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2.6 Ionic and Molecular Compounds - In ordinary chemical reactions, the atom's nucleus is not involved. Electrons are transferred between atoms or they are shared.

The Periodic Table is helpful in predicting whether an atom will gain or lose electrons. Metals (and Hydrogen) on the left will lose electrons . . . non-metals will gain electrons. If the metal or nonmetal's atomic number is within 3 of the nearest Noble gas, they will lose or gain electrons until their number of electrons equals the noble gas' number of electrons. Carbon, being in the middle, can either gain 4 electrons (to be like Ne) or lose 4 electrons (to be like He).

If you have access to a Periodic Table (and you always will) and can identify metals and nonmetals, then you will have "learned" H⁺, Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Sr²⁺, B³⁺, Al³⁺ and H^-, F^-, Cl^-, Br^-, I^-, O^2-, S^2-, N^3- You can now write the formula for 104 (13 × 8) ionic compounds . . . . compounds formed when there is a transfer of electrons between two atoms. One atom . . . the loss of a negative electron makes the atom positive. It becomes a metal cation. and the other atom . . . the gain of a negative electron makes the atom negative. It becomes a nonmetal anion. Since compounds (ionic and molecular) are neutral, the electrons lost must equal the electrons gained. Recall that electrons are directly related to charge . . . . so, an easier way to think about this is that the positive charges must equal the negative charges.

Lithium and Chlorine react to form Lithium chloride, LiCl. Lithium forms the Li⁺ cation and Chlorine forms the Cl^- anion. Only LiCl represents the smallest, whole-number ratio (1:1) where the overall charge is zero.

Aluminum and Bromine react to form Aluminum bromide, AlBr₃. Aluminum forms the Al³⁺ cation and Bromine forms the Br^- anion. Only AlBr₃ represents the smallest, whole-number ratio (1:3) where the overall charge is zero.

Magnesium and Nitrogen react to form Magnesium nitride, Mg₃N₂. Magnesium forms the Mg²⁺ cation and Nitrogen forms the N^3- anion. Only Mg₃N₂ represents the smallest, whole-number ratio (3:2) where the overall charge is zero.

In addition to the monoatomic ions there are several polyatomic ions that are used frequently in chemistry.

Initially, it may seem like a lot of memorization, but the Periodic Table comes to the rescue again. Each polyatomic ion above contains Oxygen and/or Hydrogen. We know Oxygen has a -2 charge and Hydrogen has a +1 charge . . . . we can use this information to "calculate" the charge on these polyatomic ions. Be aware that this "calculation trick" only works for the ionic compounds listed in the table above, it does not work for all ionic compounds.

When to use this "trick" . . . .

• if you remember that sulfate is SO₄ but you don't remember if the charge is ^-, 2^- or 3^-
• if you remember that sulfate has a 2- charge, but you're not sure if it is SO₂^2-, SO₃^2-, or SO₄^2-

How to use this "trick". Let's use the hydroxide ion as an example. Suppose you know that the hydroxide ion is "OH", but can't remember its charge. Since the oxide anion has a charge of -2 and the hydrogen cation has a charge of +1,  the charge of a hydroxide ion equals -1 (-2 + 1) .

Ionic compounds containing Oxygen and another element: since Oxygen has a charge of -2, the other element must have a positive charge that is based on its position in the Periodic Table. For example, the nitrate ion (NO₃^-) has three Oxygens (-2 × 3 = -6), so the Nitrogen must have a positive charge. We know that the element Nitrogen wants to gain 3 electrons to become N^3- and resemble Neon, but it can also lose 5 electrons to be like Helium, N⁵⁺. The charge on the nitrate ion equals -1 (+5 – 6).

Carbonate, CO₃^2-: 3 O^2- + C⁴⁺ =  -2 charge 
Sulfate, SO₄^2-: 4 O^2- + S⁶⁺ =  -2 charge 
Phosphate, PO₄^3-: 4 O^2- + P⁵⁺ =  -3 charge 
Chromate, CrO₄^2-: 4 O^2- + Cr⁶⁺ =  -2 charge    (the transition metal, Cr, must lose 6 electrons to resemble Argon)
Permanganate, MnO₄^-: 4 O^2- + Mn⁷⁺ =  -1 charge    (the transition metal, Mn, must lose 7 electrons to resemble Argon)

Ionic compounds containing Hydrogen and another element: the other element must have a negative charge that is based on its position in the Periodic Table. For example, the ammonium ion has four Hydrogens (+1 × 4 = +4), so the Nitrogen must have its typical negative charge (-3).  The charge on the ammonium ion equals +1 (+4 -3) .

Ionic compounds containing Oxygen, Hydrogen and another element: the other element must have a positive charge that is based on its position in the Periodic Table. For example, the bicarbonate ion (HCO₃^-) has three Oxygens (-2 × 3 = -6) and one Hydrogen (+1), so the Carbon must have a positive charge. Carbon will lose 4 electrons to become C⁴⁺, like Helium.  The charge on the bicarbonate ion equals -1 (-6 + 1 + 4) .

Hydrogen sulfate, HSO₄^-: 4 O^2- + H⁺ + S⁶⁺ =  -1 charge 
Hydrogen phosphate, HPO₄^2-: 4 O^2- + H⁺ + P⁵⁺ =  -2 charge 
Acetate, C₂H₃O₂^-: 2 O^2- + C⁴⁺ + C^4- + 3 H⁺ =  -1 charge    (this one has a twist in that one Carbon is +4 and one is –4)

Of course, once you have memorized these polyatomic ions and their charges, this "trick" will not be needed.

When an ionic compound contains a polyatomic ion that needs to be represented two or more times, the ion is encased in parentheses and the subscript follows the closing parentheses. Ammonium phosphate is written as (NH₄)₃PO₄ while Aluminum sulfate is written as Al₂(SO₄)₃ .