Even though the number of protons must be the same for all atoms of an element, the number of
neutrons, and hence the mass number, can be different. Atoms of the same element with different
masses are known as isotopes of one another. Isotopes are referred to either by the convention
described above or, more commonly, by the name of the element followed by the mass number. For
example, carbon-12 is a carbon atom with 6 protons and 6 neutrons, while carbon-14 is a carbon
atom with 6 protons and 8 neutrons. Since isotopes have the same number of protons and
electrons, they generally exhibit the same chemical properties. Chapter 15 describes nuclear
chemistry in greater detail.
ELECTRONS
Electrons carry a charge equal in magnitude but opposite in sign to that of protons. An electron has
a very small mass, approximately 1/1837 the mass of a proton or neutron, which is negligible for
most purposes. The electrons farthest from the nucleus are known as valence electrons. The farther
the valence electrons are from the nucleus, the weaker the attractive force of the positively charged
nucleus and the more likely the valence electrons are to be influenced by other atoms. Generally,
the valence electrons and their activity determine the reactivity of an atom. In a neutral atom, the
number of electrons is equal to the number of protons. A positive or negative charge on an atom is
due to a loss or gain of electrons; the result is called an ion. A positively charged ion (one that has
lost electrons) is known as a cation; a negatively charged ion (one that has gained electrons) is
known as an anion.
Some basic features of the three subatomic particles are summarized in Table 3.1 on the next page.
Example: Determine the number of protons, neutrons, and electrons in a nickel-58 atom and in a
nickel-602+ cation.
Solution:^58 Ni has an atomic number of 28 according to the periodic table and a mass number of
- Therefore, ^58 Ni will have 28 protons, 28 electrons (since it is a neutral atom), and 58
– 28, or 30, neutrons.
In the ^60 Ni2+ species, the number of protons is the same as in the neutral ^58 Ni atom.
However, ^60 Ni2+ has lost two electrons and thus will only have 26 electrons: this is what
gives it the +2 charge. Also, the mass number is two units higher than for the ^58 Ni atom,
and this difference in mass must be due to two extra neutrons; thus it has a total of 32
neutrons.