Chapter 5: 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 the nearest 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.
