LESSON 9
Glossary: Bonding
Anion
a negatively charged ion.
Cation
a positively charged ion
Covalent bond
a chemical bond formed by the sharing of a pair of electrons
between atoms
Double bond
a covalent bond in which two pairs of electrons are shared by
two atoms
Electronegativity
a measure of the ability of an atom in a molecule to draw
bonding electrons to itself
Ion
an electrically charged particle obtained from an atom or a
chemically bonded group of atoms by addin- or removing zn
electrons
Ionic bond
a chemical bond formed by the electrostatic attraction
between positive and negative ions
Lewis electron-dot
a formula using dots to represent valence electrons
system
Octet rule
the tendency of atoms in molecules to have eight electrons
in their valence shells (two for hydrogen atoms).
Polar covalent bond
a covalent bond in which the bonding electrons spend more
time near one atom than near the other
Resonance
a representation in which we describe the electron structure
of a molecule, having delocalized bonding by, writing all
possible electron-dot formulas
Triple bond
a covalent bond in which three pairs of electrons are shared
by two atoms
Valence electron
an electron in an atom outside the noble gas or pseudonoble-gas core
Lewis Dot Notation
Helpful Link: Visit this link to view a video on the Lewis Dot Notation
http://www.backpack.tv/video/chemistry/bozeman-science/paul-andersen/drawing-lewisdot-diagrams
Lewis Dot notation is a way of describing the outer shell (also called the valence
shell) of an atom's electrons. Dots are drawn around the elements symbol to
represent the electrons in the valence shell of the atom. For example Helium,
which has two electrons in its valence shell, would be written as:
He has two electrons in an s orbital.
Al has 2 electrons in an s orbital and 1 in a p orbital
The three dots around the symbol stand for the three electrons.
Al has 3 valence electrons (1s22s22p63s23p1). You can determine this by writing
out the electron configuration or simply by the Roman numeral III at the top of
the family on the periodic table containing Al. The dots are placed on all four sides
of the symbol before repeating dots on a side. An example of this is the carbon
atom which only has four electrons in its valence shell.
Note: C always makes four bonds; so one electron is promoted from the s
orbital to the p orbital and C is represented as this:
Atoms will always promote electrons so they can make more bonds, thus
increasing stability.
When the number of electrons in the valence shell is more than four electrons the
dots are written as pairs on the side until all the electrons are accounted for.
Examples are Fluorine and Sulfur.
Ionic Bonds
Helpful Link: Visit this link to view a video on ionic bonds.
http://www.khanacademy.org/video/ionic--covalent--and-metallic-bonds
http://www.backpack.tv/video/chemistry/bozeman-science/paul-andersen/chemical-bondscovalent-vs-ionic
An ionic bond is a chemical bond formed by the electron attraction between
positive and negative ions. Ionic bonds are made when an electron from the
valence shell of one atom is transfered to the valence shell of another atom. The
atom that lost an electron becomes a positive ion and the atom that gains the
electron becomes a negative ion.
In NaCl the sodium ion has one less electron than protons so it has a positive
charge. The chlorine ion has one more electron than protons so it has a negative
charge. Since postives are attracted to negatives the two ions are attracted to
each other. The atom that loses an electron becomes a cation which is positive,
and the atom that gains an electron becomes an anion which is negative. The
nature of the ionic bonds facilitates the formation of ionic solids by attracting
other charged atoms to form a solid. The ions are arranged in a crystalline
structure with each Na+ ion attracted to several Cl- ions and each Cl- ion attracted
to several Na+ ions. There are no NaCl molecules.
Covalent Bonds
Helpful Link: Visit this link to view a video on covalent bonding.
http://www.khanacademy.org/video/ionic--covalent--and-metallic-bonds
Covalent Bonds are chemical bonds formed by the sharing of a pair of electrons
between atoms. The nuclei of two different atoms are attracting the same
electrons. Therefore, unlike ionic bonds where an electron is moved from one
atom to another the electrons are shared.
The Octet Rule is a tendency of atoms in molecules to have eight electrons in their
valence shells. (Two for hydrogen atoms.) The octet rule is a general rule, but is
not followed by all molecules.
