Chemistry Reference
In-Depth Information
Electrical discharge
Figure 5.3
Welding Torch
Hydrogen gas is piped into a special
tube in which many of the H
2
molecules are separated into atoms
by an electrical discharge. The gas
flow is adjusted to the rate at which
most of the H atoms recombine into
molecules just as they exit from the
torch. The heat produced at that point
is intense enough to weld pieces of
steel together.
H
2
gas
H atoms
Recombination point
Dissociation
2 H H
2
heat
Electrical
energy
H
2
2 H
Some other free (uncombined) elements also occur in different forms. Dif-
ferent forms of the same element are called
allotropes
of each other. Except
for oxygen, the elements that form diatomic molecules when uncombined do
not form allotropes, but many other nonmetals do. The allotropes of carbon—
diamond and graphite—are perhaps best known to the general public. Sulfur
and phosphorus are also notable for forming allotropes.
Binary Compounds
Atoms of two or more different nonmetals can bond together to form molecules
of a compound. H
2
O, CH
4
, and CH
4
O are examples.
In the formula for a
binary compound
(a compound containing only two
elements), the element that attracts electrons less is usually written first. The
elements are assigned an
electronegativity
that reflects their affinity for elec-
trons in chemical bonds. The elements that attract electrons most are said to
have the highest electronegativities or to be the most
electronegative.
Fluorine,
the most electronegative element, is assigned an electronegativity of 4.0, and
the other elements have values relative to that of fluorine. The elements that
attract electrons least are said to have the lowest electronegativities or to be the
most
electropositive.
Values for the electronegativities of the main group elements are presented
in Figure 5.4. Except for those of the noble gases, the electronegativities of the
H
2.1
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
He
Figure 5.4
Electronegativities
of the Main Group Elements
Be
1.5
Mg
1.2
B
2.0
Al
1.5
C
2.5
Si
1.8
N
3.0
P
2.1
O
3.5
S
2.5
F
4.0
Cl
3.0
Ne
Ar
Ca
1.0
Ga
1.6
Ge
1.8
As
2.0
Se
2.4
Br
2.8
Kr
3.0
Sr
1.0
Ba
0.9
Ra
0.9
In
1.7
Tl
1.8
Sn
1.8
Pb
1.9
Sb
1.9
Bi
1.9
Te
2.1
Po
2.0
I
2.5
At
2.2
Xe
2.6
Rn
