Isotopes are atoms of the same element that have the same number
of protons and electrons, but different numbers of neutrons.
• Isotopes of an element have the same chemical properties because
the number of electrons in the outermost shell is the same.
• Som e isotopes have stable nuclei, while others have unstable nuclei.
Unstable nuclei change or decay into stable ones by emitting
radioactive particles and energy. These unstable isotopes are called
radioactive isotopes, radioisotopes, or radionuclides.
• Isotopes are usually named by giving the element number and the
m ass number. Hydrogen-2 and hydrogen-3 are isotopes that have
other names as well.
Isotopes of hydrogen
d. Radioisotopes in medicine
Examples of radioisotopes used in medicine
Chrom ium -51
Labeling red blood cells
Treating cancer, asse ssin g kidney function
Im aging thyroid, treating thyroid cancer
Treating bone cancer pain
A sse ssin g lung ventilation
D iagnosing heart conditions
scan of a normal
produced in nuclear
ingested or injected
dium, with 11 protons and 12 neutrons in its nucleus, has
an atomic number of 11 and a mass number of 23.
Even though their exact positions cannot be predict-
ed, specific groups of electrons are most likely to move
about within certain regions around the nucleus. These
regions are called
electron sh e lls
they are depicted as circles in Figure 2.3a, b. The electron
shells can hold different numbers of electrons. The out-
ermost shell of an atom is called the
valence sh e ll
number of electrons in the valence shell determines the
atom’s chemical reactivity; the maximum number of elec-
trons that the valence shell can hold is eight.
Atoms of almost all elements have some variations in
which the number of protons in the nucleus is the same
but the number of neutrons within the nucleus is differ-
ent. These variations are called
Figure 2 .3 c
Isotopes have the same atomic number but different
atomic masses. Many isotopes have more neutrons than
the nucleus can hold and remain stable. The nucleus of
such unstable isotopes changes or decays to a stable nu-
cleus by releasing radioactive particles and energy; such
unstable isotopes are called
rad io iso to p es
are used in nuclear medicine for diagnostic imaging and
cancer treatment (
Matter Is Made of Elements and Atoms 25