Outer Electrons and Groups of Elements
You can see from the shell diagrams in the last activity that the elements in each
group show some differences. The atomic mass increases as you go down a
group, and so too does the number of elec^on shells. For example, in Group 2,
calcium has a higher atomic mass and more electron shells than magnesium or
beryllium, and the radius of a calcium atom is larger. In spite of these differ- y
ences, all the elements in the groups you have studied do have one thing in .
common: the number of electrons in the outer shell is the same.
Table 8.1 illustrates this idea using four groups: halogens, noble gases, alkalj
metals, and a group you have not yet studied, the alkaline earth metals
(Group 2). These groups are displayed in a special format to help you compare
electron arrangements. Refer to Table 8.1 frequently as you read the rest of
this section.
Table 8.1 Electron Arrangements of Four Families
10
11
12
(Ve)
v
17
1&
a
35
19
Ar
3
Br
53
Ca
Kr
Rb
o5
Xe
Sr
56
Cs
Ba
86 | 87
At
260 Atoms and Elements
38
37
54
85
20
rtn
i
Fr
Ra
The Noble Gases: A Stable Outer Shell
Chemists find the noble gases interesting because they are so unreactive. As you
will find out, mu-eactivity can be a very usefid property.
Figure 8.2A The earliest light bulbs
burnedout rapidly, as oxygen in the
air reacted with the glowing filament.
Figure 8.2B Modem light bulbs are filled with the
unreactive gas argon.
:'^li
DidYouKnow?
Before 1963, all chemistry textbooks claimed
Consider an ordinary light bulb. If the slightest leak allows air to get inside the
glass covering, the filament will burn out in a bright flash of light and the bulb will
go dark. Exposure to oxygen makes the tungsten filament burn. This is because
that the noble gases were
entirely inert-that is,
they would never form
chemical compounds.
tungsten's outer shell of electrons is affected by oxygen's outer shell, especially
Then Canadian chemist
Modern light bulbs are usually 'filled with argon, an imreacrive gas, to keep the
Neil Bartlett startled the
scientific world by
preparing noble gas
when tungsten is hot. If oxygen is excluded, however, tungsten cannot react.
tungsten filament from burning out.
Argon is a noble gas. The high-intensity discharge (fflD) headUghts that you read
abK>utat the begilmmg of Chapter 7 contain another noble gas: xenon. Xenon'srolein
compounds in a labora-
tory at the University of
HIDheadlights is quite different fi-om argon's role in Ught bulbs, however. The new
British Columbia. The
methods he used to
like the discharge tubes that Crookes and Thomson used to stody atoms. The head-
rearrange the electrons
headlights are actuaUy gas discharge tubes, which have no mament. Instead, they work
light is fiUed with low-pressure xenon, and the inside surface of the glass is coated
wiA metallic salts. As the current arcs through the tube, it energizes the xenon atoms
and causes them to glow with an intense ultraviolet light. This Ught energizes the
metallic salts wliich, in turn, glow with their own bright blue Ught.
Numerous laboratory experiments have confirmed that all noble gases are chemaUy "stable." This means that they are highly unlikely to take part in a chemical
were not exceptionally
difficult, but the com-
pounds proved to be
unstable. The noble gas
atoms are definitely more
stable when uncombined.
change. In fact, only the very largest noble gas atoms can be made to react chenu-
eaUy at all. Even when they do react, their compounds soon decompose, ailowmg
^noble gas to separate into single atoms again.
Based on these and other observed properties, chemists have reasoned that the
electron arrangement of the noble gas atoms must be exceptionally stable. For
example, helium has only two electtons m total. The shell that holds them has
room for only two, so it is full. When a helium atom bumps into an atom of
;ln°Aer element, its outer shell is unchanged.
The same is true for all the other noble gases. Their outer shells have eight elec-
GgSHECT
Check in a dictionary to
find the root word of
"octet" and the language
it comes from. List as
many words as you can
find that contain this root
word. Then make a word
frons instead of two but these outer shells are unchanged when they comde with
"search puzzle with these
elght electrons ijar-ab outer shell - is often caUed a stable octet. The stable octet
tally, vertically, and
diagonally. Challenge a
ser atoms'even i^the other atoms are usually reactive. This arrangement -
®animpor^aE^concept because it can also help to explain why the halogens and
lij^etals are so unstable or reactive, and what happens when they do react.
words written horizon-
partner to find all the
words.
OC-1
i '
DidYouKnow?
The Halogens: One Electron Short of Stability
Pure chlorine gas wasused as a chemical
Think about this question as you read:
weapon in World War I.
Where can a fluorine atom get the extra electron it needs to make a stable octet?
Because chlorine is twice
as dense as air, it sank
into the trenches and set-
As you saw in Table 8.1, aU halogen a.toms have seven outer electrons, regardless of
tied there. Many soldiers
died on the spot. Of those
who survived, most suf-
and harmful.
their mass, number of shells, or total electrons. Halogen atoms react vigorously
with nearly everything. Even the least reacdve halogens are extremely corrosive
fered lifelong respiratory
problems. The poet Wilfred
Owen described a gas
attack in the trenches in
these words:
Dim, through the misty
panes and thick green
light,
As under a green sea I
saw him drowning.. .
