Polyatomic Compounds
Molecular Compounds

Polyatomic Compounds

Polyatomic compound are formed when more than two atoms combine. Two atom compounds are called binary compounds. The most common polyatomic compounds are those that contain oxygen as the third and usually the last element. Even though there are three atoms in the compound it behaves as far as naming and writting the formula, very similar to binary compounds. The last two elements are grouped together, have their own name and a unique valence value. Criss crossing is the same as binary however you must put brakets around the last two elements.

NaNO₃ is called sodium nitrate
Na⁺¹ and (NO₃)^-1 and the valence values are criss-crossed and the dropped.

Na, sodium has a valence of ⁺1. It is univalent having only one valence so a Roman Numeral is not needed.

Nitrate has the formula (NO₃)^-1 with a valence value of -1.Try and remember this.

Ca₃(PO₄)₂ is called calcium phosphate
Ca⁺² and (PO₄)^-3 and the valencies are criss-crossed to obtain the correct formula.

The postive cation and negative polyatomic anion have the same valence, when they are criss-crossed they do not appear in the formula.

A list of polyatomic anions is found on page 196 or your book and listed at You may use this list on tests.

Three non-oxyacid polyatomic ions are

ammonium
hydroxide
cyanide

To obtain the formulae of these go to

and carry out the roll-overs.
Each of these ions must be bonded to either a cation or anion
Example: What is the formula for sodium hydroxide, ammonium sulfide and copper (II) cyanide? (Answers provided in class).

Worksheets would be in order here, so make sure you get one.

Molecular Compounds

Molecular compounds are made of molecules.

each molecule contains anywhere from two atoms (diatomic molecules) to thousands (biological molecules).
each molecule has the same element composition and properties as the compound.
synonym: covalent compound

Common molecular compounds examples: H₂O, CO₂, C₆H₁₂O₆, NH₃, CH₄
which are respectively water, carbon dioxide glucose ammonia & methane
Some of these names are what is called a common name and do not follow any nomencalture rules.

Formulas for molecular compounds

The simplest types are the diatomic molecules: N₂ H₂ O₂ & Cl₂
A molecular formula shows the type and number of atoms in a molecule
type of atom indicated by element symbol
number of atoms per molecule indicated by subscripts (if greater than one)
- H₂O contains 2 hydrogen atoms and one oxygen atom per molecule.
- Carbon tetrafluoride, CF₄, contains four fluorine atoms and one carbon atom per molecule.
atoms in formulas are sometimes grouped to show how they're connected in the molecule
- Methanol is usually written as CH₃OH to show that 3 hydrogens are bound to the carbon and another hydrogen is bound to the oxygen.
- Acetic acid can be written as CH₃COOH or as or C₂H₄O₂. The first formula shows how the molecule is put together; the second formula is the least informative because it shows only the numbers and types of atoms in the molecule.
groups that appear more than once in the molecule are enclosed in parentheses CH₃(CH₂)₃CH₃ could be written as C₅H₁₂, but all information about the structure of the molecule would be lost. (CH₃CH₂)₄P₂O₇ molecules contain 8 carbons, 20 hydrogens, two phosphoruses, and seven oxygens.
Molecular compound are based on shared pairs of electrons between two nonmetal atoms. When two nonmetals combine they do so in order that each atom has its outer shell filled to eight. Since nonmetals do not give away electrons they can share electrons between each other to achieve the octet of eight electrons.
Formula can be derived the same way as the binary ionic compounds by criss-crossing valence numbers and reducing does not take place. The order of elements in the formula usually follows the column order on the periodic table. See examples page 204.
Being nonmetals a prefix system is used in naming these molecules. You just have to know the appropriate prefix for the number of atoms which is listed in the chart below.

Value	Prefix	Value	Prefix
1	mono	5	penta
2	di	6	hexa
3	tri	7	hepta
4	tetra	8	octa

Remember that these prefixes may only be used with nonmetals !

For a short additional note on Molecular Compounds

Polyatomic Compounds Molecular Compounds