Context: A covalent bond is the force of attraction that holds together two atoms that share a pair of electrons. The shared electrons are attracted to the nuclei of both atoms. Covalent bonds form only between atoms of nonmetals. The two atoms may be the same or different elements. If the bonds form between atoms of different elements, a covalent compound forms. Covalent compounds are described in detail later in the lesson. To see a video about covalent bonding, go to this URL: (6:20). MEDIA Click image to the left or use the URL below. URL: Figure 7.7 shows an example of a covalent bond forming between two atoms of the same element, in this case two atoms of hydrogen. The two atoms share a pair of electrons. Hydrogen normally occurs in two-atom, or diatomic, molecules like this (di- means "two"). Several other elements also normally occur as diatomic molecules: nitrogen, oxygen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the hydrogen atoms in Figure 7.7. Alone, each hydrogen atom has just one electron. By sharing electrons with another hydrogen atom, it has two electrons: its own and the one in the other hydrogen atom. The shared electrons are attracted to both hydrogen nuclei. This force of attraction holds the two atoms together as a molecule of hydrogen. Some atoms need to share more than one pair of electrons to have a full outer energy level. For example, an oxygen atom has six valence electrons. It needs two more electrons to fill its outer energy level. Therefore, it must form two covalent bonds. This can happen in many different ways. One way is shown in Figure 7.8. The oxygen atom in the figure has covalent bonds with two hydrogen atoms. This forms the covalent compound water. In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. Figure 7.9 shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. For another example of polar bonds, see the video at this URL: (0:52). MEDIA Click image to the left or use the URL below. URL: In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral. Figure 7.10 shows an example of nonpolar bonds. Covalent bonds between atoms of different elements form covalent compounds. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates. These are compounds in living things. Helpful Hints Naming Covalent Compounds Follow these rules in naming simple covalent compounds: The element closer to the left of the periodic table is named first. The second element gets the suffix ide. Prefixes such as di- (2) and tri- (3) show the number of each atom in the compound. These are written with subscripts in the chemical formula. Example: The gas that consists of one carbon atom and two oxygen atoms is named carbon dioxide. Its chemical formula is CO2 . You Try It! Problem: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? What is its chemical formula? Covalent compounds have different properties than ionic compounds because of their bonds. Covalent compounds exist as individual molecules rather than crystals. It takes less energy for individual molecules than ions in a crystal to pull apart. As a result, covalent compounds have lower melting and boiling points than ionic compounds. Many are gases or liquids at room temperature. Covalent compounds have shared electrons. These are not free to move like the transferred electrons of ionic 
 Question: force of attraction holding together two atoms that share a pair of electrons
Output: covalent bond