Chapter 2.1 -2.2
Ionic Compounds
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recall, from Chapter 1, that atomic theory describes electrons moving about the
nucleus of the atom in energy levels, and that the electrons in the outermost energy
level are called the valence electrons.
It is the valence electrons of an atom that form chemical bonds. chemical bond: the
forces of attraction holding atoms or ions together
recall that all matter is either an element or a compound
element: a pure substance that cannot be broken down into simpler substances by
chemical means (empirical definition); a substance composed entirely of one kind
of atom (theoretical definition)
compound: a pure substance that can be broken down by chemical means to produce
two or more pure substances (empirical definition); a substance containing atoms of
more than one element combined in fixed proportions (theoretical definition)
ionic compound: a pure substance formed from a metal and a nonmetal
molecular compound: a pure substance formed from two or more different nonmetals
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Ionic Compounds
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Ionic Compounds
Ionic Bonding forming Ionic Compounds
According to atomic theory, ionic compounds are formed when one or more
valence electrons are transferred from a metal atom to a nonmetal atom.
This leaves the metal atom as a positive ion, or cation, and the nonmetal atom
as a negative ion, or anion.
The two oppositely charged ions are attracted to each other by a force called
an ionic bond.
Explaining the Properties of Ionic Compounds
• They are solids at SATP with high melting points, often hard and brittle, and
they are electrolytes.
1. Solid at SATP with High Melting Point
• form what is called a crystal lattice – a regular, order arrangement of atoms,
ions or molecules.
• Although composed of ions, pure ionic compounds are electrically neutral.
• (i.e. the sum total of the electrical charges on all the ions must be zero)
• To achieve neutrality - anions and cations in an ionic compound are locked in
a regular structure, held by the balance of attractive bonds and electrical
repulsion.
• The most common model of ions shows them as spheres arranged in a
regular three dimensional pattern called a crystal lattice (Figure 2, page 70)
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Ionic Compounds
Explaining the Properties of Ionic Compounds (cont’d)
• not all crystal lattices are square
• depends on the sizes and charges of the ions that make up the substance
• all lattices are arranged so that each ion has the greatest possible number of
oppositely charged ions close, while keeping ions with the same charge as far
away as possible.
• In all cases, each ion is surrounded by ions of opposite charge.
• In theory, this arrangement of ions creates strong attractions.
• This theory is supported by empirical evidence such as the hard surfaces and
high melting and boiling points of ionic solids.
2. Hard and Brittle
• It requires a great deal of energy to break the strong electrostatic attractions
within a crystal lattice.
• The ions resist any movement, as even a slight shift would cause positive ions
to move closer to other positive ions, and negative ions closer to other
negative ions, resulting in strong repulsion.
• Once the lattice is broken, repulsions between ions of the same charge will
cause the substance to split into two crystals.(i,e, brittle)
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Ionic Compounds
Explaining the Properties of Ionic Compounds (cont’d)
3. Electrolytes
• When ionic compounds are dissolved in water, the positive and negative ions
dissociate.
• The ions are responsible for carrying current when charged electrodes are
placed in the ionic solution.
Formula Units for Ionic compounds
• Ionic compounds are made up of a fixed proportion of positive and negative
ions.
• Consequently, ionic compounds can only be identified in terms of the smallest
unit of the compound, known as a formula unit, that would still have the
properties of the compound.
• In the case of sodium chloride, the sodium and chloride ions are present in a
1:1 ratio, as indicated by its chemical formula, NaCl.
• Note the formula unit is the same as the chemical formula.
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Ionic Compounds
Ionic Compounds
The Formation of Ionic Compounds
The Formation of Ionic Compounds (cont’d)
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Groups 1 and 2 form ionic compounds with oxygen producing oxides:
The oxides formed by Group 1 elements have the general formula M2O while
those of Group 2 elements (e.g., magnesium oxide, MgO) have the general
formula MO, where M represents a metal ion.
Because of its high melting point, magnesium oxide is used to make objects
that are exposed to very high temperatures, such as crucibles, furnace linings,
and thermal insulation.
Group 1 metals readily react with the elements in Group 17 to form ionic
compounds with the general formula MX.
These compounds, which are composed of a metal and a halogen, are
collectively referred to as ionic halides. Sodium chloride, an example of an
ionic halide, is found in large underground deposits in various parts of the
world. It is mined from these deposits and used as road salt, table salt, and as
a reactant in many industrial processes.
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The elements in Group 2 show a similar trend, as they also react with the
halogens to produce ionic halides with the general formula MX2
In general, the addition of a metal from Group 1 or Group 2 to water will
produce hydrogen gas and a basic ionic compound.
Calcium hydroxide, Ca(OH)2, is an example of an ionic compound that can be
produced in this way.
However, the reaction of calcium and water is quite vigorous.
A safer means of producing this ionic compound is to react calcium oxide with
water.
Calcium hydroxide is also referred to as slaked lime and is used to make
mortar and plaster for buildings.
The metals in Groups 13 to 15, except mercury, will form ionic oxides
(compounds composed of a metal and oxygen) when burned in air.
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Ionic Compounds
Ionic Compounds
Predicting Common Ions of Atoms
Representing Ionic Bonds
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By looking at an element’s position in the periodic table, we can predict the
charge on that element’s most stable ion.
Rule to use is the stable octect rule – an atom will become stable if it loses or
gain electrons to get to the nearest noble gas.
Noble gases are stable and virtually inert. Similarly, ions with eight valence
electrons appear to have a special stability.
This arrangement of electrons is known as a stable octet. stable octet: a full
shell of eight electrons in the outer energy level of an atom.
To reach this stable state, metal atoms of elements in Groups 1, 2, and 3 will
lose electrons to form cations, while elements in Groups 15, 16, and 17 will
gain electrons to form anions.
Electron dot diagram or Lewis symbol: a representation of an atom or ion,
made up of the chemical symbol and dots indicating the number of electrons in
the valence energy level
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Ionic Compounds
Ionic Compounds
Representing Ionic Bonds (cont’d)
Representing Ionic Bonds
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Using electron dot diagrams, we can show the formation of an ionic bond
between sodium and chlorine.
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we place square brackets around the ion to indicate that the charge is not
associated with any particular electron and that all the electrons in the valence
shell are equivalent.
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Electron dot diagrams also illustrate the theory that ionic bonds tend to
produce full outer orbits of electrons: a configuration exactly the same as that
of the noble gases.
Sodium has one valence electron. By transferring this electron to another
entity that has a stronger attraction for the electron, the resulting sodium ion
will have the same electron configuration as neon.
A chlorine atom has seven valence electrons. By attracting an electron from
another entity, the resulting chloride ion will have eight electrons in its valence
shell and the same number of electrons as its nearest noble gas, argon.
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