s8pe-30802-ca
12/1/05
11:16 AM
MAZER
Page 250
Atoms can transfer electrons.
Ions are formed when atoms gain or lose electrons. Gaining electrons
changes an atom into a negative ion. Losing electrons changes an atom
into a positive ion. Individual atoms often do not form ions by themselves. Instead, ions typically form in pairs when one atom transfers
one or more electrons to another atom.
Reminder
Remember that the elements in columns of the
periodic table show similar
chemical properties.
1
2
Li+
Be2+
Na+ Mg2+
+
K
2+
Ca
An element’s location on the periodic table can give a clue as to
the type of ions the atoms of that element will form. The illustration
to the left shows the characteristic ions formed by several groups.
Notice that all metals lose electrons to form positive ions. Group 1
metals commonly lose only one electron to form ions with a single
positive charge. Group 2 metals commonly lose two
electrons to form ions with two positive charges. Other
16
17
metals, like the transition metals, also always form positive
O2–
F–
ions, but the number of electrons they may lose varies.
S2–
Cl –
Se2–
Br –
Rb+ Sr2+
I–
Cs+ Ba2+
Fr+
Nonmetals form ions by gaining electrons. Group 17
nonmetals, for example, gain one electron to form ions
with a 1– charge. The nonmetals in Group 16 gain two
electrons to form ions with a 2– charge. The noble gases do
not normally gain or lose electrons and so do not normally
form ions.
Ra2+
check your reading
What type of ions do metals form?
Ionic Bonds
What happens when an atom of an element from Group 1, like sodium,
comes near an atom of an element from Group 17, like chlorine?
Sodium is likely to lose an electron to form a positive ion. Chlorine is
likely to gain an electron to form a negative ion. An electron, therefore, moves from the sodium atom to the chlorine atom.
sodium atom (Na)
chlorine atom (Cl)
sodium ion (Na+) chloride ion (Cl–)
Remember that particles with opposite electrical charges attract
one another. When the ions are created, therefore, they are drawn
toward one another by electrical attraction. This force of attraction
between positive and negative ions is called an ionic bond.
250 Unit 3: Chemical Interactions
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