Chemically unstable atoms enter more stable configurations by forming chemical bonds with each other. When two or more atoms bond together, they create a molecule. There are three main types of chemical bonds: ionic bonds, covalent bonds, and metallic bonds. All three types of chemical bonds are formed when atoms enter a lower energy state with a more stable electron configuration by losing, accepting, or sharing electrons.
Elements in the alkali metal group have one valence electron in their outer shell. Elements in the halogen group have seven valence electrons in their outer shell. According to the octet rule, atoms tend to lose, accept, or share electrons until they have an electron configuration with a full outer shell. If an alkali metal atom, such as sodium, is paired with a halogen atom, such as chlorine, then the sodium atom will typically lose an electron and the chlorine atom will typically accept it.
Once the sodium atom loses an electron, it becomes a positively charged ion (it has eleven protons and only ten electrons), and its second electron shell, which is full, becomes the outermost electron shell (the third electron shell no longer has any electrons in it). And once the chlorine atom accepts an electron, it becomes a negatively charged ion (it has only seventeen protons, but eighteen electrons), and its outermost electron shell is now filled with eight valence electrons.
At this point, the sodium ion and the chlorine ion both have more stable electron configurations, but they are still separate ions and have not formed a chemical bond yet. A chemical bond forms because the positively charged sodium ion is attracted to the negatively charged chlorine ion, and an electromagnetic force holds them together. This chemical bond is called an ionic bond.
Ionic bonds form when positively charged and negatively charged ions are held together by electrostatic attraction (the attraction between oppositely charged particles). It actually does not matter if a chlorine ion is attracted to the sodium ion it accepted an electron from, or if it is attracted to some other random sodium ion. An ionic bond has nothing to do with the exchange of electrons; it only has to do with the attraction between ions.
Ions will continue to attract oppositely charged ions until their own charge is balanced. So, while a sodium Na+ ion will form an ionic bond with one Cl- ion, a Mg2+ ion will form ionic bonds with two Cl- ions.
A Ca2+ ion will form an ionic bond with one O2- ion.
And an O2- ion will form ionic bonds with two Na+ ions.
Atoms in the lithium and beryllium groups form positive ions fairly easily. Atoms in the fluorine and oxygen groups form negative ions fairly easily. But what about an atom like carbon? Carbon atoms are the backbone of most organic molecules, including key molecules in biochemistry such as proteins, carbohydrates, nucleic acids, lipids, and fatty acids.
A carbon atom has four valence electrons. To form an ion with a stable electron configuration, a carbon atom would have to either lose four electrons or accept four electrons. After losing one electron, it takes even more energy to lose a second and then a third, never mind a fourth. This is because a positive ion holds onto its electrons more strongly as it becomes more positive. Similarly, accepting an electron takes increasingly more energy as an ion becomes more negative. It would take 2.37 × 10-17 J of energy to remove four electrons from a carbon atom. This is almost thirty times the energy to form a Na+ ion. The probability that a carbon atom will form an ionic bond is incredibly small.
Besides losing or accepting electrons, atoms can also share electrons. This is how a carbon atom forms most of its chemical bonds. For example, a carbon atom can share an electron with a hydrogen atom. The two electrons between the carbon and the hydrogen atom (one from the carbon atom and one from the hydrogen atom) are shared between the two atoms. While the carbon atom still does not have a stable electron configuration, the hydrogen atom now does.
If the carbon atom shares its remaining three valence electrons with three other hydrogen atoms, all five atoms can achieve stable electron configurations. Chemical bonds formed between atoms by the sharing of electrons are called covalent bonds (“co-” means shared and “-valent” refers to valence electrons… shared valence electrons). The region between two positively charged atomic nuclei is incredibly stable for a negatively charged electron. This is why covalent bonds are so strong.
An oxygen atom will easily form an O2- ion, but it needs some other atom or atoms to supply it with two electrons. What happens when an oxygen atom only has other oxygen atoms to interact with? If a carbon atom is not going to lose four electrons to form a positively charged ion with a stable electron configuration, then an oxygen atom is certainly not going to lose six electrons. An oxygen atom has six valence electrons; it needs two more to achieve a stable electron configuration. To get those two additional electrons, it can accept two electrons from somewhere else, or it can share two of its electrons with some other atom or atoms.
An oxygen atom can share two electrons with another oxygen atom to form two covalent bonds. The four electrons between the two oxygen atoms (two from each atom) are shared between them. This is called a double bond. Each oxygen atom has four electrons of its own and four electrons shared with the other oxygen atom. If hydrogen is present, then an oxygen atom can also share one electron with one hydrogen atom and another electron with a second hydrogen atom. This forms two single bonds. (The way we draw the Lewis structure of a molecule is a little different than the way we draw the Lewis structure of an atom. For quantum mechanical reasons, the electrons in a molecule are almost always paired. This is why the oxygen atoms in the O2 molecule have orbitals with paired electrons and a completely empty orbital. In a free oxygen atom, each of the p-orbitals would have received on electron before any of them receive a pair.)
By analyzing the elements’ Lewis structures and applying the octet rule, you can often predict how a group of atoms will chemically bond to form a stable molecule. But keep in mind, some groups of atoms will never form a stable configuration.
I am not going to spend much time on metallic bonds. Metallic bonds generally occur in metal solids, such as a piece of solid aluminum or iron. In solid metals, some of the electrons become “delocalized.” This means that, Instead of being associated with a specific atom or a specific covalent bond, an electron occupies an orbital that extends across a group of three or more atoms. (When a covalent bond forms, the shared electrons occupy a new molecular orbit that extends across the two atoms involved.) When this happens, a metal solid is held together by the electrostatic attraction between positively charged metal ions (the metal atoms are positively charged because they have each lost a certain number of delocalized electrons) and the sea of free moving delocalized electrons. The presence of delocalized electrons is what gives metals their excellent electrical conductivity. You will learn more about delocalized electrons later in this unit.