ionic and molecular compounds - chemistry...

General, Organic, and Biological Chemistry: Structures of Life, 5/e

Karen C. Timberlake

© 2016 Pearson Education, Inc.

Ionic and Molecular CompoundsChapter 6


Karen C. Timberlake


Ch. 6 – Ionic and Molecular Compounds

• 6.1 – Ions: Transfer of Electrons

• 6.2 – Writing Formulas for Ionic Compounds

• 6.3 – Naming and Writing Ionic Formulas

• 6.4 – Polyatomic Ions

• 6.5 – Molecular Compounds: Sharing Electrons

• 6.6 – Lewis Structures for Molecules and Polyatomic Ions

• 6.7 – Electronegativity and Bond Polarity

• 6.8 – Shapes and Polarity of Molecules

• 6.9 – Attractive Forces in Compounds


Karen C. Timberlake


6.1 Ions: Transfer of Electrons

Learning Goal Write the symbols for the simple ions of the representative elements.


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Ionic and Covalent Bonds

Chemical bonds are formed when atoms lose, gain, or share valence electrons to completely fill their outermost electron shell.

• Ionic bonds occur when valence electrons of a metal atom are transferred to the atom of a nonmetal.

• Covalent bonds occur when nonmetal atoms share electrons to attain a noble gas arrangement.


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Ionic and Covalent Bonds


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Positive Ions: Metals Lose Electrons

In ionic bonding, ions form when atoms gain or lose their valence electrons to form a stable electron configuration.

Metals:

• have low ionization energies.

• readily lose one or more of their valence electrons to form ions with a positive charge.

• lose electrons until they have the same number of valence electrons as the nearest noble gas, usually eight valence electrons.


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Positive Ions: Loss of Electrons

When sodium atoms in Group 1 are neutral, they have 11 electrons and 11 protons.

• They lose one electron to have the same number of valence electrons as neon and a filled energy level.

• They will form an ion with 10 electrons, 11 protons, and an ionic charge of 1+: Na+.


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Positive Ions: Loss of Electrons

When magnesium atoms in Group 2 are neutral, they have 12 electrons and 12 protons.

• They will lose 2 electrons to have the same number of valence electrons as neon and a filled energy level.

• They form an ion with 10 electrons, 12 protons, with an ionic charge of 2+: Mg2+.


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Negative Ions: Nonmetals Gain Electrons

Nonmetals

• Have high ionization energies

• readily gain one or more valence electrons to form ions with a negative charge.

• gain electrons until they have the same number of valence electrons as the nearest noble gas, usually eight valence electrons.


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Negative Ions: Gain of Electrons

When chlorine atoms in Group 17 are neutral, they have17 electrons and 17 protons.

• They will gain one electron to have the same number of valence electrons as argon.

• They form an ion with 18 electrons, 17 protons, and a charge of 1−: Cl−.


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Formulas, Names of Common Ions


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Ionic Charges, Group Numbers

We can use the group numbers in the periodic table to determine the charges for the ions.


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Chemistry Link to Health: Important Ions in the Body

Ions are important in regulating body functions.

Ion Occurrence Function Source

Na+ Principal cation

outside the cell

Regulation and control of

body fluids

Salt, cheese,

pickles

K+ Principal cation

outside the cell

Regulation of body fluids

and cellular functions

Bananas, potatoes,

orange juice, milk

Ca2+ Cation outside the

cell; found in bones

Major cation in bones,

needed for muscle

contractions

Milk, yogurt,

cheese, greens,

spinach

Mg2+ Cation outside the

cell; found in bones

Essential for certain

enzymes, muscles, and

nerve control

Chlorophyll, nuts,

grains


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Study Check

Write the formula and symbol of an ion with16 protons and 18 electrons.


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Study Check

Consider the elements calcium and chlorine.

A. Identify each as a metal or a nonmetal.

B. State the number of valence electronsfor each.

C. State the number of electrons that must be lost or gained for each to acquirean octet.

D. Write the symbol, including its ionic charge, and name of each resulting ion.


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Karen C. Timberlake


6.2 Writing Formulas for Ionic Compounds

Learning Goal Using charge balance, write the correct formula for an ionic compound.


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Properties of Ionic Compounds

Ionic compounds

• consist of positive and negative ions.

• have attractions called ionic bonds between positively and negatively charged ions.

• have high melting points.

• are solids at room temperature.


