Page 59 - Theory and Problems of BEGINNING CHEMISTRY

48 ATOMS AND ATOMIC MASSES [CHAP. 3

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

(a) 78 Se 34 78 34 34 44
34
(b) 44 Ca 20 44 20 20 24
20
(c) 14 6 C 6 14 6 6 8

(d) 50 Cr 24 50 24 24 26
24
(e) 80 Se 34 80 34 34 46
34

PERIODIC TABLE
3.26. How many electrons are there in the outermost shell of (a) H and (b) He?
Ans. (a) One. There is only one electron in the whole atom. The shell can hold two electrons, but in the free
hydrogen atom, there is only one electron present. (b)Two.

3.27. How are the 19 electrons of the potassium atom arranged in shells?
Ans. 2881
The first shell of any atom holds a maximum of 2 electrons, and the second shell holds a maximum of 8.
Thus, the first 2 electrons of potassium fill the first shell, and the next 8 fill the second shell. The outermost
shell of any atom can hold at most 8 electrons. In potassium, there are 9 electrons left, which would fit into
the third shell if it were not the outermost shell; however, if we put the 9 electrons into the third shell, it would
be the outermost shell. Therefore, we put 8 of the remaining electrons in that shell. That leaves 1 electron in
the fourth shell.

3.28. How many electrons are there in the outermost shell of each of the following elements? (a) Li, (b) Be,
(c) Al, (d) Si, (e)N,(f)O,(g) Cl, and (h)Ar.
Ans. (a)1 (b)2 (c)3 (d)4 (e)5 ( f )6 (g)7 (h)8

3.29. Write the electron dot symbol for each atom in Problem 3.28.
Ans. Li Be Al Si N O Cl Ar

Supplementary Problems

3.30. Distinguish clearly between (a) neutron and nucleus, (b) mass number and atomic mass, (c) atomic number and mass
number, (d) atomic number and atomic mass, and (e) atomic mass and atomic mass unit.
Ans. (a) The nucleus is a distinct part of the atom. Neutrons are subatomic particles that, along with protons,
are located in the nucleus. (b) Mass number refers to individual isotopes. It is the sum of the numbers of
protons and neutrons. Atomic mass refers to the naturally occurring mixture of isotopes, and is the relative
12
mass of the average atom compared to C. (c) Atomic number equals the number of protons in the nucleus;
mass number is defined in part (b). (d) These terms are defined in parts (b) and (c). (e) Atomic mass unit
is the unit of atomic mass.

3.31. Can you guess from data in the periodic table what the most important isotopes of bromine are? With the additional
80
information that Br does not occur naturally, how can you amend that guess?
80
Ans. The atomic mass of Br is about 80, so one might be tempted to guess that Br is the most important isotope.
Knowing that it is not, one then can guess that an about-equal mixture of 79 Br and 81 Br occurs, which is
correct.