Page 57 - Theory and Problems of BEGINNING CHEMISTRY

46 ATOMS AND ATOMIC MASSES [CHAP. 3

Ans. 75.77 24.23
(34.968 852 amu) + (36.965 903 amu) = 35.45 amu
100 100

ATOMIC STRUCTURE
3.14. If uncombined atoms as a whole are neutral, how can they be made up of charged particles?

Ans. The number of positive charges and negative charges (protons and electrons) are equal, and the effect of the
positive charges cancels the effect of the negative charges.

3.15. Explain, on the basis of the information in Table 3-1, (a) why the nucleus is positively charged, (b)why
the nucleus contains most of the mass of the atom, (c) why electrons are attracted to the nucleus, and (d)
why the atom may be considered mostly empty space.
Ans. (a) The only positively charged particles, the protons, are located there. (b) The protons and neutrons, both
massive compared to the electrons, are located there. (c) The nucleus is positively charged, and they are
negatively charged. (d) The nucleus is tiny compared to the atom as a whole, and the nucleus contains
most of the mass of the atom. Thus, the remainder of the atom contains little mass and may be thought of as
mostly empty space.

3.16. (a) How many protons are there in the nucleus of a neon atom? (b) How many protons are there in the
neon atom?
Ans. (a) and (b) 10 (all the protons are in the nucleus, so it is not necessary to specify the nucleus). Here are two
questions that sound different, but really are the same. Again, you must read the questions carefully. You
must understand the concepts and the terms; you must not merely memorize.

ISOTOPES
3.17. Complete the following table (for uncombined atoms):

Atomic Mass Number of Number of Number of
Symbol Number Number Protons Electrons Neutrons

(a) 3 He
(b) 8 18
(c) 24 28
(d) 107 47
(e) 29 36

Ans.
Atomic Mass Number of Number of Number of
Symbol Number Number Protons Electrons Neutrons

(a) 3 He 2 3 2 2 1
(b) 18 O 8 18 8 8 10
(c) 52 Cr 24 52 24 24 28
(d) 107 Ag 47 107 47 47 60
(e) 65 Cu 29 65 29 29 36

(a) Atomic number is determined from the identity of the element and is equal to the number of protons
and to the number of electrons. Its value is given in the periodic table. The mass number is given, and the
number of neutrons is equal to the mass number minus the atomic number. (b) The atomic number and
the number of protons are always the same, and these are the same as the number of electrons in