Topic 7 Topic 7 Topic 7: Types of Compounds Table of Contents Topic 7 Topic 7 Basic Concepts...

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Topic 7: Types of Compounds

Table of ContentsTable of ContentsTopic 7Topic 7

Basic Concepts

Additional Concepts

• The submicroscopic structure of ionic compounds helps explain why they share certain macroscopic properties such as high melting points, brittleness, and the ability to conduct electricity when molten or when dissolved in water.

Ionic Compounds

Types of Compounds: Basic ConceptsTypes of Compounds: Basic Concepts Types of Compounds: Basic ConceptsTypes of Compounds: Basic Concepts

• You have learned that ionic compounds are made up of oppositely charged ions held together strongly in well-organized units.

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Ionic Compounds

• Because of their structure, they usually are hard solids at room temperature and are difficult to melt. Look at the structure of magnesium oxide.

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Ionic Compounds• When ionic compounds melt or dissolve in

water, their three-dimensional structure breaks apart, and the ions are released from the structure.

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Ionic Compounds

• These charged ions are now free to move and can conduct an electrical current.

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Binary Ionic Compounds

• Formulas are part of the language that is used to communicate information about substances.

• As a first step in studying this new language, you will learn how to name and write formulas for ionic compounds.

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• Sodium chloride (NaCl) contains only sodium and chlorine, and potassium iodide (KI) contains only potassium and iodine.

• Each is an example of a binary compound, which is a compound that contains only two elements.

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• Binary ionic compounds can contain more than one ion of each element, as in CaF2, but they are not composed of three or more different elements, as are more complex compounds.

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• To name a binary ionic compound, first write the name of the positively charged ion, usually a metal, and then add the name of the nonmetal or negatively charged ion, whose name has been modified to end in -ide.

• The compound formed from potassium and chlorine is called potassium chloride.

• Magnesium combines with oxygen to form a compound called magnesium oxide.

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• You are already familiar with one formula for an ionic compound—NaCl.

• You have learned that compounds are electrically neutral.

• Sodium chloride contains sodium ions that have a 1+ charge and chloride ions that have a 1– charge.

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• This means that the sum of the charges in an ionic compound must always equal zero.

• Thus, one Na+ balances one Cl– in sodium chloride.

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• When you write a formula, you add subscripts to the symbols for the ions until the algebraic sum of the ions’ charges is zero.

• The smallest subscript to both ions that results in a total charge of zero is 1.

• However, no subscript needs to be written because it is understood that only one ion or atom of an element is present if there is no subscript.

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• The formula NaCl indicates that sodium chloride contains sodium and chloride ions, that there is one sodium ion present for every chloride ion in the compound, and that the compound has no overall charge.

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• If more than one ion of a given element is present in a compound, the subscript indicates how many ions are present.

• The mineral known as fluorite is calcium fluoride, which has the formula CaF2.

• This formula indicates that there is one calcium ion for every two fluoride ions in the compound.

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• In an ionic compound, a formula represents the smallest ratio of atoms or ions in the compound.

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Binary Ionic Compounds• In a covalent compound, the smallest unit of

the compound is a molecule, so a formula represents a single molecule of a compound.

• This simplest ratio of ions in a compound is called a formula unit.

• Each formula unit of calcium fluoride consists of one calcium ion and two fluoride ions.

• Each of the three ions has a stable octet configuration of electrons, and the formula unit has no overall charge.

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Predicting Charge on Ions

• The noble gases each have eight electrons in their outer-energy levels.

• Metals have few outer-level electrons so they tend to lose them and become positive ions.

• Sodium must lose just one electron, becoming an Na+ ion.

• Calcium must lose two electrons, becoming a Ca2+ ion.

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• Most nonmetals, on the other hand, have outer-energy levels that contain four to seven electrons, so they tend to gain electrons and become negative ions.

