Unit 4

Chemistry Unit 4 - Understanding & Naming Compounds

Elements can join together to form compounds in 2 ways. One atom can take electrons from another atom (ionic bond), or the two atoms can share electrons (covalent bond). The two different types of compounds are named using two separate naming systems. It is important that you recognize what kind of compound you have, and that you know the naming system for both sets of compounds.

Chemical formula: a short-hand to represent compounds using symbols and oxidation numbers.

Binary compounds are compounds composed of two elements.

Ionic Compounds

Ionic compounds are formed when two or more ions join together. This is most easily seen when either a metal is the first listed element, or ammonium is present. Every ion in a compound has a charge or oxidation number which is based upon the total number of electrons that are gained or lost by the atom.

Polyatomic ion - An ion that consists of more than one atom.

Although compounds containing polyatomic ions usually have more than two atoms, we will write the formulas of these compounds like binary compounds. The polyatomic ion will be either the positive or the negative half of the compound.

Never change the subscripts in a polyatomic ion.

If more than one polyatomic ion is needed in a formula, put parenthesis around the ion and the subscript needed outside the parenthesis. Example:

	The cyanide ion is CN ^-. To balance oxidation numbers, two cyanide ions are needed to combine with calcium, which has an oxidation number of +2.
	The formula for calcium cyanide is Ca(CN)₂.

The sum of the oxidation numbers in a polyatomic ion is equal to the ion's charge.

Indicating numbers of atoms and molecules:

	Subscripts, small numbers to the lower right of a symbol, represent the number of atoms of that element in the compound.
	Coefficients, large numbers to the left of formulas in equations, represent the number of molecules.
	Subscripts and coefficients of 1 are "understood" - NEVER WRITTEN

A simple way to learn to write chemical formulas for ionic compounds:

Write the oxidation number above each element.
Cross these and write the oxidation number (the absolute number) of one element as the subscript of the other element.
Reduce the subscripts to their simplest form, if needed.

Three steps for writing ionic formulas

Naming Ionic Compounds

Ionic compounds either start with a metal or with ammonium

Naming Binary Compounds

1st - Write the name of the positive element

2nd - Write the name of the negative element with an -ide ending

Naming Compounds containing polyatomic ions

1st - Write the name of the positive ion

The only positive polyatomic ion in this class is ammonium, NH₄⁺

2nd - Write the name of the second ion

The following list shows the names of some common polyatomic ions that you should know.

hydroxide ion, OH^-

nitrate ion, NO₃^-

acetate ion, C₂H₃O₂^-

carbonate ion, CO₃^2-

sulfate ion, SO₄^2-

phosphate ion, PO₄^3-

In addition to these ions, there is also a pattern for naming other polyatomic ions. That pattern deals only with Oxyanions, or negative ions that include one non-metal and some oxygens. The pattern works like this:

Number of oxygens	Name	Examples
Most	per______ate	ClO₄^- , perchlorate
	______ate	ClO₃^- , chlorate
	______ite	ClO₂^- , chlorite
Least	hypo_____ite	ClO^- , hypochlorite

This pattern works for all of the oxyanions. For example, NO₃^- is nitrate while NO₂^- is nitrite, SO₄^2- is sulfate while SO₃^2- is sulfite.

Names containing Roman Numerals

Some elements have more than one possible oxidation number. This occurs in most transition metals and the p block elements in period 4 and later. You can tell if an element has more than one oxidation number if it has a roman numeral following the ion name on your sheet of ions. When naming a compound with an ion that has Roman numerals, you must include the Roman numeral in your name. The following periodic chart shows charges of some common monatomic ions.

Naming Covalent Compounds

Covalent compounds start with a non-metal

Covalent compounds, unlike ionic compounds, can be made with different proportions of elements. For example, carbon and oxygen can combine as CO, or CO₂. Because of this, when we name the compounds, they must be named differently. We use prefixes to tell how many of each atom are present in the compound. Prefixes will be used on all elements, with the exception of the first element if it is 1.

The first element does not need a prefix if it has one atom.

Prefixes

1 - mono	4 - tetra	7 - hepta
2 - di	5 - penta	8 - octa
3 - tri	6 - hexa	9 - nona

Naming Acids

All acids start with a Hydrogen.

Acids are named based upon the name of the negative ion. Binary acids and ternary acids are named differently.

Binary Acids

       Binary acids include hydrogen and only one other element. These acids are named hydro________ic         acid.

                Example:      HCl - Hydrochloric acid

Ternary Acids

    Ternary acids are formed from hydrogen bonding to a polyatomic ion. These acids follow a naming system, where the ending of the polyatomic ion is changed. Hydro is not included in the name.

If the ending of the polyatomic ion is:	Then the ending of the acid is:
ate	ic
ite	ous

Flowchart to Naming any compound
Look at the first element Is it a Metal, non-metal, or hydrogen?
Metal		Non-metal		Hydrogen
Does it have more than one charge?		Is it the ammonium ion (NH₄)⁺		Is the negative ion monatomic or polyatomic
Yes Use the negative ion to determine which positive ion is being used	No Write the name of the compound without Roman numerals	Yes Treat it as ionic and don't use prefixes.	No Write the name of the compound using prefixes.	Monatomic the name will be hydro____ic acid	Polyatomic The name will be _____ic/ous acid
Example: Cu(NO₃)₂ - Copper (II) nitrate	Example: Mg₃(PO₄)₂ - Magnesium phosphate	Example: NH₄NO₃ - Ammonium nitrate	Example: N₂O - Dinitrogen Monoxide	Example: HI - Hydroiodic acid	Example: H₂SO₃ - Sulfurous acid