Chemistry 10050 - Ions and Ionic Compounds

Periodic Table

The periodic table can be used to predict the ground state electron configuration of atoms and the charge of many atoms in ionic compounds. Metals have a relatively small number of electrons in their outermost (valence) shell. In ionic compounds, metals lose these electrons to form positively-charged ions called cations. The valence shell of non-metals is almost filled, so non-metals tend to gain electrons to form negatively-charged ions called anions.

Charge +1 +2 ? ? ? ? ? ? ? ? ? ? (+3) ? -3 -2 -1 0
  1   18
Row 1 H 2 13 14 15 16 17 He
Row 2 Li Be B C N O F Ne
Row 3 Na Mg 3 4 5 6 7 8 9 10 11 12 Al Si P S Cl Ar
Row 4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Row 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Row 6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Row 7 Fr Ra Ac Db Jl Rf Bh Hn Mt 110 111
.
s1 s2 d1 d2 d3 d4 d5 d6 d7 d8 d9 d10 p1 p2 p3 p4 p5 p6
s-block (row-1) d-block p-block

For example, calcium (Ca) has an electron configuration of
... 4s2

Loss of these two electrons gives the Ca+2 ion.

Polyatomic ions

Polyatomic ions are combinations of two or more atoms held together by covalent bonds that have a charge. The following table needs to be memorized.

Ammonium NH4+1   Hydroxide OH-1      
Nitrite NO2-1   Sulfite SO3-2   Phosphate PO4-3
Nitrate NO3-1   Sulfate SO4-2   Phosphate PO4-3
      Bisulfate HSO4-1   Hydrogen Phosphate HPO4-2
Carbonate CO3-2   Bicarbonate HCO3-1   Dihydrogen Phosphate H2PO4-1

Ionic Compounds

Definitions

Ion
Charged particle (element or small molecule).
Cation
Positively charged ion. Formed by loss of one or more electrons (e-).
Anion
Negatively charged ion. Formed by gaining one or more electrons (e-).
Salt
Ionic compound, containing both anions and cations. Total charge must equal zero. Physical properties of salts include high melting point, crystalline solid, function as electrolytes (solutions in water conduct electricity), and are typically water soluble.
Ionic Bond
Electrostatic forces between charged particles. Attractive for ions of opposite charges; Repulsive for ions having the same charge.
Octet Rule
Whenever possible, main group elements tend to gain or lose electrons to form ions that have a noble gas electron configuration. Main group elements typically do not gain or lose more than 3 electrons to achieve this. The octet rule does NOT generally apply to transition metals.

Nomenclature (Naming compounds)

Salts
Cation name given first, followed by anion.
Main Group Cations
Use element name (unchanged).
Transition Metal Cations
Follow element name with Roman number in parentheses to indicate charge of metal. [Ex. Fe+3 named as iron (III)].
Main Group Anions
Change suffix of element name to end in -ide. [Ex. S-2 is sulfide].
Polyatomic Ions
Memorize ions listed above

Determining the formula of ionic compounds

Although ionic compounds contain charged paticles, they have a total charge of zero. For this to happen, the total positive charge must be equal in magnitude to the total negative charge.

For example, in calcium chloride, the ions Ca+2 and Cl-1 are present. To balance the +2 charge of Ca+2, two chloride (Cl-1) ions are required. Thus, the formula of calcuim chloride is CaCl2.

Determining Charge on Transition Metal Ions

All salts have a total charge of zero. To determine the charge on the transition metal in a salt of known formula. Determine the total negative charge. The total positive charge will be equal to this, which should be divided equally between all of the transition metals present.

For example, in V2O5, the oxygen anion has a charge of -2 (O-2). The total negative charge is 5*(-2)=-10. Thus, the total positive charge must be +10. Dividing this positive charge on each of the two vanadium atoms must be 10/2 = +5 (V+5).

For polyatomics, the same procedure can be used. In Ni3(PO4)2, the total negative charge is 2*(-3)=-6. Thus, the total positive charge is +6, and the charge on each nickel ion must be +6/3=2 (Ni+2).