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• Because all elements in a given group have the same number of electrons in their outer-energy level, they must lose or gain the same number of electrons to achieve a noble-gas electron configuration.

• Metals always lose electrons and nonmetals always gain electrons when they form ions.

• The charge on the ion is known as the oxidation number of the atom.

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Predicting Charge on Ions• The oxidation numbers for many elements in

the main groups are arranged by group number.

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Predicting Charge on Ions• Oxidation numbers for elements in Groups 3

through 12, the transition elements, cannot be predicted by group number.

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• Aluminum is in Group 13, so it loses its three outer electrons to become an Al3+ ion; oxygen is in Group 16 and has six valence electrons, so it gains two electrons to become an O2– ion.

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• Notice that one of aluminum’s three electrons has not been taken up by the oxygen atom.

• Because all the electrons must be accounted for, more than one oxygen atom must be involved in the reaction.

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Predicting Charge on Ions• But, oxygen cannot gain only one electron,

so a second aluminum atom must be present to contribute a second electron to oxygen.

• In all, two Al3+ ions must combine with three O2– ions to form Al2O3.

• Remember that the charges in the formula for aluminum oxide must add up to zero.

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Compounds Containing Polyatomic Ions

• The ions you have studied thus far have contained only one element.

• However, some ions contain more than one element.

• An ion that has two or more different elements is called a polyatomic ion.

• Although the individual atoms have no charge, the group as a whole has an overall charge.

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Compounds Containing Polyatomic IonsTopic 7Topic 7



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• Ionic compounds may contain positive metal ions bonded to negative polyatomic ions, such as in NaOH; negative nonmetal ions bonded to positive polyatomic ions, such as in NH4I; or positive polyatomic ions bonded to negative polyatomic ions, such as in NH4NO3.

Compounds Containing Polyatomic IonsTopic 7Topic 7


• To write the formula for an ionic compound containing one or more polyatomic ions, simply treat the polyatomic ion as if it were a single-element ion by keeping it together as a unit.

• Remember that the sum of the positive and negative charges must equal zero.


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• Multiples of a polyatomic ion in a formula can be indicated by placing the entire polyatomic ion, without the charge, in parentheses.

• Write a subscript outside the parentheses to show the number of polyatomic ions in the compound.

• Never change the subscripts within the polyatomic ion.


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• To do so would change the composition of the ion.

• The formula for the compound that contains one magnesium ion and two nitrate ions is Mg(NO3)2.


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• To name a compound containing a polyatomic ion, follow the same rules as used in naming binary compounds.

• Name the positive ion first, followed by the negative ion.


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• The name of the compound composed of calcium and the carbonate ion is calcium carbonate.

• However, do not change the ending of the negative polyatomic ion name.


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• To form a neutral compound, one Ca2+ ion must combine with one CO3

2– ion to give the formula CaCO3.

• Calcium is in Group 2, so its ion has a 2+ charge. The carbonate ion has a 2– charge.


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Compounds of Transition Elements

• Transition elements form positive ions just as other metals do, but most transition elements can form more than one type of positive ion.

• Elements known as transition elements are located in Groups 3 through 12 in the periodic table.

• In other words, transition elements can have more than one oxidation number.

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• Zinc and silver are two exceptions to the variability of other transition elements; each forms one type of ion.

• For example, copper can form both Cu+ and Cu2+ ions, and iron can form both Fe2+ and Fe3+ ions.

• The zinc ion is Zn2+ and the silver ion is Ag+.

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• They do this by using a Roman numeral to indicate the oxidation number of a transition element ion.

• Chemists must have a way to distinguish the names of compounds formed from the different ions of a transition element.



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• No additional naming system is needed for zinc and silver compounds because their formulas are not ambiguous.

• This Roman numeral is placed in parentheses after the name of the element.



Hydrates

• A compound in which there is a specific ratio of water to ionic compound is called a hydrate.

• Many ionic compounds are prepared by crystallization from a water solution, and water molecules become a part of the crystal.

