What is a chemical formula? Section 16-1

Chemical Formulas A way of writing the name of a compound using chemical symbols. The chemical formula for water is H 2 0. Chemical formulas use subscripts to indicate how many atoms of each type of element are in a compound. If there is no subscript, it is assumed to be 1.

Writing Chemical Formulas In many compounds, a metallic element is combined with a non-metallic element. The symbol for the metallic element is written first. Ex. NaCl, sodium chloride.

What is an oxidation number? Section 16-2

Electrons Oxidation Number Number of electrons an atom gains, loses, or shares when it forms a chemical bond. An atom which gains electrons has a negative oxidation number, an atom which loses electrons has a positive oxidation number. Valence Electrons The electrons in an atom’s outer energy level. The first ring can have 2 and each ring afterwards can have 8 electrons. If an atom has 6 valence electrons, it will have an oxidation number of -2. If an atom has 2 valence electrons, it will have an oxidation number of +2.

Writing Chemical Formulas. You must know the oxidation numbers. The oxidation numbers of the elements in a compound must add up to 0 (zero).

How are chemical compounds named? Section 16-3

Binary Compounds Compounds containing two elements. NaCl (sodium chloride) is an example of a binary compound. The name of the element with the positive oxidation number is written first followed by the second element’s name being changed to end in –ide. Some elements have more than 1 oxidation number. A Roman numeral is used to denote this. Ex. FeCl 2 is iron (II) chloride and FeCl 3 is iron (III) chloride.

Other Elements Most elements have more than 1 oxidation number. The Roman numeral in the name of the compound shows the oxidation number of the element.

What is a polyatomic ion? Section 16-4

Polyatomic Ions A group of atoms that acts as a single atom when combining with other atoms. The group of atoms is called an ion because it has an electrical charge. The atoms are joined together by covalent bonds. Ex.

What are diatomic molecules? Section 16-5

Diatomic Molecules Molecules made up of 2 atoms of the same element formed by a covalent bond. There are 7. These 7 elements only appear in nature as a pair of atoms. So oxygen gas is always seen as O 2. The other 6 are also seen as a subscript 2.

What is formula mass? Section 16-6

Formula Mass The sum of the mass numbers of all the atoms in a molecule. This number represents the total of all the protons and neutrons. Sometimes called molecular mass.

Formula Mass Steps 1. Write the chemical formula. 2. Find atomic mass of each element. 3. Multiply the mass number of each element by its subscript. 4. Add the total masses of all the elements.

Formula Mass Ex. Ethyl chloride C 2 H 5 Cl C = 12 H = 1 Cl = 35 2(12) + 5(1) + 1(35) is formula mass.