chapter 7: ionic and metallic bonding what is a chemical bond? a mutual attraction between the...
TRANSCRIPT
Chapter 7Ionic and Metallic bonding
Chapter 7: Ionic and Metallic BondingWhat is a chemical bond?
• a mutual attraction between the nuclei and valence electrons of different atoms that bind the atoms together.
• Why?
REMEMBER: valence electrons are involved in bonding!!
Types of Bonds
1. Ionic Bond an electrical attraction due to an exchange of electrons.
2. Covalent Bond a bond that results from the sharing of electrons.
How do you determine the bond type?
IonicBond
BondFormation
Covalent
Electronegativity
BondLength
Bond Type Using Electronegativities
Nonpolar(0 – 0.3)
CovalentPolar
(0.3 – 1.7)
Ionic Ionic(1.7 – 3.3)
Covalent Bonds
• atoms are joined by sharing of electrons.
1. Nonpolar-covalent: an equal sharing of electrons/an even distribution of electron clouds.
2. Polar-covalent: an unequal sharing of electrons/an uneven distribution of electron clouds.
Ionic Bonds: a bond that results from a mutual electrical attraction between a cation and an anion.
Na + Cl
“molecule”: a neutral group of atoms held together by covalent bonds.
“diatomic molecule”: a molecule containing only two atoms.
ex: O2, H2, N2
Electron Dot Diagrams
• shows how an atom’s valence electrons become involved in bonding.
Hydrogen (1 valence electron)
Beryllium (2 valence electron)
Carbon (4 valence electron)
Nitrogen (5 valence electron)
Oxygen (6 valence electron)
Neon (8 valence electron)
UnsharedPairs/
Lone Pairs
Bonding site
Lewis Structures
Shows how atoms are bonded in a molecule.
Unshared pairs (lone pairs): a pair of electrons that are not involved in bonding (exclusive only to that atom).
Draw the dot diagrams for each element first, then “connect the dots”.
Ex: H2O
Ex: BF3
Ex: C2H4
Formation of Ionic Compounds
Formation of Ionic Compounds
What is the electrical charge of an ionic compound?
Formation of Ionic Compounds
Sodium chloride, or table salt, is an ionic compound consisting of sodium cations and chloride anions.
• An ionic compound is a compound composed of cations and anions.
Formation of Ionic Compounds
Although they are composed of ions, ionic compounds are electrically neutral.• The total positive charge of the cations equals
the total negative charge of the anions.
Formation of Ionic Compounds
• The electrostatic forces that hold ions together in ionic compounds are called ionic bonds.
Anions and cations have opposite charges and attract one another by means of electrostatic forces.
Fool’s gold is composed of iron(II) cations (Fe2+) and disulfide anions (S2
2–). Write the electron configuration of the Fe2+ ion.
Fe: 1s22s22p63s23p63d64s2
Fe2+: 1s22s22p63s23p64s23d4
Fe2+: 1s22s22p63s23p63d6
Fool’s gold is composed of iron(II) cations (Fe2+) and disulfide anions (S2
2–). Write the electron configuration of the Fe2+ ion.
How does a cesium atom form a cation?
A. By losing 2 electrons
B. By gaining 1 electron
C. By losing 1 electron
D. By gaining 2 electrons
How does a cesium atom form a cation?
A. By losing 2 electrons
B. By gaining 1 electron
C. By losing 1 electron
D. By gaining 2 electrons
Formation of Ionic Compounds
• Sodium and chlorine atoms combine in a one-to-one ratio, and both ions have stable octets.
When sodium and chlorine react to form a compound, the sodium atom transfers its one valence electron to the chlorine atom.
Ionic Bonds
Formation of Ionic CompoundsIonic Bonds
• Each bromine atom has seven valence electrons and readily gains one additional electron.
• Three bromine atoms combine with each aluminum atom.
Aluminum metal (Al) and the nonmetal bromine (Br2) react violently to form the ionic solid aluminum bromide (AlBr3).
Formation of Ionic CompoundsFormula Units
A chemical formula shows the numbers of atoms of each element in the smallest representative unit of a substance.
• NaCl is the chemical formula for sodium chloride.
Formation of Ionic CompoundsFormula UnitsIonic compounds do not exist as discrete units, but as collections of positively and negatively charged ions arranged in repeating patterns.
Formation of Ionic CompoundsFormula Units
The chemical formula of an ionic compound refers to a ratio known as a formula unit.
• A formula unit is the lowest whole-number ratio of ions in an ionic compound.
Formation of Ionic CompoundsFormula Units
For sodium chloride, the lowest whole-number ratio of the ions is 1:1 (one Na+ ion to each Cl– ion).
• The formula unit for sodium chloride is NaCl.
• Although ionic charges are used to derive the correct formula, they are not shown when you write the formula unit of the compound.
Use electron dot structures to predict the formulas of the ionic compounds formed from the following elements:
Predicting Formulas of Ionic Compounds
a.potassium and oxygen
b.magnesium and nitrogen
Analyze Identify the relevant concepts.1
• Atoms of metals lose valence electrons when forming an ionic compound.
