Page 52 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 3] ATOMS AND ATOMIC MASSES 41

Ans. (a)17(1 amu) + 18(1 amu) = 35 amu
(b)17(1 amu) + 20(1 amu) = 37 amu
The nuclei differ in mass (but not in charge).

Uncombined atoms as a whole are electrically neutral.
EXAMPLE 3.6. Refer to Table 3-1 and deduce which two of the types of subatomic particles in an uncombined atom occur
in equal numbers.
Ans. The number of positive charges must equal the number of negative charges, since the atom has a net charge of zero.
The number of positive charges, as shown in the table, is equal to the number of protons. The number of negative
charges, also from the table, is equal to the number of electrons. Therefore, in an uncombined atom, the number of
protons must equal the number of electrons.

The number of protons in the nucleus determines the chemical properties of the element. That number is
called the atomic number of the element. Atomic number is symbolized Z. Each element has a different atomic
number. An element may be identified by giving its name or its atomic number. Atomic numbers may be specified
by use of a subscript before the symbol of the element. For example, carbon may be designated 6 C. The subscript
is really unnecessary, since all carbon atoms have atomic number 6 and all atoms with atomic number 6 are
carbon atoms, but it is sometimes useful to include it. Atomic numbers are listed in the periodic table, page 350,
and in the Table of Elements, page 349.

EXAMPLE 3.7. (a) What is the charge on a magnesium nucleus? (b) What is the charge on a magnesium atom?
Ans. (a) +12, equal to the atomic number of magnesium (from the Table of Elements, p. 349). (b) 0 (all uncombined
atoms have a net charge of 0). Note that these questions sound very much alike, but are very different. You must read
questions in chemistry very carefully.
3.5. ISOTOPES
Atoms having the same number of protons but different numbers of neutrons are called isotopes of one
another. The number of neutrons does not affect the chemical properties of the atoms appreciably, so all isotopes
of a given element have essentially the same chemical properties. Different isotopes have different masses
(contrary to Dalton’s second postulate) and different nuclear properties, however.
The sum of the number of protons and the number of neutrons in the isotope is called the mass number of
the isotope. Mass number is symbolized A. Isotopes are usually distinguished from one another by their mass
numbers, given as a superscript before the chemical symbol for the element. Carbon-12 is an isotope of carbon
with a symbol 12 C.

12
EXAMPLE 3.8. (a) What is the sum of the number of protons and the number of neutrons in C? (b) What is the number
12
12
of protons in C? (c) What is the number of neutrons in C?
Ans. (a) 12, its mass number. (b) 6, its atomic number, given in the periodic table. (c)12 − 6 = 6. Note that the mass
numbers for most elements are not given in the periodic table.
EXAMPLE 3.9. Choose the integer quantities from the following list: (a) atomic number, (b) atomic mass, and (c) mass
number.
Ans. Atomic number and mass number are integer quantities; atomic mass is not in general equal to an integer.

EXAMPLE 3.10. Choose the quantities that appear in the periodic table from the following list: (a) atomic number,
(b) mass number, and (c) atomic mass.
Ans. Atomic number and atomic mass appear in the periodic table. The mass numbers of only those few elements that do
not occur naturally appear there in parentheses.
The atomic mass of an element is the weighted average of the masses of the individual isotopes of the
element. (Thus, unless an element consists of only one stable isotope, the atomic mass is not equal to the mass
of any atom.)