires in a way that uses least energy. Yet it is the fundamental reason why chemical changes occur. Chemical change is all about particles reacting to the energies of their surround environment and forming structures that more efficiently satisfy their energy desires in that environment.
If we ask why hydrogen and oxygen exist as the molecules H2 and O2 we are told they are more stable. But what is this stability? It is the result of particles coming together in more energy efficient, energy exchanging structures. There is less total energy (node energy and photon energy) in the molecule than in the two separated atoms. Particle desires for energy are responsible for photon absorptions, emissions and their energy flows. Such desires are what hold the particles of a structure at a distance and cause them to vibrate.
Water H2O is a more energy efficient combination of hydrogen and oxygen. We use this fact in fuel cells where we harvest, as electrical energy, the energy released in combining the two. Simply bringing oxygen and hydrogen together in the right proportions in a container will not of itself create water because the molecules in the mixture are content with their energy interactions. However, if we input a spark of photon energy we agitate some of the particle energy exchanges. They re -combine as water and release photon energy which acts like the spark and converts more gas to water. We get a rapid explosive energy releasing change to water. The burning of paper, wood, domestic gas and fuel in car engines are other examples where an activation energy is required to trigger chemical changes that release heat in the process of forming more energy efficient structures of particles.
Changes in states of matter are not regarded as chemical change but they do involve the rearrangement of a single structures particles. Consider the changes involving ice, water and steam. Come the depths of winter when photon energies from the sun are much reduced, many photon energies passing between earth objects and between particles within those objects are reduced. Water particles desire more of the lower energy photons being exchanged and so energy collecting electrons move marginally closer to the energy desiring nucleons making the water slightly less fluid. We describe this by saying cold water is more viscous.
In most cases there are impurities and/or circulations in the water. They, at freezing point, are the trigger that enables the water to make a change to the less energy consuming but more rigid structure that is ice and in the process releases photon energies. Unlike most solid structures, ice is less dense than its liquid water state and will float on it. Its molecules are denser than those of water but the way they link together as ice is more space consuming.
Input photon energy to ice and it will melt back to a water structure that can better and more efficiently accommodate the increased energy. Continue adding heat and that water will eventually turn to the steam that is its vapour form. The diagram below shows energy is absorbed as ice changes to water and water to steam. The reverse processes of condensing and freezing give up energy.
The steady temperatures recorded by a thermometer at the changes of state transitions tells us the photon energies passing between the molecules are not rising so where are the input energies going? They are being absorbed internally into each molecule, where our thermometer cannot reach, and and are passing between particles as increased photon energies. Those increased photon energies have increased the distances between the particles (particularly electron to proton distances) within the molecules so as not to overload particle energy desires.
By now you should understand why the photon pressures of your hand can’t part solids like ice, can feel the pressures when they part the molecules of liquids like water and feel even less pressure when they part gases like those of water vapour and steam.
In chemistry it is not uncommon now to be introduced to reactivity diagrams like that shown for metals. The diagram enables us to say that iron added to copper sulphate will replace the copper and make for iron sulphate and that magnesium added to zinc chloride will replace the zinc and create magnesium chloride. The diagram also enables us to say that the reverse reactions will not naturally occur.
Rarely are we told why the reactivity series works. It does so because some metal particles better combine with the particles of water, acids and oxygen to make more energy efficient structures than others do. The iron displaced copper and magnesium displaced zinc in the above examples because in doing so they made for more energy efficient structures. They released photon energies in the process.
Try not to be confused by the term binding energy much used to explain the stability of a structure. The word binding makes it sound like it is energy within the structure but that is just not so. A more stable structure is a more desirable structure with less energy in it and which is more resistant to change. The badly named binding energy is the external energy we have to input into that structure to break its bonds. More energy efficient structures need greater inputs of binding energy to break their particle bonds. The reversal of many chemical reactions, like that of say a chargeable battery, requires input energies to break and rearrange particle bonds to accommodate the increased energy.
In all chemical changes the least attached outer valence electrons play a major role. That is because they are more sensitive to the energy desires of surround structure protons. If it is more energy efficient to do so they will transfer their allegiance in full or in part to surround structures and thereby form bonds with them. Of course any such changes also brings rearrangements of associated electrons and nuclei.
The groups and periods of the periodic table of elements tell us much about the distribution of the electrons around the nucleus of each element. The groups in the table tell us how many electrons are in the outer valence shell. The transition metals that occupy the large section in the middle all have, like the alkali metals, two valence electrons. As outer electrons are key particles in the formation of new energy links we should not be surprised that elements in the groups have many common behaviours.
The inert gases in group 8 are most energy stable. With the exception of helium with its full valence shell of 2 electrons, the others have full valence shells of 8 electrons. They are highly content with that state and desire no change. Where there are small numbers of electrons in an elements outer shell they are more free to wander and thereby more likely to make more efficient energy links with other structures. In elements where outer valence shells are nearly full there are opportunities for those wandering electrons to fill such shells making for more energy stable arrangements. It is for this reason that so many of the metals with 1 and 2 electrons in their outer shells are keen to bond with non metals and halogens in groups 6 and 7.