RULES FOR WRITING FORMULAS OF BINARY COVALENT COMPOUNDS
CHEMISTRY 1411 HANDOUT
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The name of the element farther to the left of the periodic table is written first.
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If both elements belong to the same group, then the element with the higher atomic number is
named first.
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The name of the second element is written with an ide ending. For example,
Element
Oxygen
Names
Oxide
Hydrogen
Hydride
Nitrogen
Nitride
Chlorine
Chloride
Bromine
Bromide
Fluoride
Fluoride
Iodine
Iodide
Sulfur
Sulfide
Selenium
Selenide
Phosphorus
Phosphide
Greek prefixes are used to indicate the number of atoms for each element.
Prefix
Number of atoms denoted
Mono
1 atom
Di
2 atoms
Tri
3 atoms
Tetra
4 atoms
Penta
5 atoms
Hexa
6 atoms
Hepta
7 atoms
Octa
8 atoms
Nona
9 atoms
Deca
10 atoms
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If the first element has only one atom, then the prefix mono is not used.
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If the prefix ends with an “a or o” and the name of the element begins with a vowel such as
oxide, then the “a or o” is dropped. For example, the formula Cl2O is named as dichlorine
monoxide instead of monooxide.
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Example 1 : SO
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2
Both sulfur and oxygen belong to the same group and sulfur has a higher atomic number
compared to oxygen so the name begins with Sulfur.
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One atom of sulfur and two atoms of oxygen are present so the the prefix di is used (We do not
use the prefix mono because the name does not start with mono).
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The compound is named as sulfur dioxide.
Example 2: PCl
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5
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Phosphorus is further to the left compared to chlorine. So the name begins with phosphorus.
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The compound contains one atom of phosphorus and five atoms of chlorine and hence the name
ends with pentachloride.
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The compound is named phosphorus pentachloride.
Example 3 : N O
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2
3
Nitrogen is further to the left compared to oxygen. So the name of the compound begins with
nitrogen.
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The compound contains two atoms of nitrogen, hence dinitrogen and 3 atoms of oxygen, hence
trioxide.
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The compound is named dinitrogen trioxide.
Example 4 : SiBr
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4
Silicon is further to the left compared to bromine. So the name of the compound begins with
silicon.

The compound contains one atom of silicon, hence the name begins with silicon and not
monosilicon. And four atoms of bromine are present, hence the name tetrabromide.
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The name of the compound is silicon tetrabromide.
Example 5: S Cl
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2
2
Sulfur is further to the left compared to chlorine. So the name of the compound begins with
sulfur.
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The compound contains two atoms of sulfur, hence the name begins with disulfur. And two
atoms of chlorine are present, hence the name dichloride.
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The name of the compound is disulfur dichloride.