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NaCl, An Ionic Compound

Sodium chloride is more commonly known as table salt. The magnification of NaCl crystals shows the arrangement of Na+ and Cl− ions in an NaCl crystal.


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Formulas of Ionic Compounds

In a chemical formula,

• the symbols and subscripts are written in the lowest whole-number ratio of the atoms or ions.

• the sum of ion charges equals zero.

• the total positive charge = total negative charge.


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Subscripts in Formulas


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Writing Ionic Formulas from Ion Charges

To balance ionic charge in an ionic compound,

total positive charge = total negative charge


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Study Check

Write the ionic formula of the compound formedwith Ba2+ and Cl− ions.


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Study Check

Select the correct formula for each of the following ionic compounds.

1. Na+ and O2−

A. NaO B. Na2O C. NaO2

2. Al3+ and Cl−

A. AlCl3 B. AlCl C. Al3Cl

3. Mg2+ and N3−

A. MgN B. Mg2N3 C. Mg3N2


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Karen C. Timberlake


6.3 Naming and Writing Ionic Compounds

Learning Goal Given the formula of an ionic compound, write the correct name; given the name of an ionic compound, write the correct formula.


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Naming Ionic Compounds

When naming an ionic compound,

• the name of the metal is written first and is the same as the name of the element.

• the name of the nonmetal is the first syllableof the nonmetal name + ide ending and iswritten second.

•a space is placed between the name of the metal and nonmetal ion.


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Names of Some Ionic Compounds


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Naming Ionic Compounds, K2O

Name the ionic compound K2O.


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Study Check

Write the names of the following compounds.

A. CaO

B. Al2O3

C. MgCl2


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Metals with Variable Charge

Transition metals except for Zn2+, Cd2+, and Ag+ form two or more positive ions (cations).

A Roman numeral equal to the ion charge is placed in parentheses immediately after the metal name.

Cu2+ copper(II) Pb2+ lead(II)Cu+ copper(I) Pb4+ lead(IV)Fe2+ iron(II) Cr2+ chromium(II)Fe3+ iron(III) Cr3+ chromium(III)


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Metals with Variable Charge


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Determination of Variable Charge

Determine the charge of the metal ion in: MnF2


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Ion Charges, Periodic Table


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Naming Ionic Compounds with Variable Charge Metals, FeCl2

Name the ionic compound FeCl2.


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Study Check

Name the following ionic compound containing a variable charge metal.

SnO2


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Writing Formulas from the Name of an Ionic Compound

The formula for an ionic compound is written from,

• the first part of the name that describes the metal ion.

• the second part of the name that specifies the nonmetal ion.

Subscripts are added to balance the charge.


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Writing Formulas from the Name of an Ionic Compound

Write the formula for iron(III) chloride.


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Study Check

Write chemical formulas for the following compounds:A. nickel(II) sulfideB. aluminum chlorideC. iron(III) oxide


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Karen C. Timberlake


6.4 Polyatomic Ions

Learning Goal Write the name and formula for an ionic compound containing a polyatomic ion.


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Polyatomic Ions

An ionic compound may contain a polyatomic ion (many-atom ion) as one its cations or anions.

• A polyatomic ion is a group of covalently bonded atoms that has an overall ionic charge. • Most polyatomic ions are made up of nonmetals (SO4

2−, NO3−, ClO4

−)

• Almost all of them have negative charges (-1, -2, -3, etc)

• NH4+


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Polyatomic Ions


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Polyatomic Ions, Names, and Formulas


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Names of Polyatomic Ions

1. Names of polyatomic ions end in ate.

SO42− sulfate

PO43− phosphate

NO3−nitrate

2. When a related ion has one less oxygen, its name ends in ite.

SO32− sulfite

PO33− phosphite

NO2−nitrite


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Names of Polyatomic Ions

3. Exceptions to these rules are the following:

CN− cyanide

OH− hydroxide

4. Halogens form 4 polyatomic ions with oxygen.

Each has a −1 charge.

ClO4− perchlorate

ClO3− chlorate

ClO2− chlorite

ClO− hypochlorite


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Writing Formulas for Compounds Containing Polyatomic Ions

When writing formulas for ionic compounds containing polyatomic ions, we use the same rules of charge balance as those for simple ionic compounds.

Magnesium nitrate


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Writing Formulas with Polyatomic Ions

Write the formula for aluminum nitrite


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Study Check

Write the formula for sodium phosphate.


