Welcome to week 15 of Science Friday. Last week, we covered the four quantum numbers that allow us to assign unique volumes where it is most likely to find an electron within an atom. This week, I am going to describe how individual atoms come together to form the molecules that compose virtually every material in the universe. Fundamentally, a molecule is a collection of atoms that have interactions between their outermost electrons—called valence electrons. The valence electrons reside in the highest principal energy level of the atom. There are three important types of interaction between atoms’ valence electrons: ionic, metallic, and covalent. Ionic bonds are perhaps the simplest type of valence interaction (bond) to describe. In ionic compounds, a certain chemical species has its electron(s) stripped and a different chemical species receives the electron(s). To understand this process better, let’s consider the example of sodium chloride, more commonly known as table salt. Sodium chloride is composed of sodium and chloride as the name implies. Sodium is found in the first family (column) of the periodic table, which means it has one valence electron (in the n = 3 energy level). The laws of physics prescribe species (in the physical and chemical sense, not the biological sense) tend toward their lowest energy state in order reach a stable condition. For example, a ball situated on the top of a hill will tend toward the bottom of the hill (from high potential to low potential). In the same way, the sodium atom’s most stable state would be to have a full outermost valence shell. In order to achieve this state, the sodium atom must lose its outermost electron. Now let’s consider chlorine. It also wishes to achieve a full valence shell; to do so, the it must gain one more electron. The natural conclusion to be drawn is that the sodium atom will give up its valence electron to chlorine. This is indeed what occurs. The chlorine atom’s high electronegativity (attraction of electrons due to protons in the nucleus) causes the sodium atom to lose its valence electron and the chlorine atom to gain the same electron. As a result, both atoms are now in a stable state and are bounded together by the electrostatic attraction of positive and negative charge (negative chloride, positive sodium). This is the basic principle behind all ionic interactions. Metallic bonds involve the interaction of multiple metal atoms. In these frameworks, the atoms come together and allow their electrons to freely roam around from one atom to another. This model of metals is referred to as the electron sea model, and it explains many of metals’ characteristics. For example, the delocalized (freely roaming) electrons provides an explanation for the electrical conductivity of metals. Perhaps the most significant type of bonding is covalent bonding. Indeed, an entire science friday can be dedicated to this type of bonding, which is why many of the interesting details must be left out in this post. Covalent bonds occur when atoms interact in such a way that the valence electrons between them are shared. To understand this better, let’s look at an example. The hydrogen atom has one proton and one electron in its n = 1 energy level. It would be energetically favorable for the hydrogen atom to have two electrons in this shell. In order to achieve this, two hydrogen atoms can “share” their one electron with one another so that, in effect, they both have two electrons. This results in the molecular hydrogen, H2. In fact, when we refer to “hydrogen gas,” we are referring to H2 since it is the most stable and common form the element in the universe. The covalent bond between two hydrogen atoms is a perfect sharing of two electrons between atoms. If two different atoms partake in covalent bonding, there is a chance that one of the atoms will have a higher electronegativity and therefore “hog” the electrons due to the presence of more protons (positive charge). This unshared distribution of electrons is called polar covalent bonding. The dynamics of the electron orbitals during covalent bonding are quite interesting; they effectively provide a theoretical foundation for the necessary bond angles between atoms in covalent molecules. One of these theories is called orbital hybridization and is too important to tack on to the end of a post. I encourage those interested to look up hybridization. So there we have it. These three bond types—and especially covalent bonds—describe the materials we encounter all around us. They are the reason we exist as organic and sentient creatures. It all boils down to the dynamics of atomic bonds. Without them, there would be no proteins, DNA, oxygen gas, or carbohydrates. I hope you found this week’s Science Friday enjoyable and informative to read. As always, leave comments and questions below. Tune in next week for more science!