CHE Q2 Lesson 1: Chemical Formulas and Chemical Compounds

Chemical Formula

• Indicates the number of each kind of atom in a chemical compound.

• When there is no subscript next to an atom, the subscript is understood to be 1.

Cations

Atoms with 1, 2, or 3 valence electrons tend to lose them to form positive ions.

Anions

Atoms with 5, 6, or 7 valence electrons tend to gain more in order to have an octet (8 electrons ) in their outer shell.

Gaining extra electrons forms negative ions.

Monoatomic

Are ions formed from a single atom.

Some main-group elements tend to form covalent bonds instead of ions (ex. C and Si.)

Binary Ionic Compounds

These are composed of two elements.

Alfred Stock

Created the Stock system

1919

The year when the stock system was designed.

The Stock System

• Most d-block elements (transition metals) can form 2 or more ions with different charges.

• To name ions of these elements, scientists use the Stock system, designed by Alfred Stock in 1919.

• The system uses Roman numerals to indicate an ion's charge.

Polyatomic Ions

• It is a charged group of covalently bonded atoms.

• Common endings are -ate or -ite, but there are exceptions.

The Prefix System

• Molecular compounds are composedof covalently-bonded molecules.

• The old prefix system is still used for molecular compounds.

• Name the prefix, then the element. Anions end in -ide.

The prefix mono- usually isn't used for cations

Acid

•is a certain typeof molecular compound.All acids start with H (e.g. HCl, H2SO4).

•Acids can be divided into two categories

Binary Acid

Oxyacids

Two categories of Acid

Binary acids

These are acids that consist of H and a non-metal. (e.g. HCl.)

Oxyacids

These 1.are acids that contain H and a polyatomic ion that includes O (e.g. H2SO4.)

1.Begin with the prefix hydro-.

2.Name the anion, but change the ending to -ic.

3.Add acid to the name.

Examples:

HCl, hydrochloric acid.

HBr, hydrobromic acid.

H2S, hydrosulfuric acid.

Binary Acid Rules

1.Name the polyatomic ion.

2.Replace -ate with -ic or -ite with -ous

3.Add acid to the name.

Examples:

H2SO4, sulfuric acid.

H2SO3, sulfurous acid.

HNO3, nitric acid.

HNO2, nitrous acid.

Oxyacids Acid Rules

Oxidation Number

In order to indicate the general distribution of electrons among covalently bonded atoms, ________ numbers are assigned to the atoms.

1.The sum of the oxidation numbers for a neutral compound equals zero.

2.The sum of the oxidation numbers for an ion equals the charge of the ion.

3.Atoms in a pure element are zero.

4.The most electronegative element in a compound is assigned a negative number equal to the charge it would have as an anion.

5.Hydrogen is always either +1 or -1.

Rules for Assigning Oxidation Numbers

Formula Mass

It is the sum of the masses of all the atoms in its formula.

It is numerically equal to its formula mass. Only the units are different. (Ex: Molar mass of H2O = 18.0 g.)

Percent Composition

The percentage by mass of each element a compound is known as the _____________ of the compound.