Chemistry: The Central Science (13th Edition)

Published by Prentice Hall
ISBN 10: 0321910419
ISBN 13: 978-0-32191-041-7

Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 75: 2.27c


Protons: 31 Neutrons: 39 Electrons: 31

Work Step by Step

The atomic mass number, 70, of the isotope is given in the problem. The symbol Ga identifies the isotope as being a Gallium isotope, which the periodic table shows has 31 protons (the atomic number of the element is equal to the number of electrons). Because the atomic mass number is 70, there must be 39 neutrons. After all, electrons essentially weigh 0 amu while protons and neutrons weigh 1 amu, meaning that the number of neutrons and protons add up to the atomic mass number 70 (31 protons+39 neutrons=70). Finally, there is no indication of charge in the symbol, so the isotope is neutral. Because neutrons have no charge, the number of protons and electrons (which have opposite charges) must be equal, meaning that there are 31 electrons.
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