Chapter 7Ionic and Metallic Bonding

valence ElectronsScientists 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 electronsvalence 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 ElectronsThe 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 RuleNoble 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 RuleThe 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 CationsAn 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 CationsAn 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 CationsUsing electron dot structures, you can show the ionization of some elements more simply.

Na· Na+ + e- Sodium atom Sodium ion electron

neutral 1 unit of + charge 1 unit of - charge

·Mg· Mg2+ + 2e- Magnesium atom Magnesium ion electron

neutral 2 unit of + charge 2 units of - charge

Transition MetalsFor 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 - 1s22s22p63s23p63d104s4s224p4p664d105s5s11

To achieve the structure of krypton, which is the preceding noble gas, a silver atom would have to lose eleven electrons.

Transition MetalsIons 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, (4s4s224p4p664d10) 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 pseudo noble-gas electron configuration. electron configuration.

Ag forms a positive ion (Ag+) in this way.

Formation of AnionsThe 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.-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 AnionsChlorine 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 ionsHalide 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-.

QuestionsHow can you determine the number of valence electrons in an atom of a representative element?

Look up the group number 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 CompoundsCompounds composed of cations and anions are called ionic compoundsionic compounds.

Ionic compounds are usually composed of metal cations and nonmetal anions. Ex: NaCl is formed from Na+ + Cl-

Although 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 bondsionic bonds.

Formation of Ionic CompoundsCompounds composed of cations and anions are called ionic compoundsionic compounds.

Ionic compounds are usually composed of metal cations and nonmetal anions. Ex: NaCl is formed from Na+ + Cl-

Although 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 bondsionic bonds.

Formation of Ionic CompoundsLook 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 11A of the representative elements, thus has 1 1 valence electron)

Cl has seven valence electrons and can easily gain one electron. (Cl is in group 77A of the representative elements, thus has 77 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)

Formation of Ionic CompoundsWhen 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+ Cl-

1s22s22p6 1s22s22p63s23p6

Formula UnitsChemists represent the composition of substances by

writing chemical formulas. A chemical formulachemical 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 unitFormula 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

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 1- charge) as Mg2+ (each with a 2+ charge), the compound is electrically neutral.

Another example: Al3+ + Br- combine to form AlBr3.

QuestionsUse 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 CompoundsMost 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 CompoundsIonic compounds can conduct an electric current Ionic compounds can conduct an electric current

when melted or dissolved in water.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

QuestionsHow 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

QuestionsWrite the correct chemical formula for the compounds

formed from each pair of ions.

K+ , S2- Ca2+ , O2-

Na+ , 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 for form ionic compounds?

Cl, Br Li, Cl K, He I, Na

Li, Cl I, Na

End of Section 7.2

Metallic Bonds & PropertiesMetals 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 bondsMetallic 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 MetalsThere are several closely packed arrangements that are possible.

• body-centered cubic arrangement

• face-centered cubic arrangement

• hexagonal close-packed arrangement

Body-centered cubicBody-centered cubicEvery atom (except those on theSurface) has eight neighbors.

Crystalline Structure of MetalsFace-centered Face-centered cubic arrangement

• every atom has twelve neighbors.

Crystalline Structure of MetalsHexagonal close-packedHexagonal close-packed arrangement

• every atom also have twelve neighbors. Because of the hexagonal shape, the pattern is different from the face-centered.

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.

AlloysBronze – 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 n the crystal. (substantial alloy)(substantial alloy)

If the atomic sizes are different, the smaller atoms can fit into the spaces between the larger atoms. (interstitial (interstitial alloy)alloy)

QuestionsHow 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