Chemistry - A Molecular Science

Chapter 1 The Early Experiments

1.9

SUBATOMIC PARTICLES, ISOTOPES, AND IONS

As summarized in Table 1.1, scientists have discovered that atoms are composed of three subatomic particles: protons, neutrons, and el

ectrons. The number of protons identifies the

nucleus, and the number of protons and neutrons determines its mass.

Atomic number, Z

, is the number of protons in t

he nucleus. It is the number that

characterizes an element. If the atomic number

of an element is known, then the identity of

the element is known and

vice versa

.^

Mass number, A

, is the sum of the number of protons

and the number of neutrons in the

nucleus. Because the mass of each of these par

ticles is nearly 1 amu, the mass number is

an integer that closely approx

imates the nuclear mass in atomic mass units (amu).

The atomic mass is sometimes included with the symbol of the element. For example, the notation

63 Cu (read copper-63) indicates that the mass number of Cu is 63. There is no

reason to include Z because it is implied in

the element’s symbol. Cu means Z = 29

because Cu always has 29 protons. Note that the number of neutrons is A minus Z. Thus, 63 Cu has 63 - 29 = 34 neutrons.*

Isotopes

are different forms of the same element that have the same atomic numbers

(Z) but different masses (A). C

onsequently, they differ only in the number of neutrons (A -

Z). For example:

35 Cl and

37 Cl, chlorine-35 and chlorine-37, are two isotopes of chlorine.

Both contain 17 protons because both are chlorine atoms. They differ in that

35 Cl contains

18 neutrons (35 - 17) while

37 Cl contains 20 neutrons (37 - 17). Naturally occurring

chlorine is 75.8%

35 Cl and 24.2%

37 Cl. It is this combination of the two that gives a mole

of chlorine a mass of 35.5 grams (the molar mass of chlorine).

Table 1.1 Subatomic particles

Particle Mass (amu) Charge

†^

electron 5.49 x 10

-4 -1

proton 1.00728 +1 neutron 1.00867 0

†

Charge at the atomic and molecular level is usually expressed as multiples of the fundamental unit

of charge as determined by

Millikan. Thus, the charge on the electron is designated as -1, which means that it carries a charge of (-1)(1.6 x 10

-19

C). The

charge on the proton is then (+1)(1.6 x 10

-19

C ).

* A and Z are sometimes shown in the atom’s symbol. Thus, the

element whose symbol is X and whose atomic number and mass are Z and A, respectively would be represented as follows:

AX Z

⇒

# protons

(# protons + # neutrons)

(^) X
This notation is redundant because the symbol (X) identifies the
element and its atomic number (Z).
However, the notation is used
when balancing nuclear equations.
Charge is not continuous; it comes in bundles
§ of 1.6x10
-19
C

. Electrons are bundles of

negative charge and protons are bundles of positiv

e charge. All charge is comprised of an

integral number of these bundles, and the net charge on a particle at the atomic and molecular level is usually given as the differe

nce between the positive and negative charge

carriers rather than the actual charge in coulombs.

§ Quantities that are not continuous

but come in discrete units are

said to be

quantized

, and the bundles that carry the units are

called quanta. Thus, electrons

and protons are quanta of negative

and positive electrical charge. In Chapter 2, we show that electromagnetic energy is also quantized.

charge on particle = number of protons – number of electrons

Atoms have no charge because the number of protons and electrons are equal. Thus, Al (Z=13) contains 13 protons, 13 electrons, a

nd no net charge. While the number of protons

in an atom is fixed, electrons can be added

or removed to produce charged particles called

ions

. Positively charged ions, which are called

cations,

are produced when electrons are

lost, and negatively charged ions, which are called

anions,

are produced when electrons

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