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Lewis Dot Structures Molecules, such as Cl2 and SO2, and multi ...

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<strong>Lewis</strong> <strong>Dot</strong> <strong>Structures</strong><br />

<strong>Molecules</strong>, <strong>such</strong> <strong>as</strong> Cl 2 <strong>and</strong> SO 2 , <strong>and</strong> <strong>multi</strong>-atomic ions, <strong>such</strong> <strong>as</strong> NO 3<br />

-<br />

<strong>and</strong> SO 4<br />

2-<br />

,<br />

are held together by chemical bonds. These bonds involve the sharing of pairs of<br />

electrons between the atoms that are bonded. Electro-static forces that might be<br />

called physical bonds hold together salts, <strong>such</strong> <strong>as</strong> NaCl.<br />

Lew dot structures (or models) provide a simplified but very useful picture of<br />

chemical bonding, particularly for molecules or ions formed from atoms in the first<br />

two rows in the periodic table.<br />

Recall that the structure of an atom can be indicated <strong>as</strong> shown. X is the<br />

symbol for the atom (may be one of two letters), A is the m<strong>as</strong>s number,<br />

<strong>and</strong> Z is the number of protons in the atomic nucleus. It is also the<br />

number of electrons in the neutral atom.<br />

We can imagine building up a neutral atom by adding Z electrons,<br />

one by one, to the bare nucleus. These electrons go into shells<br />

starting with the one closest to the nucleus. When a particular shell<br />

is filled, additional electrons will go into the shell next furthest from<br />

the nucleus.<br />

A<br />

Z<br />

X<br />

The first shell h<strong>as</strong> room for only two electrons. The second shell h<strong>as</strong> room for<br />

eight electrons, <strong>as</strong> does the third. These shells actually have two sub-shells. The<br />

eight electrons in these shells are not distributed uniformly, but they are found in<br />

pairs at the corners of a tetrahedron.<br />

Filled shells have exceptional stability. These occur for Z values of 2, 10, <strong>and</strong> 18;<br />

these are the atomic numbers for the first three rare, or noble, g<strong>as</strong>es, He, Ne,<br />

<strong>and</strong> Ar. Beyond argon, the shells get larger <strong>and</strong> can hold more electrons. The<br />

next rare g<strong>as</strong> is Kr, with Z=36. Electrons in the outer-most shell are called the<br />

valence electrons. They are involved in chemical bonding. The electrons in the<br />

inner shells are called the core electrons.<br />

When a molecule or <strong>multi</strong>-atomic ion is formed, the component nuclei <strong>and</strong><br />

electrons arrange themselves in the most stable configuration. Ideally, each<br />

atom, with its core electrons, will be surrounded by eight valence electrons in the<br />

tetrahedral configuration mentioned above. Bonding between atoms occurs when<br />

electron pairs must be shared. When one electron pair is shared, the resulting<br />

bond is called a single bond. When two pairs are shared, a double bond is<br />

formed. IN this c<strong>as</strong>e, the two electron-tetrahedra will share an edge. In a triple<br />

bond, three electron pairs are shared <strong>and</strong> two tetrahedral share a face. It should<br />

be fairly e<strong>as</strong>y to see that a quadruple bond cannot be formed without ab<strong>and</strong>oning<br />

the tetrahedral electron configuration.


The <strong>Lewis</strong> <strong>Dot</strong> Model helps us to underst<strong>and</strong> <strong>and</strong> predict what these stable<br />

configurations might be. More accurate but more complicated models are<br />

discussed in the chapters on Chemical Bonding.<br />

In the lab work, we will construct a number of <strong>Lewis</strong> <strong>Dot</strong> Models to illustrate the<br />

ide<strong>as</strong> mentioned above <strong>and</strong> to gain some familiarity with the molecules <strong>and</strong> ions<br />

we will be studying <strong>and</strong> working with.<br />

Model <strong>as</strong> many of the following <strong>as</strong> you have time for. Do at le<strong>as</strong>t four molecules<br />

<strong>and</strong> four ions. For each, draw your result in the margin <strong>and</strong> indicate the numbers<br />

of valence electrons <strong>and</strong> the numbers of single, double, <strong>and</strong> triple bonds.<br />

N 2 nitrogen<br />

2-<br />

CO 3 carbonate<br />

H 2 O water CN - cyanide<br />

NH 3 ammonia OH - hydroxide<br />

CH 4 methane<br />

-<br />

NO 3 nitrate<br />

C 2 H 4 ethylene<br />

-<br />

NO 2 nitrite<br />

C 2 H 2 acetylene<br />

2-<br />

O 2 peroxide<br />

CO 2 carbon dioxide<br />

3-<br />

PO 4 phosphate<br />

CO carbon monoxide<br />

2-<br />

SO 4 sulfate<br />

SO 2 sulfur dioxide<br />

2-<br />

SO 3 sulfite<br />

BF 3 boron trifluoride SCN - thiocyanate<br />

O 3 ozone<br />

+<br />

NH 4 ammonium<br />

N 2 O nitrous oxide H 3 O + hydronium<br />

NaCl sodium chloride<br />

BeCl 2 beryllium chloride<br />

Notes:<br />

1. <strong>Molecules</strong> or ions with the same numbers of atoms <strong>and</strong> valence electrons<br />

have the same structures. This means that the atoms could be rearranged<br />

without changing the bonding or structures. Usually, one of these<br />

rearrangements will be much more stable than the others. The re<strong>as</strong>ons for<br />

this will be discussed in a later chapter.<br />

2. Be considering the tetrahedral configuration for the eight electrons, it is<br />

possible to predict the shapes of these molecules or ions in three<br />

dimensions. Examples: H 2 O is not linear, CO 2 is linear, but SO 2 is not.<br />

NO 3<br />

-<br />

is flat but ClO 3<br />

-<br />

is not.<br />

3. In the <strong>Lewis</strong> <strong>Dot</strong> notation, the double bond is shown <strong>as</strong> X::Y. This does not<br />

imply that one pair is closer to X <strong>and</strong> the other to Y. The pairs are<br />

horizontal rather than vertical. The double bond is commonly indicated <strong>as</strong><br />

X=Y.

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