Page 58 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 3] ATOMS AND ATOMIC MASSES 47

uncombined atoms. (c) The sum of the number of protons and neutrons equals the mass number. In general,
if we use Z for the atomic number, A for the mass number, and n for the number of neutrons, we get the
simple equation:
A = Z + n
Once we are given two of these, we may calculate the third. Also,
Z = number of protons = number of electrons in uncombined atom

3.18. What subscripts could be used in the symbols in the first column of the answer to Problem 3.17?

Ans. The atomic numbers can be used as subscripts to the left of the chemical symbols.
3.19. (a) Which columns of the answer to Problem 3.17 are always the same as other columns for uncombined
atoms? (b) Which column is the sum of two others? (c) Which column is the same as the superscripts of
the symbols?

Ans. (a) Atomic number, number of protons, and number of electrons
(b) Mass number = number of protons + number of neutrons
(c) The superscripts in the first column are repeated in the column of mass numbers

3.20. The numbers of which subatomic particle are different in atoms of two isotopes?
Ans. The number of neutrons differs.

18
16
3.21. Consider the answer to Problem 3.20 and the fact that O and O have very similar chemical properties.
Which is more important in determining chemical properties, nuclear charge or nuclear mass?
Ans. Since the charge is the same and the mass differs, the chemical properties are due to the charge.

3.22. What is the net charge on an atom that contains 8 protons, 8 neutrons, and 10 electrons?
Ans. The charge is −2. There are 10 electrons, each with −1 charge for a total of −10, and there are 8 protons,
each with a +1 charge, for a total of +8. It does not matter how many neutrons there are, since they have no
charge. +8 + (−10) =−2. If the initial letter of the particle stands for the number of that particle, then
Charge = (+1)p + (−1)e + (0)n = (+1)(8) + (−1)(10) + (0)(8) =−2

3.23. Two uncombined atoms are isotopes of each other. One has 17 electrons. How many electrons does the
other have?
Ans. The second isotope also has 17 electrons. The only difference between isotopes is the number of neutrons.

3.24. What is the mass number of an atom with 8 protons, 8 neutrons, and 8 electrons?
Ans. The mass number is the number of protons plus neutrons; the number of electrons is immaterial.
8 + 8 = 16

3.25. Complete the following table (for uncombined atoms):

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

(a) 78 Se
34
(b) 20 44
(c) 14 8
(d) 50 24
(e) 80 34