Covalent bond

A covalent bond is created when two nonmetal atoms join by sharing electrons. In general, the chemical compounds created by covalent bonds do not conduct electricity and have low boiling and melting points.

Structure of an Atom

Understanding the structure of an atom is crucial to understanding how covalent bonds form. At the center of the atom is the nucleus, which contains protons and neutrons. Protons are subatomic particles that have a positive charge, and neutrons are subatomic particles that have no charge. Electrons, which have a negative charge, are another important part of the atom.

Electrons orbit the nucleus in energy levels, also called shells. Each electron shell can hold only a certain number of electrons. For example, in some atoms, the first shell can hold only two electrons, and the second and third shells can hold up to eight.

Types of Chemical Bonding

There are two main types of chemical bonding: ionic bonding and covalent bonding. It is important to understand how ionic bonding works to demonstrate how covalent bonding differs.

Ionic Bonding

Most atoms want to have full outer electron shells because this makes them stable. In some cases, atoms will try to gain or give up electrons so their outer shells are full. For example, if an atom has only one electron in its outer shell, it will try to give up its electron to another atom. By losing an electron, the second shell, which is full, will become the outermost shell.

When a metal and a nonmetal trade electrons so that both have full outer shells, they create an ionic bond. In the process of ionic bonding, the two atoms become two ions. The metal, which loses an electron, becomes a positively charged cation. Conversely, the nonmetal, which gains an electron, becomes a negatively charged anion. Atoms can trade more than one electron in an ionic bond, and the number of electrons gained or lost will determine the overall charge of the newly created ions.

An example of ionic bonding can be observed when potassium, a metal, forms a compound with iodine, a nonmetal. Potassium has one electron in its outer shell, and iodine has seven. Both are unstable because their outer shells are not full. When the two elements bond, potassium gives up the electron in its outermost shell to iodine, which then has eight electrons. Potassium's third electron shell is empty, so its second shell becomes its outermost shell. Now, both the potassium and the iodine atoms are stable. In addition, the iodine atom has become an anion with an overall charge of 1 -, and the potassium atom has become a cation with an overall charge of 1 +.

Covalent Bonding

Another form of chemical bonding is covalent bonding. Atoms of nonmetals form covalent bonds by sharing pairs of electrons rather than trading them. This sharing creates a molecule instead of an ion.

When a single pair of electrons is shared between two atoms, a single covalent bond is created. Usually, each atom contributes one electron to form this bond.

A water molecule contains two single covalent bonds. In a water molecule, two atoms of hydrogen combine with one atom of oxygen. The two hydrogen atoms each only have one electron that occupies the first electron shell, which can hold a maximum of two electrons. Both hydrogen atoms need one more electron to fill their shells. The oxygen atom has six electrons in its outermost shell, so it needs two additional electrons to fill this shell. By forming two single covalent bonds, the hydrogen atoms and the oxygen atom will gain the electrons they need to become stable.

The oxygen atom shares one electron with one hydrogen atom and another with the other hydrogen atom. The two hydrogen atoms also share their electrons. Now, both hydrogen atoms each have two electrons in their first electron shells, and the oxygen atom has eight electrons in its outermost shell.

Multiple covalent bonds are also possible. This occurs when atoms share more than one pair of electrons. A good example of multiple bonds can be seen when two nitrogen atoms combine. Each nitrogen atom has five electrons in its outer shell, meaning that it would need three electrons to become stable. When two nitrogen atoms bond, they share three electron pairs so that they each have eight electrons in their outermost shells. There are now three covalent bonds holding the two nitrogen atoms together.

Covalent bonds may result in molecules that have a neutral charge. This is because no electrons were gained or lost in the creation of the molecule. However, sometimes the atoms in a molecule do not share electrons equally. Due to various factors, including the size of each atom and the distance of the shared electrons to the atoms' nuclei, one atom may have a greater attraction for electrons. This attraction is the atom's electronegativity. The electrons in the covalent bond will often stay closer to the atom with the greater electronegativity.

This unequal sharing creates a polar covalent bond, in which one atom has a slightly positive charge and the other has a slightly negative charge. For example, a fluorine atom has a greater electronegativity than a hydrogen atom does. In a compound of these two atoms, the shared electrons stay closer to the fluorine atom, giving it a slightly negative charge, and stay away from the hydrogen atom, which gives it a partial positive charge. This creates a dipole, which means there are two separate areas, or poles, with opposite charges. When two atoms share electrons equally, they create a nonpolar covalent bond.

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