Chemistry Reference
In-Depth Information
This chapter examines many aspects of atomic and molecular properties. These
properties affect the properties of the substances that the atoms and molecules
constitute. We begin with the sizes of atoms and ions, especially the periodic
variation of sizes (Section 13.1). We then discuss the effects the sizes have
on the energies associated with the gain or loss of an electron by an atom—
ionization energy and electron affinity, respectively (Section 13.2). From that
more theoretical discussion of gaseous atoms and ions, we progress to elec-
tronegativity, a more practical, semiquantitative measure of the electron-attracting
ability of atoms in molecules or ionic substances (Section 13.3). The atoms'
varying abilities to attract electrons in molecules lead to polar and nonpolar
covalent bonds (Section 13.3). Section 13.4 discusses molecular shapes.
Depending on the molecular shapes, polar bonds can lead to dipolar or nondipo-
lar molecules (Section 13.5). Intermolecular forces, the small forces between
molecules that result from molecular polarity and other causes, are discussed in
Section 13.6.
These discussions will enable us to consider the nature of the solid and liquid
states in Chapter 14.
13.1
Atomic and Ionic Sizes
Before we discuss the sizes of atoms and ions, let us review briefly some basic
scientific principles:
1. Oppositely charged particles attract each other.
2. Like charged particles repel each other.
3. The greater the charge(s), the greater the attraction or repulsion.
4. The closer the two charged bodies, the stronger the force of attraction or
repulsion.
5. Electrons are negative and lie outside the nucleus, whereas protons are pos-
itive and lie within the nucleus.
The greater the number of electrons in an atom or its ions, all other factors
being equal, the larger the
atomic
or
ionic size.
The more electrons, the greater
the electronic repulsion. With the same nuclear charge but greater electronic
repulsion, the electrons occupy a larger volume of space.
EXAMPLE 13.1
Co
2
,
Co
3
.
Compare the relative sizes of Co,
and
Solution
In each case, the nucleus has 27 protons and, therefore, 27 positive charges.
This quantity of positive charge can attract the 24 electrons in
Co
3
better than
Co
2
the 25 electrons in
or the 27 electrons in Co. Hence, Co is the largest,
Co
3
and
is the smallest.
Practice Problem 13.1
Compare the relative sizes of Cl and Cl
.
