Chemistry - A Molecular Science

are gained. Al

3+ is a cation with a charge of +3. An Al atom must lose three electrons to

form Al

3+, so Al

3+

contains 13 protons and 10 electrons

. Similarly, an O atom has 8

protons, so the O

2- anion must have 10 electrons.

†^

† Note: +1 and -1 charges are normally indicated as simply ‘+’

and ‘-’ as in Na

+ and Cl

-. However, the charge is easily

overlooked when written in this manner, so 1+ and 1- are used for emphasis in this book. Thus, Na

1+ = Na

+ and Cl

l- = Cl

-.

Example 1.9

H

He

Li

Be

C B

N

O

F

Na Ne

Mg

Si Al

P

S Cl

K Ar

Ca

)Co s( point Melting -1000

0

4000 3000 2000 1000

Order

number

Figure 1.6 Periodic behavior of a physical property The melting points of the first

20 elements in order of increasing

atomic mass. Atoms that are show

n in the same color are in the

same chemical group.

order number

(^3210)
H He
Be Li
C B
N
O F
Ne
Na
Mg
Al
P Si
S Cl
Ar
Ca K
Maximum number of
O atoms per atom
Figure 1.7 Periodic behavior of a chemical property The maximum number of oxygen atoms that bind to an atom is given as a function of the order number
of the atom. Elements shown in
the same color, such as N (N
O 2
) and P (P 5
O 2
), are in the same 5
chemical group, while elements between the atoms in red are in the same chemical period.
Indicate whether the energy of the system would increase, decrease, or remain the same during each of the following processes.
a) An anion approaches a cation.
An anion is negatively charged and a cation is
positively charged. According to Coulomb’s
law (Equation 2.4) opposite charges lo
wer their energy when they approach.
b) One cation is moves away from another.
Cations are positively charged. Moving like
charges away from one another lowers their
energy.
c) A neutron moves toward an electron.
Neutrons carry no charge, so there is no interaction and the energy remains the same.
1.10
DIMITRI MENDELEEV AND THE PERIODIC LAW (1869) Well after Dalton’s atomic theory had been
accepted, but before T
homson’s experiments,
Dimitri Mendeleev, a Russian chemist, was writing a chemistry book describing the reactions and properties of the elements. Ho
wever, he could not decide on a good way to
organize the chapters. As he sought to arrange and label the elements in an orderly fashion, he decided to use their atomic mas
ses because that was the only known sequential
number that was available. Consequently, he arranged the elements in order of increasing atomic masses. As Mendeleev studied the elemen
ts in this arrangement, he noticed that
their chemical and physical properties seemed to cycle (Figures 1.6 and 1.7).
He arranged the elements in rows of a length such that elements of similar properties
fell directly beneath one another to form chemical
families
or
groups
(see the caption to
Figure 1.6). The rows are referred to as
periods
(see the caption to Figure 1.7)

. Thus, the

properties of the elements change gradually as you progress across a period, but they remain similar as you progress down a group. The elements on the left side of any period tend to be lustrous solids (have a luster or shin

e) that are malleable (can be beaten into a

form), ductile (capable of being drawn into wires), and good conductors of heat and

Chapter 1 The Early Experiments

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