Atoms can be combined to create new substances through chemical bonding. Bonds are generally classified in Chemistry as “strong” or “weak” bonds. Strong bonds involve an exchange or sharing of electrons and include covalent bonds and ionic bonds. Weak bonds are a result of electrostatic forces and include dipole interactions, London dispersion forces, and hydrogen bonds.
Note: The following discusses chemical bonding as relevant only to the physical sciences section on the MCAT.
In a covalent bond, atoms share electrons. By definition, the elements involved in a covalent bond are of relatively similar electronegativity. One example of covalent bonding is the diatomic gas formed by chlorine, a member of the halogens. Convalent bonds are particularly important in organic compounds, where the magnitude of difference in electronegativity differentiates polar and non-polar covalent bonds.
In an ionic bond, atoms are held together by an electron exchange, where one atom loses electrons while the other gains electrons. By definition, ionic bonds occur between atoms of greatly differing electronegativity. Ionic bonds typically occur between atoms with a complementary number of valence electrons in the outer shell. For example, if an atom has a p orbital with only 5 valence electrons in its outer shell, it will most likely bond with another atom that has a single valence electron.
Ionic bonds include a positively-charged cation and a negatively-charged anion.
Other Chemical Bonds
Other chemical bonds include those that are a result of electrostatic forces between elements and compounds. When electrons are shared between different atoms, the sharing is often unequal. Atoms with greater electronegativity will possess a greater pull on the shared electrons than the others. Since electrons carry a negative charge, the resulting compounds have sides that are more positive and more negative.
Hydrogen bonds, for example, occur between hydrogen atoms that are part of a compound. In water, the oxygen atom is more electronegative, causing the electrons to remain closer to the nucleus of the oxygen atom. As a result, the hydrogen side of the water molecule is more positive than the oxygen side of the water molecule. Conversely, the oxygen side of the molecule has a relatively more negative charge. Ultimately, the hydrogen atoms of one molecule will hold a weak bond with the oxygen atom of another molecule – the hydrogen bond.
Next: Phases and Gas Equillibria.