• In a hydrate, the water molecules are chemically bonded to the ionic compound.

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Hydrates

• These compounds are called hygroscopic substances, and one example is sodium carbonate (Na2CO3).

• Some ionic compounds can easily become hydrates by absorbing water molecules from water vapor in the air.

• In a hydrate, the water molecules are chemically bonded to the ionic compound.

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Hydrates

• Substances that are so hygroscopic that they take up enough water from the air to dissolve completely and form a liquid solution are called deliquescent.

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Hydrates• To write the formula for a hydrate, write the

formula for the compound and then place a dot followed by the number of water molecules per formula unit of compound.

• The dot in the formula represents a ratio of compound formula units to water molecules.

• For example, CaSO4 • 2H20 is the formula for a hydrate of calcium sulfate that contains two molecules of water for each formula unit of calcium sulfate.

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Hydrates

• To name hydrates, follow the regular name for the compound with the word hydrate, to which a prefix has been added to indicate the number of water molecules present.

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Hydrates

• The name of the compound with the formula CaSO4 • 2H2O is calcium sulfate dihydrate.

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Hydrates

• Heating hydrates can drive off the water.

• This results in the formation of an anhydrous compound—one in which all of the water has been removed.

• In some cases, an anhydrous compound may have different color from that of its hydrate.

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Properties of Molecular Substances

• You know that ionic compounds share many properties.

• The properties of a molecular substance—a substance that has atoms held together by covalent rather than ionic bonds—are more variable than the properties of ionic compounds.

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• Molecular substances usually have lower melting points, and most are not as hard as ionic compounds.

• In addition, most molecular substances are less soluble in water than ionic compounds and are not electrolytes.

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• A molecule that forms when atoms of the same element bond together is called a molecular element.

• Note that molecular elements are not compounds—they contain atoms of only one element.



• Seven nonmetal elements are found naturally as molecular elements of two identical atoms.

• When they bond together, each atom achieves the stability of a noble-gas electron configuration.



• The elements whose natural state is diatomic are:

• hydrogen,

• bromine,• nitrogen,

• oxygen,

• fluorine,

• and iodine

• chlorine,



• Their formulas can be written as:

• H2,

• Br2, • N2,

• O2,

• F2,

• and I2, respectively

• Cl2,



• If two chlorine atoms combine, they share a single pair of electrons, and each atom attains a stable octet configuration.



• Two oxygen atoms share two pairs of electrons to form O2, and two nitrogen atoms share three pairs of electrons to form N2.



Allotropes

• The structure of ozone is different from that of diatomic oxygen.

• Although the diatomic form of oxygen, O2, is most common in our atmosphere, oxygen also exists as O3—ozone.

• It consists of three atoms of oxygen rather than the two atoms in diatomic oxygen.

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Allotropes

• Molecules of a single element that differ in crystalline or molecular structure are called allotropes.

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Allotropes

• The properties of allotropes are usually different even though they contain the same element.

• This is because structure can be more important than composition in determining properties of molecules.

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Allotropes

• Phosphorus has three common allotropes: white, red, and black.

• All are formed from P4 molecules that are joined in different ways, giving each allotrope a unique structure and properties.

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Formulas and Names of Molecular Compounds

• Chemists have devised a naming system for molecular compounds that is based on a much smaller number of rules than there are compounds.

• Substances are either organic or inorganic. Compounds that contain carbon, with a few exceptions, are classified as organic compounds.

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• Compounds that do not contain carbon are called inorganic compounds.

• To name these compounds, write out the name of the first nonmetal and follow it by the name of the second nonmetal with its ending changed to -ide.

Formulas and Names of Molecular Compounds

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Naming Organic Compounds

• You have learned that most compounds that contain carbon are organic compounds.

• Organic compounds make up the largest class of molecular compounds known.

• This is because carbon is able to bond to other carbon atoms in rings and chains of many sizes.

