Writing Chemical Formulas

When the elements are combined compounds are formed. Compounds are not considered to be elements because compounds can be decomposed into elements. The elements which combine to form compounds do so in fix ratios of atoms. For example, water contains two atoms of hydrogen and one atom of oxygen.

Substances, which are not elements, can be classes as either ionic or covalent compounds. Although we will spend time later in the semester discussing the physical and chemical properties of ionic and covalent compounds. Let's consider two familiar examples one from each group. The two compounds are sodium chloride (NaCl) and sucrose (C₁₂H₂₂O₁₁ cane sugar). NOTE: One of the students in our class reminded me that cane sugar is initail brown and is then bleached to give its familiar white color.) Both are white crystalline solids. However, sodium chloride melts at 801 degrees C and boils at 1465 degrees C. Sucrose melts at 185 degrees C and at higher temperatures, decomposes.

A key difference between these two classes of compounds is apparent when each is dissolved in water and the resulting solution tested for its ability to conduct an electric current. Sodium chloride, which dissolves to an extent of 36 g in 100 g of water, conducts electricity very well. However, sugar, which dissolves to an extent of 180 g in 100 g of water, does not conduct electricity at all.

An explanation of this behavior was proposed by Svante Arrhenius in 1884. Arrhenius suggested that when sodium chloride dissolved the NaCl dissociated into ions. Ions are atoms or groups of atoms with an electrical charge. Arrhenius postulated that sodium formed Na⁺ and chlorine formed Cl^- ions. So sodium chloride consists of positively charged sodium atoms and negatively charged chloride ions which are released into the solution when NaCl dissolves.

Sucrose, while it dissolves in water, does not dissociate into ions in solution.

Sodium chloride is an example of an ionic compound and sucrose is an example of a covalent compound.

So ionic compounds contain ions, which carry either a positive or negative charge. Covalent compounds contain neutral molecules, which are not electrically charged.

An ion is a positively or negatively charged atom or molecule. Ions are formed when an atom gains or loses an electron. The alkali metals like to lose an electron. Sodium is an example. As the neutral atom sodium has 11 protons and 11 electrons. The lose of one electron give the ion 10 electrons and Na becomes Na⁺. All of the alkali metals lose an electron when they combine with other elements. Alkaline earth metals lose two electrons forming doubly charged cations, i.e. Mg²⁺. It is interesting to note that for the monatomic atoms the number of the electrons lost or gain can be correlated with the number of electrons in the nearest noble gas. When sodium loses one electron it has as many electrons as the noble gas neon. In general metals lose electrons, forming cations, to have as many electrons as a noble gas element. Chlorine, Cl, will gain one electron, Cl^-, to have as many as argon. Nonmetals generally gain electrons, forming anions, to have as many electrons as the nearest noble gas element.

Compounds containing metallic elements and nonmetallic elements are called ionic compounds. Writing formulas consists of balancing the charges on the cation and anion to produce a formula with a balance of positive and negative charge. Sodium and chlorine form a compound sodium chloride whose formula is NaCl. Sodium likes to lose one electron and chlorine likes to gain one electron. A formula of NaCl balances Na⁺ and Cl^-. A compound containing calcium and bromine would have a formula of CaBr₂.

So the key is; metals lose electrons to form cations, nonmetals gain electrons to form anions. Metals generally lose a number of electrons equal to their group number. Group IA elements lose one electron, Group IIIA elements lose three electrons. Nonmetals gain electrons equal to the group number minus 8. Chlorine, in Group VIIA, gains 8 - 7 = 1 electron. Oxygen gains two electrons.