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Naming Compounds with Polyatomic Ions

When naming ionic compounds containing polyatomic ions,

• first write the positive ion, usually a metal.

• write the name of the polyatomic ion second.

Recognizing polyatomic ions in a chemical formula helps to name it correctly.


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Compounds with Polyatomic Ions


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Study Check

Select the correct formula for each.

1. aluminum nitrate

A. AlNO3 B. Al(NO) 3 C. Al(NO3) 3

2. copper(II) nitrate

A. CuNO3 B. Cu(NO3)2 C. Cu2 (NO3)


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Study Check

Select the correct formula for each.

3. iron(III) hydroxide

A. FeOH B. Fe3OH C. Fe(OH)3

4. tin(IV) hydroxide

A. Sn(OH)4 B. Sn(OH)2 C. Sn4(OH)


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Flow Chart, Naming Ionic Compounds


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Study Check

Name the following ionic compounds:

A. Ca(NO3)2

B. FePO4


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Study Check

Name each of the following compounds containing polyatomic ions.

A. MgSO3

B. MgSO4

C. Pb3(PO3)2


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Study Check

Name each of the following compounds:

A. Fe2(SO4)3

B. Ba3(PO3)2

C. NiCO3


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Karen C. Timberlake


6.5 Molecular Compounds: Sharing Electrons

Learning Goal Given the formula of a molecular compound, write its correct name; given the name of a molecular compound, write its formula.


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Covalent Bonds

Molecular compounds form when

• atoms of two or more nonmetals share electrons and form a covalent bond.

• valence electrons are shared by nonmetal atoms to achieve stability.

A molecule forms when two or more atoms share electrons.


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Names and Formulas, Molecular Compounds

When naming a molecular compound, the

• first nonmetal in the formula is named by its element name.

• second nonmetal is named using the first syllable of the name followed by ide.

When a subscript indicates two or more atoms of an element, a prefix is shown in front of its name.


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Names and Formulas, Molecular Compounds

Several compounds may be formed from the same two nonmetals:

CO2 = carbon dioxide

CO = carbon monoxide

• The number of oxygen atoms present is indicated by the prefix.

• When the vowels o and o or a and o appear together, the first vowel is omitted.

NO = nitrogen monoxide,


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Common Molecular Compounds


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Naming Molecular Compounds, NCl3

Name the molecular compound NCl3.


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Study Check

Name the molecular compound B2O3.


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Study Check

Write the molecular formula for diphosphorus pentoxide.


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Compounds, Ionic or Covalent?

A compound is ionic if a metal and a nonmetal are bound together OR the compound begins with 𝑁𝐻4

+

K2O: potassium oxideNH4NO3: ammonium nitrate

A compound is covalent if 2+ nonmetals are bound together.

N2O: dinitrogen oxide


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Flowchart, Naming Compounds


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Study Check

Select the correct name for each compound.

1. SiCl4 A. silicon chloride

B. tetrasilicon chloride

C. silicon tetrachloride

2. P2O5 A. phosphorus oxide

B. phosphorus pentoxide

C. diphosphorus pentoxide

3. Cl2O7 A dichlorine heptoxide

B. dichlorine oxide

C. chlorine heptoxide


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Study Check

Identify each compound as ionic or covalent and give its correct name.

A. SO3

B. BaCl2

C. (NH4)3PO3

D. Cu2CO3

E. N2O4


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Karen C. Timberlake


6.6 Lewis Structures for Molecules and Polyatomic Ions

Learning Goal: Draw the Lewis structures for molecular compounds or polyatomic ions.


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The Simplest Molecule, H2

When two H atoms are far apart, they feel no attraction or repulsion to each other.

As they approach each other, the electron on one H is attracted to the proton (nucleus) of the other. And vice versa.

The attraction is stronger than the repulsion between the two electrons.

A covalent bond forms from each H sharing its electron with the other, creating the molecule: H2.

Each H acts as if it has 2 electrons.

This gives it the electron configuration of He and therefore is very stable.


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The Octet Rule• Atoms are the most stable when they have the

electron configuration of a noble gas.

• For H and He, that’s 2 electrons

• For n=2 and n=3 (Li through Ar) that is 8 electrons

• (n=4 etc is higher. We won’t worry about that now.)

• The Octet Rule: Most atoms want to have 8 valence electrons.• Exceptions:

• H, He (2 valence electrons)• B (can only have 6)• n=3 can have more than 8

due to access to the 3d orbital.