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Naming Organic Compounds

• The name of even the most complex organic compound is based on the name of a hydrocarbon, an organic compound that contains only the elements hydrogen and carbon.

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Basic Assessment QuestionsBasic Assessment Questions

Question 1

Determine the ratio of the atoms in the ionic compound formed in each case.

A. aluminum (Al) and fluorine (F)

B. lithium (Li) and oxygen (O)

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Answers

A. aluminum (Al) and fluorine (F)

B. lithium (Li) and oxygen (O)

one AL for every three F

two Li for every O

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Write the correct formula for the ionic compound formed between atoms of each of the following pairs of elements.

A. sodium (Na) and sulfur (S)

Question 2

B. magnesium (Mg) and nitrogen (N)

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A. sodium (Na) and sulfur (S)

Answers

B. magnesium (Mg) and nitrogen (N)

Na2S

Mg3N2

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Types of Compounds: Additional ConceptsTypes of Compounds: Additional Concepts Types of Compounds: Additional ConceptsTypes of Compounds: Additional Concepts Topic 7Topic 7

Additional Concepts

Types of Compounds: Additional ConceptsTypes of Compounds: Additional Concepts Types of Compounds: Additional ConceptsTypes of Compounds: Additional Concepts

Forming Chemical Bonds• Elements tend to react so as to achieve the

stable electron configuration of a noble gas, typically an octet of electrons.

• A cation, or positive ion, is formed when an atom loses one or more electrons.

• An anion, or negative ion, is formed when an atom gains one or more electrons.

• The periodic table is useful in predicting the charges of ions typically formed by various atoms.

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Properties of Ionic Compounds and Lattice Energy

• In a solid ionic compound, the positive ions are surrounded by negative ions, and the negative ions by positive ions.

• The resulting structure is called a crystal lattice and contains a regular, repeating, three-dimensional arrangement of ions.


• This arrangement, which involves strong attraction between oppositely charged ions, tends almost always to produce certain properties, such as high melting and boiling points and brittleness.



• Ionic compounds are always nonconductors of electricity when solid but good conductors when melted.

• They also act as electrolytes, substances that conduct electric current when dissolved in water.



• The combination of these conductivity characteristics is a very good identifier of ionic compounds, although each characteristic separately is not very reliable.



• The energy required to separate one mole of the ions of an ionic compound is called lattice energy, which is expressed as a negative quantity.

• The greater (that is, the more negative) the lattice energy is, the stronger is the force of attraction between the ions.



• Lattice energy tends to be greater for more-highly-charged ions and for small ions than for ions of lower charge or large size.



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Naming Ionic Compounds

• Certain polyatomic ions, called oxyanions, contain oxygen and another element.

• If two different oxyanions can be formed by an element, the suffix -ate is used for the oxyanion containing more oxygen atoms, and the suffix -ite for the oxyanion containing fewer oxygens.

• In the case of the oxyanions of the halogens, the following special rules are used.


Naming Ionic Compounds

• four oxygens, per + root +-ate (example: perchlorate, ClO4

–)

• three oxygens, root + -ate (example: chlorate, ClO3

–)

• two oxygens, root + -ite (example: chlorite, ClO2

–)

• one oxygen, hypo- + root + -ite (example: hypochlorite, ClO–)


Additional Assessment QuestionsAdditional Assessment Questions

For each of the following atoms, write the formula of the ion the atom is most likely to form and identify that ion as a cation or an anion.

Questions 1

A. bromine (Br), element 35

B. gallium (Ga), element 31

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Answers

A. bromine (Br), element 35 Br– anion

B. gallium (Ga), element 31 Ga3 cation

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Name the ionic Compounds that have the following formulas.

Question 2

B. KHSO4

A. Mg(NO3)2

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Answers

B. KHSO4

A. Mg(NO3)2 magnesium nitrate

potassium hydrogen sulfate

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