Download 6.7 – Ionic Compounds

6.7 – Ionic Compounds
Formation of Ions
Valence Electrons – Electrons found in the outermost shell of an atom. These electrons determine the atom’s
chemical properties.
Octet Rule – Atoms will tend to lose electrons (metals) or gain electrons (nonmetals) in order to achieve a
noble gas electron configuration. Therefore, atoms will lose or gain electrons to either have 0 or 8 valence
electrons.
Example: Magnesium is in Group 2A and therefore has 2 valence electrons. Magnesium will lose those 2
valence electrons in order to achieve the octet rule (have 0 valence electrons). Fluorine is in Group 7A and
therefore has 7 valence electrons. So Fluorine will gain 1 valence electron in order to achieve the octet rule
(have 8 valence electrons).
Cation – A metal that has lost its valence electrons, and is a positively charged ion. Alkali metals will lose 1
valence electron and become + (Na+), alkaline earth metals will lose 2 valence electrons and become 2+ (Mg2+),
and Group 3A will tend to lose 3 valence electrons and become 3+ (Al3+). Transition metals will often have
different charges.
Anion – A nonmetal that has gained valence electrons, and is a negatively charged ion. Halogens will gain 1
valence electron and become - (F-), the oxygen family will tend to gain 2 valence electrons and become 2- (O2-),
and the nitrogen family will tend to gain 3 valence electrons and become 3- (N3-).
6.7 – Ionic Compounds
Ionic Compounds
Ionic Compounds – A metal and nonmetal that has transferred electrons. A cation (positively charged ion) and
an anion (negatively charged ion) that has a force of attraction in order to become a neutral charge overall.
Properties of Ionic Compounds – Most ionic compounds are crystalline solids at room temperature. Ionic
attractions result in high melting points. Most ionic compounds can dissolve in water to become an aqueous
solution with ions that are free to move around. Therefore, ionic compounds do not conduct electricity when
they are solids, but when melted or dissolved in water, they conduct electricity (called an electrolyte).
6.7 – Ionic Compounds
Chemical Formulas for Ionic Compounds
Criss-Cross Method – To write the chemical formula for an ionic compound, simply criss-cross the numbers
on the charges in order to cancel them out and place those as subscripts (reduced to the lowest whole number
ratio).
Example: What is the formula for sodium fluoride?
Na has a 1+ charge and F has a 1- charge
So you only need one Na and only 1 F.
Chemical Formula = NaF
Example: What is the formula for magnesium chloride?
Mg has a 2+ charge and Cl has a 1- charge
So you only need one Mg and 2 Cl’s.
Chemical Formula = MgCl2
Example: What is the formula for calcium oxide?
Ca has a 2+ charge and O has a 2- charge.
So although after the criss-cross you would get Ca2O2, you would reduce it to the lowest-whole number ratio.
Chemical Formula = CaO
Naming Ionic Compounds – An ionic compound will be named with the cation followed by the monatomic
(one atom) anion ending in –ide.
Example: What are the names for K3N, NaBr, and CaS? K3N is potassium nitride, NaBr is sodium bromide,
and CaS is calcium sulfide.
6.7 – Ionic Compounds
Ions of Transition Metals
Transition Metals – Since transition metals have often different positive charges, the charge will be given in a
parenthesis after the element name. An example would be copper (II) would be Cu2+ where copper (I) would be
Cu+).
Example: What is the chemical formula for iron (III) oxide?
Fe3+ O2-
Chemical Formula = Fe2O3
Polyatomic Ions
Polyatomic Ions – Ions that are composed of more than one atom. These names often end in –ite or –ate.
When writing a chemical formula with a polyatomic ion, treat the polyatomic as one big thing and place a
parenthesis around it and do the criss-cross the same way you did previous ionic compounds.
List of Common Polyatomic Ions
1- Charge




Hydroxide
Acetate
Nitrate
Bicarbonate
OH –
C2H3O2 –
NO3 –
HCO3 –
2- Charge


Sulfate
Carbonate
SO4 2CO3 2-
3- Charge

Phosphate
PO4 3-
Example: What is the chemical formula for Sodium Carbonate?
Na+ CO3 2-
Chemical Formula = Na2CO3
Example: What is the chemical formula for Magnesium Phosphate?
Mg2+ PO4 3-
Chemical Formula = Mg3(PO4)2