What Are Chemical Bonds, and Why Do They Form?
A chemical bond is the result of an attraction between atoms or ions. The types of bonds that a molecule contains will determine its physical properties, such as melting point, hardness, electrical and thermal conductivity, and solubility. How do chemical bonds occur? As we mentioned before, only the outermost, or valence, electrons of an atom are involved in chemical bonds. Let’s begin our discussion by looking at the simplest element, hydrogen. When two hydrogen atoms approach each other, electron-electron repulsion and proton-proton repulsion both act to try to keep the atoms apart. However, proton-electron attraction can counterbalance this, pulling the two hydrogen atoms together so that a bond is formed. Look at the energy diagram below for the formation of an H–H bond.
As you’ll see throughout our discussion, atoms will often gain, lose, or share electrons in order to possess the same number of electrons as the noble gas that’s nearest them on the periodic table. All of the noble gases have eight valence electrons (s2p6) and are very chemically stable, so this phenomenon is known as the octet rule. There are, however, certain exceptions to the octet rule. One group of exceptions is atoms with fewer than eight electrons—hydrogen (H) has just one electron. In BeH2, there are only four valence electrons around Be: Beryllium contributes two electrons and each hydrogen contributes one. The second exception to the octet rule is seen in elements in periods 4 and higher. Atoms of these elements can be surrounded by more than four valence pairs in certain compounds.
Types of Chemical Bonds
You’ll need to be familiar with three types of chemical bonds for the SAT II Chemistry exam: ionic bonds, covalent bonds, and metallic bonds.
Ionic bonds are the result of an electrostatic attraction between ions that have opposite charges; in other words, cations and anions. Ionic bonds usually form between metals and nonmetals; elements that participate in ionic bonds are often from opposite ends of the periodic table and have an electronegativity difference greater than 1.67. Ionic bonds are very strong, so compounds that contain these types of bonds have high melting points and exist in a solid state under standard conditions. Finally, remember that in an ionic bond, an electron is actually transferred from the less electronegative atom to the more electronegative element. One example of a molecule that contains an ionic bond is table salt, NaCl.
Covalent bonds form when electrons are shared between atoms rather than transferred from one atom to another. However, this sharing rarely occurs equally because of course no two atoms have the same electronegativity value. (The obvious exception is in a bond between two atoms of the same element.) We say that covalent bonds are nonpolar if the electronegativity difference between the two atoms involved falls between 0 and 0.4. We say they are polar if the electronegativity difference falls between 0.4 and 1.67. In both nonpolar and polar covalent bonds, the element with the higher electronegativity attracts the electron pair more strongly. The two bonds in a molecule of carbon dioxide, CO2, are covalent bonds.
