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Ionic and Covalent Compound Help

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By — McGraw-Hill Professional
Updated on Aug 28, 2011

Ionic Compounds

Ionic compounds are held together by strong ionic bonds. For example, when a metal reacts with a non-metal, an ionic compound forms. Ionic compounds are commonly hard crystalline solids with high melting points due to the strength of their ionic bonds. Ionic compounds serve as electrolytes (they conduct electricity), when dissolved in water to form solutions.

Naming Ionic Compounds

When naming an ionic compound, the first element gets the name root and the second adds on the ending “ ide .” The easiest example to remember is when sodium (Na) combines with chlorine (Cl) to form NaCl or sodium chlor ide . Other ionic compounds include calcium ox ide , lithium hydr ide , magnesium brom ide . ( Note : the metal cations are named first followed by the non-metal anions.)

Table 9.1 Monatomic and polyatomic anions are important to keep track of in reactions.

Atomic Number and Ions Ionic Compounds Naming Ionic Compounds

When a chemist is trying to tell the difference between two ions of the same element, there are two ways to do it: the Stock system or the common naming system.

Stock System

The first, the Stock system , invented by German Chemist Alfred Stock, at Cornell University, uses Roman numerals to show which ion is which. When referring to lead (Pb 2+ ) you would say “lead two” and write it Pb(II).

In the Stock system, FeCl 2 would be called iron(II) chloride, FeCl 3 would be iron(III) chloride, and PbBr 4 would be called lead(IV) bromide. The IUPAC naming system uses the Stock method.

Common Naming System

The second method or common naming system uses the endings (suffixes) to distinguish between chemical forms. If an experiment calls for cuprous sulfate, it is talking about a copper Cu + ion. The ending “ous” is used to name the lesser charged ion of the different copper forms. The copper ion Cu 2+ is known as a cupr ic ion, while Cu + is called the cupr ous ion, since it is the lesser charged ion of the two forms. The iron ion Fe 3+ is called a ferr ic ion, while Fe 2+ is known as a ferr ous ion.

Naming ionic compounds in chemistry is fairly simple when you remember to name the cation (+) first, followed by the anion (–). Writing formulas for ionic compounds is easy if you remember to keep the charges balanced. Then they almost name themselves.

In the compound MgCl 2 (magnesium chloride), one magnesium ion (Mg 2+ ) combines with two chloride ions (Cl ). This combination makes the final molecule electrically neutral. It is written MgCl 2 . The subscript 2 attached to the chloride shows that 2 ions of negatively charged chlorine ions are needed to balance the formula.

Simple math is involved in writing formulas of polyatomic ions, but it is not rocket science. Just take one step at a time.

To find the lowest common multiple (LCM) , multiply each charge by whatever number works to give the least common multiple.

(Tip: lowest common multiple is easiest to use when the charge from one ion is used as the multiplier for the other ion.)

When writing the formula of gallium oxide, first write the element symbols, Ga 3+ and O 2− . The charges are dropped when you write this. Look at the LCM of 3 and 2 or 6. To get 6 Ga and 6 O, you multiply

2 (Ga 3+ ) + 3 (O 2− ) = 2 ( + 3) + 3( 2) = 0

So the formula is written Ga 2 O 3 .

To write the formula for ammonium carbonate, (NH 4 ) 2 CO 3 , parentheses are used to show that two ions of ammonium are needed to balance one carbonate ion. ( Note: two NH 4 ions balance one Atomic Number and Ions Ionic Compounds Common Naming System ion.) Parentheses are used when writing the chemical formula of compounds of polyatomic ions. Parentheses are not needed unless a subscript is used. For the compound lead sulfate, PbSO 4 , only one ion of lead is used, so no parentheses are needed.

Remember, to name a compound from its formula, write the name of the positive ion (cation) first, followed by the name of the anion (negative ion). So Li 2 S is lithium sulfide.

In the case of some polyatomic ions, the naming of the different combining types can be remembered by checking the beginnings and endings of the element name. When there are only two types, the “ate” ending is used for the ion with more atoms. A sulfur atom may have four oxygen atoms around it (sulf ate ion, Atomic Number and Ions Ionic Compounds Common Naming System ) or three oxygen atoms (sulf ite ion, Atomic Number and Ions Ionic Compounds Common Naming System ). When four different types exist then the following rule applies:

  • an ion with 4 other atoms is named: “per___ate”
  • an ion with 3 other atoms is named: “___ate”
  • an ion with 2 other atoms is named: “___ite”
  • an ion with 1 other atom is named: “hypo___ite”

For polyatomic bromine ions, the naming would be per brom ate ( Atomic Number and Ions Ionic Compounds Common Naming System ), brom ate ( Atomic Number and Ions Ionic Compounds Common Naming System ), brom ite ( Atomic Number and Ions Ionic Compounds Common Naming System ), and hypobromite , (BrO 1− ). Figure 9.2 shows some clues to help with chemical naming.

The characteristics of different compounds are affected by their ionic character. It is easier to understand element bonding if you memorize or make flash cards to study these ionic forms and their types.

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