The Foundations of Chemistry

General trends in electronegativities

of A group elements with position in

the periodic table.

6-7 Hydrogen and the Hydrides 251

it has a greater tendency to attract electron density to itself than does any other element.
Oxygen is the second most electronegative element.

For the representative elements, electronegativities usually increase from left to right

across periods and decrease from top to bottom within groups.

Variations among the transition metals are not as regular. In general, both ionization ener-
gies and electronegativities are low for elements at the lower left of the periodic table and
high for those at the upper right.

EXAMPLE 6-5 Trends in ENs

Arrange the following elements in order of increasing electronegativity.

B, Na, F, O

Plan

Table 6-3 shows that electronegativities increase from left to right across a period and decrease
from top to bottom within a group.

Solution

The order of increasing electronegativity is NaBOF.

You should now work Exercise 44.

Although the electronegativity scale is somewhat arbitrary, we can use it with reason-
able confidence to make predictions about bonding. Two elements with quite different
electronegativities (a metal and a nonmetal) tend to react with each other to form ionic
compounds. The less electronegative element gives up its electron(s) to the more elec-
tronegative element. Two nonmetals with similar electronegativities tend to form covalent
bonds with each other. That is, they share their electrons. In this sharing, the more elec-
tronegative element attains a greater share. This is discussed in detail in Chapters 7 and 8.

CHEMICAL REACTIONS AND PERIODICITY

Now we will illustrate the periodicity of chemical properties by considering some reac-
tions of hydrogen, oxygen, and their compounds. We choose to discuss hydrogen and
oxygen because, of all the elements, they form the most kinds of compounds with other
elements. Additionally, compounds of hydrogen and oxygen are very important in such
diverse phenomena as all life processes and most corrosion processes.

HYDROGEN AND THE HYDRIDES

Hydrogen

Elemental hydrogen is a colorless, odorless, tasteless diatomic gas with the lowest molec-
ular weight and density of any known substance. Discovery of the element is attributed
to the Englishman Henry Cavendish (1731–1810), who prepared it in 1766 by passing
steam through a red-hot gun barrel (mostly iron) and by the reaction of acids with active

6-7

Electronegativity

Increase

Decrease

Ionization energy (Section 6-3) and

electron affinity (Section 6-4) are

precise quantitative concepts. We find,

however, that the more qualitative

concept of electronegativityis more

useful in describing chemical bonding.

The name “hydrogen” means “water

former.”