Science 9
2.7: Chemical Symbols and Formulas
Chemical Symbols
On the periodic table all the elements are represented by different symbols – letters
which are an abbreviation of the element’s name. These same symbols are used in every
country in the world to represent the different elements. Sometimes the letters may be
similar to our spelling of the word but most of the symbols come from the element’s
Latin names.
Examples:
Mercury – Hg comes from the Latin word hydrargyrum which means liquid
silver.
Sodium – Na – comes from the Latin word natrium.
If a chemical’s symbol has only one letter it is always capitalized.
If the symbol has two letters the first one is capitalized and the second one is not.
Chemical Formulas
Combinations of chemical symbols are used to represent compounds. This combination
of symbols is called a chemical formula.
The chemical formula indicates which elements are in a particular compound and also
their proportion or ratio. A chemical formula is like a recipe.
If there is only one atom of an element in the compound no number is included, if there
is more than one atom then the number of atoms is written in subscript after the symbol
for that particular atom.
Examples:
- H2O has 2 atoms of hydrogen and one atom of oxygen
- NaHCO3 has one atom each of sodium, hydrogen and carbon and 3 atoms of
oxygen.
Science 9
2.10: Names and Formulas for Compounds
How do you determine how many atoms of each type of element will combine to form a molecule?
For example why is water H2O instead of HO or H3O?
The key is knowing how atoms combine and this has to do with their combining capacity.
Combining capacity – the ability to combine with other elements – shows how many connections an
atom can make. For example, you have two arms so you can connect with 2 other people. If you only
had one arm, you could only connect with one other person.
In order for a compound to be stable, all the atoms in it must have made all the possible connections.
Also, if an element has a positive charge (mostly metals) it must connect with an element which has a
negative charge (mostly nonmetals) to be stable. Sometimes more than one positive may band together
to connect with a negative or visa versa.
On the periodic table the elements in columns 1, 2, and 13 are considered positive. (the metals in the
middle have varying combining capacities and we will talk about them later).
Column 1 has a combining capacity of +1.
Column 2 has a combining capacity of +2.
Column 13 has a combining capacity of +3.
Column 14 has a combining capacity of + or – 4.
The elements in columns 15,16 and 17 have negative combining capacities.
Column 17 has a combining capacity of -1.
Column 16 has a combining capacity of -2.
Column 15 has a combining capacity of -3.
Column 18 is the noble gases and they do not combine so have a capacity of 0.
Give the combining capacity of the following elements.
Li
N
C
O
Mg
H
Ne
Cl
B
Metals with More Than One Combining Capacity
The metals in the middle of the periodic table do not follow the same rules as
the outside columns. Some of their combining capacities can vary depending
on the compound that they are forming.
Lead, copper, iron, and tin all have 2 different combining capacities. When
you name compounds containing these elements, you must state which type
is in the compound by using a Roman numeral after the element’s name.
Example: FeO - iron (II) oxide or Fe2O3 - iron (III) oxide
Science 9
Naming Compounds
When naming a compound the first element in the compound (the metal) maintains the
same name (as seen on the periodic table); the second element (the nonmetal) has it’s
ending replaced with “ide”. (See Table 3 in your notes)
If the compound is made of two nonmetals, which both have negative charges, you add
a prefix to the second element as follows:
2 = di3 = tri4 = tetraEg. Carbon dioxide – CO2
Name the following compounds:
LiBr
K2S
Mg3P2
CaCl2
CaO
N2S3
SO3
Science 9
Hook-and-Ball Diagrams
One way of representing elements bonding together is with hook-and-ball diagrams.
Example:
calcium chloride
Write the formula for each of the following compounds.
Draw hook-and-ball diagrams.
hydrogen chloride
sodium bromide
magnesium chloride
calcium oxide
potassium iodide
water
sodium oxide
aluminum oxide
zinc chloride
silver sulfide