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Lewis Structures, Molecules

A molecule is represented by a Lewis structure in which the valence electrons of all the atoms are arranged to give octets.

• The shared electrons, or bonding pairs, are shown as two dots or a single line between atoms.

• The nonbonding pairs, or lone pairs, are placed on the outside of the atoms.


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Lewis Structures, Molecules

A molecule is represented by a Lewis structure in which the valence electrons of all the atoms are arranged to give octets.


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Elements, Diatomic Molecules

The elements hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as diatomic molecules.


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Number of bonds

The number of electrons a nonmetal atom shares and the number of covalent bonds it forms are equal to the number of electrons it needs to achieve a stable electron configuration.

# of bonds = 8 – number of valence e’s


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Guide to Drawing Lewis Structures

1. Sum the valance electrons from all the atoms.

2. Decide on atom connections and connect with a single bond. (“central atom” is usually written first in the formula.)

3. Determine the number of valence electrons remaining• Total valence electrons (step 1) minus # electrons used in bonding.

4. Complete the octets of terminal atoms (except H and He want 2)

5. Place leftover electrons on the central atom.

6. If no unassigned electrons remain, and the central atom doesn’t have an octet, use multiple bonds.


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Drawing Lewis Structures

Draw the Lewis structure for methane, CH4



3. Determine the number of valence electrons remaining

Total valence electrons (step 1) minus # electrons used in bonding.





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Molecules, Lewis Structures


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Study Check

Draw the Lewis structure for NH4+









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Study Check

Draw the Lewis structure for BF3









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Double and Triple Bonds

A double bond

•occurs when atoms share two pairs of electrons.

• forms when there are not enough electrons to complete octets.

A triple bond

•occurs when atoms share three pairs of electrons.

• forms when there are not enough electrons to complete octets.


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Study Check

Draw the Lewis structure for N2.









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Study Check

Draw the Lewis structure for HCN.









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6.7 Electronegativity and Bond Polarity

Learning Goal: Use electronegativity to determine the polarity of a bond.


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Electronegativity

The electronegativity of an atom is its ability to attract the shared electrons in a bond.


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Electronegativity• When the atoms in the bond are different, the more

electronegative atom will “hog” the shared electrons in the covalent bond.• The electrons will spend more time near the more

electronegative atom.


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Polarity of Bonds

Differences in electronegativity of atoms dictate what kind of bond forms between the atoms.

ΔEN = 0 a nonpolar covalent bond

ΔEN = between 0 and 2 a polar covalent bond

ΔEN ≥ 2 an ionic bond


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Nonpolar Covalent Bonds

A nonpolar covalent bond occurs between nonmetals.

• It is an equal sharing of electrons by the two bonding atoms.

ΔEN = 0 a nonpolar covalent bondΔEN = between 0 and 2 a polar covalent bondΔEN ≥ 2 an ionic bond


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Polar Covalent Bonds

A polar covalent bond occurs between nonmetal atoms. It

• is an unequal sharing of electrons.

• has a moderate electronegativity difference.

Electronegativity

Atoms Difference Type of Bond

O-Cl 3.5 − 3.0 = 0.5 Polar covalent

Cl-C 3.0 − 2.5 = 0.5 Polar covalent

O-S 3.5 − 2.5 = 1.0 Polar covalent



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Dipoles and Bond Polarity

A polar covalent bond becomes more polar as the difference in electronegativity increases. The separation of charges in a polar bond is called a dipole.

The positive and negative ends of the dipole are located by using

• the lowercase Greek letter delta with a positive or negative charge.

• an arrow that points from the positive to the negative end of the dipole.


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Ionic Bonds

An ionic bond

• occurs between metal and nonmetal ions.

• is a result of electron transfer.

• has a large electronegativity difference (2.0 or more).

Electronegativity

Atoms Difference Type of Bond

Cl-K 3.0 – 0.8 = 2.2 Ionic

N-Na 3.0 – 0.9 = 2.1 Ionic



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Electronegativity and Bond Types

ΔEN = 0 ΔEN = 0 to 2 ΔEN = 2+


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Study Check

Use the electronegativity difference to identify the type of bond (nonpolar covalent [NP], polar covalent [P], or ionic [I]) between the following:

A. K—N

B. N—O

C. Cl—Cl

D. H—Cl


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6.8 Shapes and Polarity of Molecules

Learning Goal: Predict the three-dimensional structure of a molecule, and classify it as polar or nonpolar.