Figure 8.38 Most swimming pools have chlorine gas bubbled into
them to kill germs.
Figure 8.3A Tincture of iodine (an
alcohol solution) was once used to kill
germs on cuts. Unfortunately the iodme
nlnn 1/illori +hQ ci irrni inHinn noltc
Fluorine is so reactive that the first person to isolate it, Henri Moissan, in
1886, won a Nobel Prize. Nearly four decades passed before safe methods
for producing large amounts ofiluorine were developed. This project
involved massive spending and intensive effort by many scientists.
From these examples ofhalogen reactivity, we can infer that an outer
shell with seven electrons is unstable. Take a close look at fluorine's electron
arrangement, however. Notice how closely the shells resemble those of ^
neon. What if fiuorine could somehow acquire one more electron? Then
fluorine's outer shell would have a stable octet, as pictured in Figure 8.4.
Figure 8.4 This is howfluorine's electron
shells would appear if fluorine could gain one
^TCS^
.^a^,
electron.
Fluorine, at the top of Group 17, is more reactive than chlorine, just below it
The more electron shells a halogen atom has, the less reactive it is. Can you
explain why?
262 Atoms and Elements
_J
The Alkali Metals: One Electron Beyond Stability
Now think about this question as you read:
How can a sodium atom get rid of its "exrra" electron to become chemically stable?
The arrangement of outer electrons in the alkali metals group differs considerably
from the arrangement in the halogen group. Regardless of atomic radius or
number of electron shells, all alkali metals have one outer electron. Alkali metals
react vigorously with many other substances (see Figure 8.5), which suggests that
the electron structure they have in common must be unstable. In Chapter 6, you
saw how reactive sodium is.
Figure 8.5A Freshly cut sodium is bright and shiny,
but only while stored in oil.
Figure 8.5B After a few minutes' exposure the
surface of the sodium has dulled because the
sodium atoms have reacted with oxygen from the air.
Figure 8.5C When a small lump of sodium is
dropped into water, a vigorous reaction takes
place.
The most reactive alkali metal atoms are the largest ones. Since
«sodium has almost the same number of electrons as neon and fluorine,
however, it is easier to compare these three elements. At first glance,
Na
the electron shells ofsodium do not seem much like those of neon.
Mean's outer sheU has eight electrons, while sodium's has only one electron. What if sodium could get rid of that one electron? Figure 8.6
pictures how sodium's electron shells would look if this happened.
In Figure 8.6, sodium's almost empty outer shell.(the red line) is now
completely empty. The next shell inward (the blue line) has become the
new outer shell, and it has eight outer electrons - the same stable octet as
neon. Therefore sodium would be chemicallystable if it could shed an
electron.
Figure 8.6 This is how sodium's electron
shells would change if sodium could lose
one electron. Theold outer shell is shown by
the broken red line.
1 p^^;n^ 9>fi3
DidYouKnow?
The Alkaline Earth Metals: Two Electrons Beyond Stability
The alkaline earth metals
The arrangement of the outer electrons of the elements in Group 2 is similar to
got their name because
early chemists named
anything that was
that of the alkali metals m Group 1. There are two electrons beyond a stable octet
in calcium, for example. Regardless of atomic radius or number of electron shells,
insoluble in water and
all the alkaline earth metals have this same structure. They react fairly vigorously.
with a number of substances, but not as vigorously as the alkali metals. For
unchanged by heat an
example, magnesium reacts with water but less vigorously than sodium does, even
"earth" (from the ancient
though they are both m the same period.
As with the alkali metals, the most reactive metals in Group 2 are the largest
ones. Notice that two electrons must be given up to achieve a stable octet. In a
reaction between calcium and fluorine, for example, two fluorine atoms would
need to be available to give up two electrons to each calcium atom.
Greek use of the term
"earth" as one of the four
"elements"). Calcium
oxide and magnesium
oxide are both slightly
sotuble in water, giving
slightly alkaline properties (an alkali is, roughly,
the opposite of an acid).
Finally, when Humphry
Davy isolated catcium
and magnesium by
electrolysis, he was able
to identify them as
alkaline earth metals.
Magnesium, which you saw in the Starting Point Activity, is also an alkaline
earth metal. You will learn more about magnesium's reaction with oxygen in the
next section as you further develop your model of compound formation.
tsa.
Check Your Understanding
1 Why is the outer shell of electrons thought to be the most important for
determining chemical properties?
Why are the noble gases generally unreactive?
2. Briefly describe the uses made of some noble gases mentioned in this section.
3. Why are the alkali metals and the halogens so reactive?
4. Take out your collecdon of element and compound cards from Chapter 5
(page 176), and add to them the new information about reactivity and applications you have learned about in this section.
5. Thinking Critically List some of the physical properties of sodium, and
then of chlorine. Are the physical properties of sodium chloride (table salt)
an average of the properties of the elements it contains? Explain.
6. Apply Compare the uses of argon, xenon, and magnesium in lighting technologies. In which technologies is chemical reactivity, or the lack of it,
important?
264 Atoms and Elements