All elements are most stable when their outermost shell is filled with electrons according to the octet rule. This is because it is energetically favorable for atoms to be in that configuration and it makes them stable. However, since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with other atoms thereby obtaining the electrons they need to attain a stable electron configuration. When two or more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water molecule, H2O, consists of two hydrogen atoms and one oxygen atom; these bond together to form water, as illustrated in Figure 1. Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer shells.
Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances used in the beginning of a chemical reaction are called the reactants (usually found on the left side of a chemical equation), and the substances found at the end of the reaction are known as the products (usually found on the right side of a chemical equation). An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction; this direction is not always a “one-way street.” For the creation of the water molecule shown above, the chemical equation would be:
2H + O → H2O
A molecule consists of atoms of an element bonded to another atom of the same element. an example is the O2 molecule seen in Figure 2. Water representatives of a subclass of molecules known as compounds: water contains atoms of more than one type of element.
Some chemical reactions, such as the one shown above, can proceed in one direction until the reactants are all used up. The equations that describe these reactions contain a unidirectional arrow and are irreversible. Reversible reactions are those that can go in either direction. In reversible reactions, reactants are turned into products or products can be turned into reactants.
For example, in human blood, excess hydrogen ions (H+) bind to bicarbonate ions (HCO3–) form carbonic acid (H2CO3). If carbonic acid were added to this system, some of it would be converted to bicarbonate and hydrogen ions.
HCO3– + H+ ↔ H2CO3
The outer electron shell dictates how readily and what type of chemical bonds a particular atom will form. The formation of compounds is often visually outlined in chemical equations which show the reactants participating in chemical reactions to form products.
Check Your Understanding
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