Chapter 4: Atoms and Elements
 Determine ion charge from numbers of
protons and electrons.
 Determine the number of protons and
electrons in an ion.
 In chemical reactions, atoms often lose
or gain electrons to form charged
particles called ions.
 Positive ions are called cations.
 Negative ions are called anions.
 The charge of an ion is shown in the upper
right corner of the symbol.
 Ion charges are usually written with the
magnitude of the charge first, followed by
the sign of the charge.
 Examples: Mg2+, O2-

The charge of an ion depends on how many
electrons were gained or lost and is given by
the following formula where p+ stands for
proton and e− stands for electron.

In reactions, lithium atoms lose one electron (e−) to
form Li+ ions.

For the Li+ ion with 3 protons and 2 electrons, the
charge is
 Ion charge = 3 − 2 = 1+

In reactions, fluorine atoms gain one electron (e−) to
form F− ions:

For the F− ion with 9 protons and 10 electrons, the
charge is
 Ion charge = 9 – 10 = 1–
 The location of the elements on the
periodic table gives clues to the number of
valence electrons an atom has.
 Valence electrons: the electrons found in the
outermost shell of an atom. There cannot be
any more than 8 valence electrons in an atom.
 In order to be stable, elements want to
have a full outer shell.
 They want 8 electrons.
 Atoms form ions in order to become
more stable.
 The key to predicting the charge acquired
by an element is its position in the periodic
table relative to the noble gases.
 Main-group elements tend to form ions
that have the same number of valence
electrons as the nearest noble gas.
-1 e-
Lose Electrons
-2 e-
-3 e-
+4 eor
-4 e-
Gain Electrons
+3 e-
+2 e-
+1 e-
 Alkali Metals (group 1) will form 1+ ions
 Alkaline Earth Metals (group 2) will
form 2+ ions
 The Boron Group (group 3) will form 3+
ions
 Transition metals commonly form 2+ or
3+ ions, but can form greater than 3+
ions.
 Halogens (group 17) will form 1- ions
 Chalcogens (group 16) will form 2- ions
 Pnictogens (group 15) will form 3- ions
 Determine the charge of each ion:
 Mg with 10 electrons
 S with 18 electrons
 Fe with 23 electrons
 Determine the number of protons and
electrons in the Ca2+ ion.
 Determine the number of protons and
electrons in the S2- ion.
 Based on their position in the periodic
table, what ions do barium and iodine
tend to form?