Elements are substances that cannot be further broken down into other substances. Each element is made up of only one type of atom. An atom is the smallest unit of matter that still retains the properties of a specific element. Three subatomic particles comprise an atom: protons that have positive electrical charges, electrons that have negative electrical charges, and neutrons that have no electrical charge. Protons and neutrons are located inside the nucleus, or core of the atom, while electrons move around the exterior of the nucleus in a cloud.
Of the three types of subatomic particles, electrons are the most important in determining the behavior of an atom when it comes into contact with another atom. Within the electron cloud, electrons move around in electron shells. Each shell corresponds to an energy level; the farther the electron is from the nucleus, the higher the corresponding energy of the electron. The innermost electron shell can accommodate two electrons while the second and third electron shells can accommodate up to eight electrons.
The outermost shell is the most critical shell in determining the chemical properties of an atom. The electrons in the outer shell are called valence electrons. In general, atoms like to have "full" shells. If their shells are full, they are considered inert and will not tend to react with other atoms. An example of an inert gas is Argon. If the outermost electron shell is not full, the element is more likely to react with other elements to form molecules. When atoms react with other atoms, they do so by transferring or sharing electrons in the outer shells: a process that forms a chemical bond between the two atoms. The three types of bonds formed include:
•Ionic bonds: These are formed when elements give up their outermost electrons to another element or atom, resulting in an ion, or charged atom. For example, chlorine and sodium are electrically neutral; however when chlorine takes a negatively charged electron from a sodium atom, the chlorine becomes negatively charged (called an anion), while sodium becomes positively charged (called a cation). Because sodium and chlorine are now oppositely charged ions, they attract one another and form an ionic bond.
•Covalent bonds: These are the strongest bonds because they involve two atoms sharing electrons. For example, H2 is formed when two hydrogen atoms share their valence electron with each other. If each atom shares one electron, it is a single bond; if each shares two electrons, it is considered a double bond; and if each shares three, it is a triple bond. The maximum number of bonds that an element can form is directly correlated to the number of electrons needed to fill the outer electron shell. For example, carbon (C) can form a maximum of four bonds as carbon atoms contain four electrons in the outer shell.
•Hydrogen bonds: These are weak bonds that form between hydrogen and other elements. An important example of hydrogen bonding is seen in water molecules. Hydrogen bonds form as a result of the structure of the water molecule. A water molecule forms a V-shape, with the oxygen atoms on one side of the molecule, and the hydrogen atom on the opposite side of the molecule. Hydrogen and oxygen form covalent bonds; however the electrons are not shared equally. Instead, the oxygen atom has a slightly stronger pull and as a result, this creates a slightly more negative charge to the oxygen atom and a slightly more positive charge on each hydrogen atom. The unequal sharing of electrons forms a polar molecule with opposite charges on each end. The charges on one water molecule result in an attraction to the opposite charges of a neighboring water molecule. Therefore, water molecules have a tendency to align