Valence is an education-focused app designed to teach the elementary concepts of chemical bonding, namely the Lewis octet rule: the premise that chemical bonding can be explained by atoms starting with some number of valence electrons, which are compelled to try to pair up by forming covalent bonds with adjacent atoms, or filling lone-pair lobes. Most main group atoms in conventional bonding environments arrange their electrons so that the shared electron count is 8 (hence the name "octet theory"), though there are many exceptions, where this number is 6, 10 or 12.
While these elementary concepts are very simple to explain and understand intellectually, professional chemists make use of various notations, such as condensed molecular formulae, and structure diagrams where each line between two atoms represents a shared electron from each atom, and implicit hydrogen atoms for carbon. These drawing shortcuts make it possible to save time and space when communicating chemical structures, but students entering the discipline can find them bewildering until familiarity is achieved after much practice. With this in mind, the Valence app was designed to provide an interactive experience, where the student is presented with a group of atoms, each of which has a number of unassigned valence electrons. By touching-and-dragging these electrons into bonds or lobes, molecules are formed. As each bond or lobe is created, the orbiting-electron metaphor visually transitions into the line-is-covalent-bond metaphor, emphasising the mapping between these two concepts. Once each of the molecules is successfully completed, the app shows a 3D model, which can be rotated and viewed, in order to observe how the arrangement of atoms and nonbonding electrons affects the geometry.
The app is available on the iTunes AppStore. It runs on iPhone, iPod and iPad devices, and is optimised for all geometries (including portrait and landscape).
When the Valence app opens, a number of categories are offered:
The numbered sections, which are decorated with chemical structures, each correspond to lessons, each of which contains a number of individual molecules which need to be detailed individually. For example, the Main group hydrides lesson consists of simple examples where an individual element is bonded to as many hydrogen atoms as necessary to fill up its valence shell:
The quintessential simple example of covalent bonding is the dihydrogen molecule, which is the first exercise. When opened, it shows two hydrogen atoms, each of which has a single valence electron (black dot) orbiting around the atom:
The atom is coloured light purple, to indicate that it has unassigned electrons that need to be dealt with. The first thing to do is to touch one of the hydrogen atoms, and drag, which moves a single electron. Moving it to the centre of the two atoms, then removing your finger from the screen, causes the electron to stay where it is for the moment:
Then, touch the other atom, and drag its electron to the same point, at the centroid between the two atoms:
The animation shows the two electrons coming together in a pair, then fading away to be replaced with a single line, which is the mnemonic that chemists use to represent a single covalent bond:
When each exercise is complete, a 3D model is shown, which provides an idea of the way the molecule is oriented in space, for example the hydrogen atom from the previous exercise:
The model can be rotated with single-finger drag, zoomed with the pinch gesture, or panned by two-finger drag.
The dihydrogen example is trivial both as an exercise as a model, but it gets more interesting with large molecules, such as methane:
In this case, the actual exercise uses the flattened planar representation of CH4, which is commonly used for convenience. Viewing the 3D model is an opportunity to demonstrate that the way structures are drawn on paper is quite different to how they exist in actual space: the geometry is clearly tetrahedral, rather than square planar. This concept persists throughout the app: the 2D exercises are arranged in conventional pedagogical form, where students are encouraged only to think about the bonding, whereas the 3D models demonstrate how these bonding arrangements translate to actual shape.
Multiple bonds are handled sequentially: for example, a double bond such as found in ethene, is created by first dragging 2 electrons into the C-C bond, then dragging in another 2:
As the second bond is created, the 4-electron metaphor fades into the conventional two-line double bond representation.
If all atoms conformed to the simple 8-electron version of the "octet" rule, the Valence app would be smaller and much less useful. There are many examples of small molecules that have more interesting properties, which present a challenge to students learning bonding theory. Ozone is a good example: it consists of 3 oxygen atoms, each of which has lone pairs, and the molecule can be written in multiple forms. The most useful is the charge-separated version:
This asymmetric representation shows the pre-assigned [+] and [-] charges on adjacent oxygen atoms, and a total of 6 lone pairs which must be filled in order to complete the octet shells:
The molecule contains a double bond and a single bond, which must also be completed before the exercise is done:
Once it is correctly assigned, the 3D geometry is quite helpful for demonstrating the bent structure of the molecule, which is consistent with the charge-separated form that retains a lone pair on the central oxygen atom.
The phosphate ion is another example of where a charged species is shown, with just a single resonance form:
Note that in this case, the oxygen atoms have their lone pair electrons suppressed, since it would be unreasonably tedious to have the user fill in all of the lone pairs. The important concepts in the exercise are the charges and the bonding patterns between the phosphorus and oxygen atoms.
Things get even more interesting with paramagnetic molecules like nitric oxide, which have a singly-occupied lobe:
A number of unusual molecules have very high "octet" values, such as sulfur hexafluoride, with a formal shared electron count of 12 about the central sulfur atom, and an octahedral geometry:
The Valence app is designed to teach some of the most elementary chemical bonding concepts, to students who have not yet had the chance to become practised at interpreting the shorthand notations used by professional chemists. The transition of understanding between the Lewis octet metaphor and chemical diagrams is illustrated interactively: the student moves the electrons, and the app shows the morphing of the representation styles. This understanding is a rate limiting step for many students, but it is absolutely critical for mastering chemistry. The goal of the app is to reduce the burden of understanding using an interactive cartoonish interface, which is also fun and has game-like properties. The app can be used as an adjunct to conventional lectures, tutorials and textbooks, or it can be used by anyone who is interested in learning core concepts of chemistry.