The atoms of the elements consist of a nucleus containing positively charged protons and neutrally charged neutrons. Negatively charged electrons are arranged outside the nucleus. The atoms of each element differ by their number of protons, neutrons, and electrons. For example, hydrogen has one proton, one electron, and no neutrons, while carbon has six protons, six neutrons, and six electrons. The number and arrangement of electrons of an atom determine the kinds of chemical bonds that it forms and how it reacts with other atoms to form molecules. There are three kinds of chemical bonds:
- Ionic bonds form between two atoms when one or more electrons are completely transferred from one atom to the other. The atom that gains electrons has an overall negative charge, and the atom that donates electrons has an overall positive charge. Because of their positive or negative charge, these atoms are ions. The attraction of the positive ion to the negative ion constitutes the ionic bond. Sodium (Na) and chlorine (Cl) form ions (Na + and Cl –), which attract one another to form the ionic bond in a sodium chloride (NaCl) molecule. A plus or minus sign following a chemical symbol indicates an ion with a positive or negative charge, which results from the loss or gain of one or more electrons, respectively. Numbers preceding the charges indicate ions whose charges are greater than one (Ca 2+, PO 4 3–).
- Covalent bonds form when electrons are shared between atoms. That is, neither atom completely retains possession of the electrons (as happens with atoms that form ionic bonds). A single covalent bond occurs when two electrons are shared (one from each atom). A double or triple covalent bond is formed when four or six electrons are shared, respectively. When the two atoms sharing electrons are exactly the same, as in a molecule of oxygen gas (two oxygen atoms to form O 2), the electrons are shared equally, and the bond is a nonpolar covalent bond. When the atoms are different, such as in a molecule of water (H 2O), the larger nucleus of the oxygen atom exerts a stronger pull on the shared electrons than does the single proton that makes up either hydrogen nucleus. In this case, a polar covalent bond is formed because the unequal distribution of the electrons creates areas within the molecule that have either a negative or positive charge (or pole), as shown in Figure 1.
- Hydrogen bonds are weak bonds that form between the partially positively charged hydrogen atom in one covalently bonded molecule and the partially negatively charged area of another covalently bonded molecule. An individual water molecule develops a partially positively charged end and a partially negatively charged end; see Figure 1. Hydrogen bonds form between adjacent water molecules. Since the atoms in water form a polar covalent bond, the positive area in H 2O around the hydrogen proton attracts the negative areas in an adjacent H 2O molecule. This attraction forms the hydrogen bond; see Figure 1(b).
Figure 1. Two examples of chemical bonds.