Multiple Bonds are sometimes found in molecules so that the molecules satisfy
the octet rule. A single bond (which was discussed earlier) is when a single pair of
electrons is shared between the two atoms. A double bond is when two pairs of
electrons are shared between two atoms. A triple bond is when three pairs of
electrons are shared between two atoms. (Notice a trend?) Double and triple
bonds mostly occur when the elements Carbon(C), Nitrogen(N), Oxygen(O) and
Sulfur(S) are involved. An example of a molecule with double bonds is Carbon
Dioxide (CO2). Notice that each element ends up with eight electrons around it.
Polar Covalent Bonds
Helpful link: Visit this link to view a video on covalent bonds.
http://www.khanacademy.org/video/ionic--covalent--and-metallic-bonds
In bonds between atoms of the same element the sharing of the electrons is
equal between the two atoms. When two atoms of different elements make a
bond, the electrons will not usually be shared equally. The electrons are pulled
more toward the more electronegative element. Electronegativity is the measure
of the ability of an atom in a molecule to draw bonding electrons to itself. In
general, electronegativity increases from bottom to top and left to right on the
periodic table. Fluorine is the most electronegative element since it has a
tendency to pick up electrons easily and hold on to them strongly. An element like
cesium has low electronegativity. The unequal sharing of electrons is called a
polar covalent bond. The definition of a polar covalent bond is a covalent bond in
which the bonding electrons spend more time near one atom than the other.
Lewis Dot Notation Revisited
Now that you understand several types of bonds, a discussion of how to draw the
different bonds using Lewis dot notation is in order. There is a generic procedure
that can be used to draw almost all molecules. This procedure is as follows:
1. Calculate the total number of valence electrons that are in the molecule.
The number of valence shell electrons can be determined by using the
periodic table.
2. Write the skeleton structure of the molecule or ion. (Connect the bonds
with two electrons or a line)
3. Distribute electrons to the atoms surrounding the central atom to satisfy
the octet rule.
4. Distribute the remaining electrons as pairs to the central atom. If there are
less than eight electrons around the central atom then a double or triple
bond may exist. If the central atom is two short then it suggests a triple
bond (or two double bonds). Note: double and triple bonds usually involve
C, O, N and S atoms.
Examples:
Ionic Bond: NaCl
Total Number of Electrons: 8
There are no extra electrons so the drawing is done.
Covalent Bond: HCl
Total Number of electrons : 8
Covalent Bond: FCl4+
Total Number of electrons: 34
7 valence electrons for F and 7 for each Cl. The +1 charge signifies that one
electron has been lost. 7 + 28 - 1 = 34
Covalent Bond: CO2
Total Number of electrons : 16
Carbon does not follow the octet rule unless double bonds are formed.
Resonance
Resonance is another kind of bond that exists between atoms. In resonance the
bond that holds the compound together is not between two atoms, it is shared
between more than two atoms. Looking at an example is the easiest way to
understand resonance. If we try to draw sulfur dioxide (SO2), there can be two
plausible formulas written. These are:
One might conclude that the molecule would be one or the other but this
hypothesis does not fit the experimental data. During experimentation the bonds
were determined to be the same between sulfur and both of the oxygen atoms.
There were two S-O bonds that were just alike. So neither of the above formulas
are correct. So the proper way to write SO2 is as follows:
The above notation symbolizes how the two bonds are the same NOT how the
molecule can be one or the other. The actual structure is a composite of these
two.
Lesson 9 Review
1.
Ammonia is polar because its shape is _____.
2.
The word to describe a molecule with an unequal charge
distribution is _____.
3.
The type of bond formed between an active metal and a
nonmetal is ______.
4.
The simultaneous attraction of electrons for the
nucleii of two or more atoms is a chemical _____.
5.
The type of covalent bond in which one atom donates
both electrons is ______.
6.
Bonding that is responsible for the relatively high boiling
point of water is _____.
7.
The type of covalent bond found in diatomic molecules is
______.
8.
Carbon dioxide is nonpolar because it is _____ .
9.
Particles formed from covalent bonding are called ___.
10.
Compounds with both ionic and covalent bonds contain
this type of ion.
11.
Type of bond found in aluminum foil
12.
The formulas of ionic compounds must be expressed as
____ formulas.
13.
The shape of a water molecule is ____.
14.
The type of bond found between nonmetals is
______.
15.
The type of covalent bonding that is found in the
diamond is ______.
16.
The type of covalent bond found between atoms of
different electronegativity values is ______.
17.
The force of attraction between nonpolar molecules is
called _______.
18.
The element with the highest electronegativity value is
________.
Submit your answers in the text box.