Hello wonderfuls! Today I am making my first foray into science communication to get a better grade in chemistry!
This post will provide a college chem 1 student’s understanding of electron orbitals, covalent bonding, orbital hybridization and molecular/electron geometry.
You’ve likely heard that an atom is made up of a nucleus and an electron cloud. Within the electron cloud there are things called orbitals. While every electron is perfectly interchangeable with every other, their positions within an atom are not. In fact, no two electrons in an atom can exist within the same orbital with the same spin. These orbitals exist within shells, which are the layers of an atom. At most one set of a kind of orbital can exist within a shell.
There are four types of orbitals.
The common orbital types are:
s, which can only be oriented one way and only one of which exists per shell.
p, can be oriented in three orthogonal ways (px, py, and pz), and thus there are three p orbitals per shell.
The d orbitals can be oriented five ways (dxy, dxz, dyz, dz^2, and dx^2-y^2), and so five of them exist per layer.
The f orbitals can be oriented seven ways (fx(x^2-3y^2), fz(x^2-y^2),fxz^2, fz^3, fyz^2, fxyz, and fy(3x^2-y^2)), and so seven of them exist per shell.
Someone called Haade provided sketches of them to wikimedia so here they are:
Interestingly these models are not computer generated.
It is worth taking a moment to admire the wild shapes formed by the d and f orbitals.
The most important electrons in an atom are its valence electrons. Valence electrons are the outermost electrons an atom has, and they’re the electrons that are used for bonding or non-bonding interactions. For an electron to be a valence electron it needs to be in the highest shell of any electron in its orbital type.
Covalent bonds are bonds between two atoms relatively close in electronegativity. In these bonds electrons are shared between the two atoms. A covalent bond forms when two orbitals—each containing one electron—overlap and form a shared electron cloud. Atoms will often form multiple bonds with one other atom. Whether a bond is single, double or triple depends on whether each atom is contributing one, two, or three electrons to the bond. Generally an atom will form bonds that bring the number of valence electrons it has access to up to eight: this is called the octet rule. This can be conceptualized as atoms trying to become like noble gases, whose valence shells are complete because they have eight valence electrons. This is why the noble gases are almost always non-reactive. There are some exceptions to the octet rule: elements with access to d orbitals can have more than eight valence electrons, and certain elements with lower atomic numbers will have less than eight valence electrons when fully bonded (for example, boron and aluminum both favor three bonds for a total of six valence electrons).
The orbital types described above do not adequately explain the shapes of molecules that scientists have found. This is because many atoms use hybrid orbitals for forming bonds. A hybrid orbital is made by combining two or more orbitals. The number of orbitals produced is the same as the number of orbitals used. Hybrid orbitals are named by simply listing the number of each type of orbital used. For example sp2 orbitals are made with one s orbital and 2 p orbitals.
Atoms that obey the octet rule want a total of four bonds and lone pairs. The atoms want all single bonds and lone pairs to be equal, which requires the s and p orbitals to be hybridized. Double and triple bonds require unhybridized p orbitals.
The arrangement of the bonds formed by these hybridized orbitals are controlled by the mutual repulsion between electron clouds. The molecules produced have the bonds just about as far away from each other as is possible. That said, lone pair electron clouds and certain bonds have more powerful repulsion so the theoretical angles will be a bit different from the actual angels. Fortunately their behavior is very similar to balloons tied to each other, so I had an excuse to make some models!
A common molecule that features sp hybridization is CO2 (carbon dioxide). The electron geometry of the carbon atom looks something like this:
Each balloon represents a bond that the carbon atom has to an oxygen atom. The bonds are offset from one another by a full 180°. This electron geometry is aptly named “straight”.
sp^2 hybridization is a bit rarer in small molecules, so we’ll have to look a bit higher in the atmosphere to find it. Ozone or O3 is an important part of our stratosphere that blocks ionizing radiation like ultraviolet light from reaching Earth's surface where it can cause health problems like cancer in high enough doses. In 1985, it was discovered that the ozone layer was thinning in some areas due to chemicals called chlorofluorocarbons or CFCs. Thankfully the danger was recognized, and on the first day of 1989, a treaty called the Montreal Protocol went into effect to phase out the usage of CFCs. Thankfully, the ozone layer is showing signs of improvements according to a United Nations press release.
Ozone’s electron geometry should looks more or less like this:
It is important to note that while the central oxygen has sp^2 hybridization, it is only bonded to two other atoms, so while its electron geometry is described as “trigonal planar”, its molecular geometry is simply labeled “bent”. The electron clouds are around 60 degrees apart at their closest.
It is interesting to note that while the oxygens in CO2 also have sp^2 hybridization and thus have trigonal planar electron geometry, they do not have a meaningful molecular geometry as each oxygen is only bonded to the carbon. When there are no lone pairs of electrons, an atom's molecular geometry and electron geometry are the same.
sp^3 hybridization is incredibly common as it is the only hybridization that allows for atoms to follow the octet rule without having double bonds. Both methane (CH4) and water (H2O) contain this hybridization. Methane is the principal component of natural gas. It is odorless, colorless, and burns relatively cleanly when pure. Upon combustion it breaks down into water and carbon dioxide. Often it will be burned when it cannot be stored because it is a more powerful greenhouse gas than the CO2 it produces when burnt.
sp^3 hybridization produces a tetrahedral electron geometry, and the electron clouds are 109° away from each other. While methane has no lone pairs, water has two, water’s molecular geometry is bent. If an atom were to have 3 bonds and one lone pair its geometry would be trigonal pyramidal.
sp^3d hybridization makes use of the d orbital and thus ignores the octet rule. This kind of structure is rarer than the others covered thus far and isn’t often seen outside of industrial reagents like phosphorus pentachloride or phosphorus pentafluoride, both of which have an electron geometry that looks like this:
This configuration is called trigonal bipyramidal. It is the only electron configuration that has two different angles, having both 120° and 90° offsets. The geometry produced by this hybridization is effectively a combination between a straight electron geometry and a trigonal planar electron geometry. With one lone pair, this hybridization produces a molecular configuration called a “sawhorse” or “see-saw” after the playground equipment. With two lone pairs the molecules form a “t shape”. Finally 3 lone pairs produces a linear arrangement, similar to the configuration of sp hybridization.
sp^3d^2 is also mostly found in industrial chemicals. One such chemical is sulfur hexafluoride, SF6. It is a non flammable non toxic electrically inuslateing gas and as such is used to prevent arc flashes in high voltage aplications.
sp^3d^2 forms an octahedral electron geometry. An octahedron is perhaps the most underrated platonic solid. Its bonds are all 90°. When it has one lone pair its molecular geometry is a square pyramid, the same shape as the famous pyramids of Egypt. With two lone pairs its molecular geometry is square planar. With three lone pairs its molecular geometry is described as a “T shape. Finally with 4 lone pairs its molecular geometry is straight. Many transition metal compounds use their d orbitals to form octahedral structures.
A big thanks to my friend Bri for checking this over! @oracleofthepast
For a full chart of electron and molecular geometries check out the wikipedia page on vesper theory.
Most of the information featured here comes from this free online chemistry textbook, so give that a look too.
