Covalent Compounds - Background Info…
Name__________________________________________
Hr_____
St#_____
What Is Electronegativity?
Electronegativity is a measure of the attraction of an atom for the electrons in a chemical bond. The higher the
electronegativity of an atom, the greater its attraction for bonding electrons.
Electronegativity and Ionization Energy
Electronegativity is related to ionization energy. Electrons with low ionization energies have low
electronegativities because their nuclei do not exert a strong attractive force on electrons. Elements with high
ionization energies have high electronegativities due to the strong pull exerted on electrons by the nucleus.
Electronegativity and Periodic Table Trends
In an element group, the electronegativity decreases as atomic number increases, as a result of increased distance
between the valence electron(s) and nucleus (greater atomic radius). An example of an electropositive (i.e., low
electronegativity) element is cesium; an example of a highly electronegative element is fluorine.
 Moving left to right across the periodic table, electronegativity increases.
 Moving top to bottom down the periodic table, electronegativity decreases.
Of the following element pairs place the most electronegative element in the blank:
Al and S
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B and F
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Cl and Si
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Cl and I
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Br and Se
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N and O
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K and Ga
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Ca and Be
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P and Na
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Writing Formulas for Covalent Compounds
Certain rules apply to the way names of covalent compounds are written:
 The more electropositive element (further left on the periodic table) is listed before the more
electronegative element (further right on the periodic table).
 The second element is given an -ide ending.
 Prefixes are used to denote how many atoms of each element are present in the compound.
Prefixes and Molecular Compound Names
Nonmetals may combine in a variety of ratios, so it is important that the name of a molecular compound indicates
how many atoms of each type of element are present in the compound. This is accomplished using prefixes. If there
is only one atom of the first element, no prefix is used. It is customary to prefix the name of one atom of the second
element with mono-. For example, CO is named carbon monoxide rather than carbon oxide.
Examples of Covalent Compound Names
SO2 - sulfur dioxide
SF6 - sulfur hexafluoride
CCl4 - carbon tetrachloride
NI3 - nitrogen triiodide
Writing the Formula from the Name
You can write the formula for a covalent compound from its name by writing the symbols for the first and second
element and translating the prefixes into subscripts. For example, xenon hexafluoride would be written XF6. It is
common for students to confuse ionic compounds and covalent compounds and then have trouble trying to write
formulae from the compounds names. You aren't balancing charges of covalent compounds; if the compound does
not contain a metal, don't try to balance this!
General Questions & Practice Problems…
1.
Most covalent bonds form between which types of atoms?
2.
Describe what occurs when a covalent bond forms and explain how it is different from when an
ionic bond forms:
3.
How do you determine which element is written first in a covalent bond?
4.
What would be the name of the following covalent compounds?
5.
6.
a.
N3O2
d.
C4F5
b.
Cl9Si7
e.
P3Br8
c.
CO2
f.
S6F
What would be the formula of the following covalent compounds? (Note: Some of the names that
have prefixes have been shortened, eg. decoxide instead of decaoxide, chemists do this for ease of
writing/speaking - this is not required!)
a.
Tetraphosphorous trisulfide
d.
Oxygen Selenide
b.
Diarsenic pentoxide
e.
Octacarbon decoxide
c.
Silicon tetrafluoride
f.
Hydrogen dioxide
Which covalent compounds (1 from each question #4 and #5 above) would not be possible?
(Hint: check the elements’ electronegativities!)