Chapter 7
Ionic and Metallic Bonding
Valence Electrons
Scientists learned that all of the elements within each
group of the periodic table behave similarly because they
have the same number of valence electrons.
Valence electrons are the electrons in the highest
occupied energy level of an element’s atom.
The number of valence electrons largely determines the
chemical properties of an element.
To find the number of valence electrons in an atom of a
representative elements, simply look at its group number
Elements of Group IA have one valence electron.
Elements in Group 4A have four valence electrons,
and so forth
Valence Electrons
The noble gases, Group 8A, are the only exceptions to
the group-number rule.
Helium has two valence electrons, and all of the other
noble gases have eight.
Valence electrons are usually the only electrons used in
chemical bonds.
As a general rule, only the valence electrons are shown
in electron dot structures.
Electron dot structures are diagrams that show
valence electrons as dots.
Electron Dot Structures
The Octet Rule
Noble gases, such as neon and argon, are unreactive in
chemical reactions. (They are stable)
Gilbert Lewis explained why atoms form certain kinds of ions
and molecules in the octet rule.
The Octet Rule - in forming compounds, atoms tend to achieve
the electron configuration of a noble gas. An octet is a set of
eight. (each noble gas except helium has eight electrons in its
highest energy level)
Atoms of the metallic elements tend to lose their valence
electrons, leaving a complete octet in the next-lowest
energy level. Atoms of some nonmetallic elements tend to
gain electron or to share electrons with another
nonmetallic element to achieve a complete octet.
Formation of Cations
An atom is electrically neutral because it has equal
numbers of protons and electrons; an ion forms when an
atoms or group of atoms loses or gains electrons.
An atom’s loss of valence electrons produced a cation, or
a positively charged ion.
For metallic elements, the name of the ion is the same as
the name of the element.
Although their names are the same, there are many
important chemical differences between metals and their
cations.
Formation of Cations
Using electron dot structures, you can show the
ionization of some elements more simply.
Na·
Sodium atom
neutral
·Mg·
Na+
+
Sodium ion
1 unit of + charge
Mg2+
+
Magnesium atom
Magnesium ion
neutral
2 unit of + charge
eelectron
1 unit of - charge
2eelectron
2 units of - charge
NEVER look at the symbol for a cation and say that it
gained electrons! The “+” superscript refers to the
charge the ion has by losing electrons.
Transition Metals
For transition metals, the charges of cations may vary.
An atom of iron (Fe) may lose two, or three electrons
forming either Fe2+ or Fe3+ ions.
Some ions formed by transition metals do not have noble
gas electron configurations and are therefore exceptions
to the octet rule.
Ag is an example - 1s22s22p63s23p63d104s24p64d105s1
To achieve the electron structure of krypton, which is the
preceding noble gas, a silver atom would have to lose
eleven electrons.
Transition Metals
Ions with charges of three or greater are uncommon, and
losing eleven electrons is highly unlikely.
If Ag loses its 5s1 electron, the configuration that results,
(4s24p64d10) with 18 electrons in the outer energy level
and all of the orbitals filled, is relatively favorable in
compounds.
Such a configuration is known as pseudo noble-gas
electron configuration.
Ag forms a positive ion (Ag+) in this way.
Formation of Anions
The gain of negatively charge electrons by a neutral atom
produces an anion.
The name of an anion of a nonmetallic element is not the
same as the element name. The name of the ion typically
ends in -ide.
Chlorine atom (Cl) forms a chloride ion (Cl-)
Oxygen atom (O) forms an oxide ion (O2-)
Because they have relatively full valence shells, atoms of
nonmetallic elements attain noble-gas electron
configurations more easily by gaining electrons than by
losing them.
Formation of Anions
The gain of negatively charge electrons by a
neutral atom produces an anion.
NEVER look at the symbol for an anion and
say that it lost electrons! The “-” superscript
refers to the charge the ion has by gaining
electrons.