• Atoms of nonmetals gain electrons.
• Enough atoms of each element must be used in the formula so that electrons lost equal electrons gained.
a.Start with the atoms.
Solve Apply the concepts to this problem.2
K and O •••
•
•
• •
a. In order to have a completely filled valence shell, the oxygen atom must gain two electrons. These electrons come from two potassium atoms, each of which loses one electron.
Solve Apply the concepts to this problem.2
K •
•••
••
•O
K •+
K+
K+
•
•
• •O
• •
• •
2–
a.Express the electron dot structure as a formula.
Solve Apply the concepts to this problem.2
• The formula of the compound formed is K2O (potassium oxide).
b.Start with the atoms.
Solve Apply the concepts to this problem.2
Mg and N •
• • • ••
•
b. Each nitrogen atom needs three electrons to have an octet, but each magnesium atom can lose only two electrons. Three magnesium atoms are needed for every two nitrogen atoms.
Solve Apply the concepts to this problem.2
••• • •N •
•
• •N
• •
• •
3–
+ Mg2+
••• • •N
Mg2+
Mg2+
•
•
• •N
• •
• •
3–
Mg•
•Mg•
•Mg•
•
b.Express the electron dot structure as a formula.
Solve Apply the concepts to this problem.2
• The formula of the compound formed is Mg3N2 (magnesium nitride).
Use electron dot structures to determine the formula of the ionic compound formed when calcium reacts with fluorine.
Use electron dot structures to determine the formula of the ionic compound formed when calcium reacts with fluorine.
•
•
• •F
• •
•+ Ca2+
F•
•
• •
• •
• •
–
Ca•
•F•
•
• •
• •
• •
–•
•
• •
F• •
•
CaF2
Properties of Ionic CompoundsProperties of Ionic Compounds
What are three properties of ionic compounds?
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.
The beauty of crystalline solids comes from the orderly arrangement of their component ions.
Properties of Ionic Compounds
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
Ionic compounds generally have high melting points.
Would you expect to find sodium chloride in underground rock deposits as a solid, liquid, or gas? Explain.
Would you expect to find sodium chloride in underground rock deposits as a solid, liquid, or gas? Explain.
Sodium chloride is found in underground rock deposits as a solid. Like most ionic compounds, sodium chloride has a high melting point (about 800°C).
Properties of Ionic CompoundsThe coordination number of an ion is the number of ions of opposite charge that surround the ion in a crystal.
• In NaCl, each ion has a coordination number of 6.
– The coordination number of Na+ is 6 because each Na+ ion is surrounded by six Cl– ions.
– The coordination number of Cl– is also 6 because each Cl– ion is surrounded by six Na+ ions.
Properties of Ionic Compounds
In CsCl, each ion has a coordination number of 8.
• Each Cs+ ion is surrounded by eight Cl– ions.
• Each Cl– ion is surrounded by eight Cs+
ions.
Properties of Ionic Compounds
Titanium dioxide (TiO2), or rutile, forms tetragonal crystals.• The coordination number for
the cation (Ti4+) is 6.
• The coordination number of the anion (O2–) is 3.
– Each Ti4+ ion is surrounded by six O2– ions.
– Each O2– ion is surrounded by three Ti4+ ions.
Properties of Ionic Compounds
Ionic compounds can conduct an electric current when melted or dissolved in water.
Properties of Ionic CompoundsWhen sodium chloride is melted, the orderly crystal structure breaks down.
• If a voltage is applied across this molten mass, cations migrate freely to one electrode and anions migrate to the other.
• This movement of electrons allows electric current to flow between the electrodes through an external wire.
Flow of electrons
To (+) electrode
Flow of electrons
Power source Current meter
To (–) electrode
Inert metal electrodes
Cl–
Na+
Properties of Ionic Compounds
Ionic compounds also conduct electric current if they are dissolved in water.
• When dissolved, the ions are free to move about in the solution.
When can ionic compounds conduct an electric current?
A. Only when melted
B. When melted or dissolved in water
C. Only when dissolved in water
D. When solid or melted
When can ionic compounds conduct an electric current?
A. Only when melted
B. When melted or dissolved in water
C. Only when dissolved in water
D. When solid or melted
Key Concepts
Although they are composed of ions, ionic compounds are electrically neutral.
Most ionic compounds are crystalline solids at room temperature.
Ionic compounds generally have high melting points.
Ionic compounds can conduct an electric current when melted or dissolved in water.
Glossary Terms
ionic compound: a compound composed of positive and negative ions
ionic bond: the electrostatic attraction that binds oppositely charged ions together
chemical formula: an expression that indicates the number and type of atoms present in the smallest representative unit of a substance
Glossary Terms
formula unit: the lowest whole-number ratio of ions in an ionic compound; in magnesium chloride, the ratio of magnesium ions to chloride ions is 1:2 and the formula unit is MgCl2
coordination number: the number of ions of opposite charge that surround each ion in a crystal