Chapter 2: Atomic Structure-Ionic and Covalent Compounds: Writing formulas of Ionic Compounds
Ionic compounds which are generally composed of a metallic
element and a nonmetallic element, 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, which have as many electrons as the
preceeding noble gas element.
Chlorine, Cl, will gain one electron, forming Cl-, to
have as many as argon. Nonmetals generally gain
electrons, forming anions, to have as many electrons as a noble gas
element. Substances with formulas 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
with a formula of 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.
Polyatomic ions are characterized as ions, charged species, with
more than one type of atom. Some examples include
NO3-, SO42-,
OH-,and NH4+. What would be the
formula of a compound containing aluminum and nitrate? Aluminum, as a
metal, loses electrons to have as many electrons as the nearest noble
gas. In this case aluminum forms a stable 3+ cation. Nitrate,
NO3-, combines with Al3+. The
formula of the compound is Al(NO3)3. The
parenthese are needed to clearly indicate the presence of three
nitrate ions.
Remember the charge on an ion is a result of the lose or gain of
electrons, 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. The formula for ionic
compounds is determined by balancing the positive charge of the
cations and the negative charge of the anion to form an electrically
neutral compound.
What is the formula for a compound containing calcium and
phosphate? Ca loses two electrons to form Ca2+. Phosphate
has the formula PO43-. Manipulating the charges
to achieve electroneutrality means three calcium ions balance two
phosphate ions and the formula is
Ca3(PO4)2.
Here is an alphabetic list of
chemical
names with the corresponding formula.