Formation of Anions
Chlorine belongs to Group 7A and has seven valence
electrons. A gain of one electron gives chlorine an octet
and converts a chlorine atom into a chloride ion.
Atoms of nonmetallic elements form anions by gaining
enough valence electrons so as to attain the electron
configuration of the nearest noble gas.
The chloride ion has the same electron configuration as
the noble gas argon.
Chloride ion (Cl-) 1s22s22p63s23p6
Argon (Ar) 1s22s22p63s23p6
Food For Thought
Chlorine atoms need one more valence electron to
achieve the electron configuration of the nearest noble
gas.
Any electron in an atom outside the noble gas core is
called a valence electron.
Various atoms of the representative elements form ions
and gain a noble-gas electron configuration
Formation of Anions
Halide ions – the ions that are produced when atom of
chlorine and other halogens gain electrons
All halogen atoms have seven valence electrons and
need to gain only one electron to achieve the electron
configuration of a noble gas.
All halide ions (F-, Cl-, Br-, and I-) have charge of 1-.
Questions
How can you determine the number of valence electrons
in an atom of a representative element?
Look up the group number (using the A/B system) of that
element.
Atoms of which elements tend to gain electrons? Atoms
of which elements tend to lose electrons?
Nonmetallic – gain
metallic - lose
How do cations form? How do anions form?
Cation – atom loses valence electrons
Anion – atom gains valence electrons
End of Section 7.1
Formation of Ionic Compounds
Compounds composed of cations and anions are called
ionic compounds.
compounds
Ionic compounds are usually composed of metal cations
and nonmetal anions. Ex: NaCl is formed from Na+ + ClAlthough they are composed of ions, ionic compounds
are electrically neutral. The total + charge of the cations
equals the total – charge of the anions.
Anions and cations have opposite charges and attract
one another by means of electrostatic forces.
The electrostatic forces that hold ions together in ionic
compounds are called ionic bonds.
bonds
Formation of Ionic Compounds
Look at the reaction of a Na atom and a chlorine atom.
Na has 1 valence electron that it can easily lose. (Na is in
group 1A of the representative elements, thus has 1
valence electron)
Cl has seven valence electrons and can easily gain one
electron. (Cl is in group 7A of the representative
elements, thus has 7 valence electrons)
If Na loses its valence electron it achieves the stable
electron configuration of neon. If Cl gains a valence
electron, it achieves the stable electron configuration of
argon. (Remember the Octet Rule, which should be
called “Look Like a Noble Gas Rule”)
Formation of Ionic Compounds
When Na and Cl react, the Na atom gives its one valence
electron to a Cl atom. They react in a 1:1 ratio and both
ions have stable octets.
+
Na+
1s22s22p6
Cl1s22s22p63s23p6
Formula Units
Chemists represent the composition of substances by
writing chemical formulas. A chemical formula shows
the kinds and numbers of atoms in the smallest
representative unit of a substance.
NaCl is the chemical formula for sodium chloride.
A Formula unit is the lowest whole-number ratio of ions in
an ionic compound. One Na+ to each Cl-, thus the
formula unit for sodium chloride is NaCl.
Even though ionic charges are used to derive the correct
formulas, they are not shown when you write the formula
unit of the compound. NEVER show charges in a formula
unit!
Formula Units
The ionic compound Magnesium chloride (MgCl2)
contains magnesium cations (Mg2+) and chloride
anions (Cl-)
In MgCl2, the ratios of Mg2+ to Cl- is 1:2 (One Mg2+ to
two Cl-). Its formula unit is MgCl2
Because there are twice as many Cl- (each with a 1charge) as Mg2+ (each with a 2+ charge), the
compound is electrically neutral.
Another example: Al3+ + Br- combine to form AlBr3.
Questions
Use electron dot structures to determine formulas of the
ionic compounds formed when
Potassium reacts with iodine
KI
Aluminum reacts with oxygen
Al2O3
Properties of Ionic Compounds
Most ionic compounds are crystalline solids at room
temperature.
The component ions in such crystals are arranged in
repeating three-dimensional patterns.
In NaCl, each sodium ion is surrounded by six chloride
ions, and each chloride ion is surrounded by six
sodium ions.
In this arrangement, each ion is attracted strongly to
each of its neighbors and repulsions are minimized.
The large attractive forces result in a very stable
structure.
Properties of Ionic Compounds
Many, but not all, ionic compounds dissolve in water
through interaction with water molecules.
Properties of Ionic Compounds
Ionic compounds can conduct an electric current
when melted or dissolved in water.
When NaCl is melted, the orderly crystal structure
breaks down.
The movement of the ions allows electricity to flow
Questions
How can you describe the electrical charge of an ionic
compound?
Electrically neutral
What properties characterize ionic compounds?
Usually solids at room temperature; have high melting
points; conduct electric current when melted or
dissolved in water.
Define an ionic bond
Electrostatic forces that hold ions together in an ionic
compound
Questions
Write the correct chemical formula for the compounds
formed from each pair of ions.
K+ , S2Ca2+ , O2Na+ , O2-
Al3+ , N3-
K2S , CaO, Na2O , AlN
Write formulas for each compound: barium chloride,
magnesium oxide, lithium oxide, calcium fluoride
BaCl2
MgO
Li2O
CaF2
Which pairs of elements are likely to form ionic
compounds?
Cl, Br
Li, Cl
K, He
I, Na
Li, Cl
I, Na
End of Section 7.2
Metallic Bonds & Properties
Metals are made up of closely packed cations rather than
neutral atoms.
The valence electrons of metal atoms can be modeled as a
sea of electrons. (they are mobile and can drift freely from
one part of the metal to another).
Metallic bonds consists of the attraction of the free-floating
valence electrons from the positively charged metal ion.
The sea-of-electrons model explains many physical
properties of metals.
– Good conductors of electrical current because
electrons can flow freely.
– Ductile – they can be drawn into wires.
– Malleable – they can be hammered or forced into
shapes.
Crystalline Structure of Metals
The crystalline structures of metals
can be compared to the stacking of
oranges in the grocery store to save
space.
Metals are crystalline and they are
arranged in very compact and orderly
patterns.
Crystalline Structure of Metals
There are several closely packed arrangements that are
possible.
• body-centered cubic arrangement
• face-centered cubic arrangement
• hexagonal close-packed arrangement
Body-centered cubic
Every atom (except those on the
Surface) has eight neighbors.
Crystalline Structure of Metals
Face-centered cubic arrangement
• every atom has twelve neighbors.
Crystalline Structure of Metals
Hexagonal close-packed arrangement
• every atom also have twelve neighbors. Because of the
hexagonal shape, the pattern is different from the facecentered.
Alloys
Very few of the metallic items that you use every day are
pure metals. Ex: spoons.
Most of the metals you encounter are alloys.
Alloys are mixtures composed of two or more elements.,
at least on of which is a metal. Ex: Brass (Cu & Zn)
Alloys properties are often superior to those of their
component elements.
Sterling silver (92.5% silver & 7.5% copper) is harder and
more durable than pure silver, but still soft enough to be
made into jewelry and tableware.
Alloys
Bronze – 7 parts copper to 1 part tin. Bronze is harder
than copper and more easily cast.
Nonferrous (non-iron) alloys are commonly used to make
coins.
The most important alloys today are steels.
Alloys can form from their component atoms in different
ways. If the atoms of the components in an alloy are
about the same size, they can replace each other in the
crystal. (substantial alloy)
If the atomic sizes are different, the smaller atoms can fit
into the spaces between the larger atoms. (interstitial
alloy)
Questions
How do chemists model the valence electrons in metal
atoms?
Metal cations surrounded by a sea of mobile valence
electrons.
How can you describe the arrangement of atoms in
metals?
Atoms in metals are arranged in a compact and orderly
manner
Why are alloys more useful than pure metals?
Their properties are often superior to their component
elements.
End of Chapter 7