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VSEPR Theory

Valence Shell Electron-Pair Repulsion Theory (VSEPR)

• describes the orientation of electron groups around the central atom.

• states that electron groups are arranged as far apart as possible around the central atom.

• states that the specific shape of a molecule is determined by the number of atoms attached to the central atom.


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Linear (2 bond locations)

In a molecule of CO2

• two electron groups are placed around the central atom, carbon.

• the repulsion is minimized by placing the two groups on opposite sides of the carbon atom, giving this a linear arrangement with bond angles of 180°.

• the shape with two electron groups around the central atom is linear.


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Triganol Planar (3 bond locations)

In a molecule of formaldehyde, H2CO3,

• three electron groups are placed around the central atom, carbon.

• the repulsion is minimized by placing the three groups as far apart as possible at bond angles of 120°.

• the shape with three electron groups around the central atom is trigonalplanar.


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Bent (2 bond locals + 1 electron pair)

In a molecule of SO2, there are three electron groups around the central atom S:

• The repulsion is minimized by placing the three electron groups as far apart as possible.

• The shape with two bonds and a lone pair on the central atom is bent with a bond angle of 120°.


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Tetrahedral (4 bond locations)

In a molecule of CH4

• four electron groups are attached to H atoms around the central atom, carbon.

• the repulsion is minimized by placing the four electron groups at corners of a tetrahedron with bond angles of 109°.

• the shape with four bonds on the central atom is called tetrahedral.


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Trigonal Pyrimidal (3 bond locals + 1 electron pair)

In a molecule of NH3

• four electron groups, three bonds to H atoms and one lone pair, are around the central atom, N.


• the shape with three bonds and a lone pair on the central atom is called trigonal pyrimidal.


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Bent (type 2) (2 bond locals + 2 e- pairs)

In a molecule of H2O

• four electron groups, two bonds to H atoms and two lone pairs, are around the central atom, O.


• the shape with two bonds and two lone pairs on the central atom is called bent.


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Molecular Shapes, Electron-Groups


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Guide to Predicting Molecular Shape

1. Draw the Lewis structure.

2. Determine electron-group geometry based on total number of electron locations on the central atom. (bonds and electron pairs)• Single, double, triple bond all equal one electron location

• Every electron pair equals one electron location.

3. Determine the molecular shape based on how many ATOMS are bonded to the central atom.


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Predicting molecular shapes

Predict the shape of a molecule of H2S.


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Study Check

State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonalpyramidal, or bent.

A. PF3

B. H2O

C. CCl4


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Polarity of Molecules

Nonpolar molecules• such as H2, Cl2, and O2 are nonpolar because they contain

nonpolar bonds.

• Polar bonds can be nonpolar if the polar bonds (dipoles) cancel in a symmetrical arrangement, such as in CO2 and CF4.


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Polar molecules such as HCl are polar because

• one end of the molecule is more negatively charged thanthe other.

• the polar bonds in the molecule do not cancel each other.

• the electrons are shared unequally in the polar covalent bond.


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In a polar molecule such as H2O, there

• are two lone pairs and two bonds around the central atom.

• are dipoles that do not cancel since the shape is bent, making the molecule positive at one end and negative at the other end.

• is a partial negative charge on the central atom.


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In a polar molecule such as NH3, there

• is one lone pair and three bonds around the central atom.

• are dipoles that do not cancel since the shape is trigonalpyramidal, making the molecule positive at one end and negative at the other end.

• is a partial negative charge on the central atom.


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Guide to Determining the Polarity of aMolecule


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Determination of Polarity

Determine if the molecule OF2 is polar or nonpolar.


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Study Check

Identify each of the following molecules as polar or nonpolar.

A. PBr3

B. HBr

C. CH4


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6.9 Attractive Forces in Compounds

Learning Goal: Describe the attractive forces between ionic compounds, polar covalent molecules, and nonpolar covalent molecules.


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Attractive Forces

• Intramolecular forces: attractive forces between atoms within compounds• Polar covalent bonds

• Nonpolar covalent bonds

• Ionic bonds

• Intermolecular forces: attractive forces between different molecules. • London dispersion forces

• Dipole-dipole interactions

• Hydrogen bonds


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London dispersion Forces

London dispersion forces are

• weak attractions between nonpolar molecules.

• caused by temporary dipoles that develop when molecules bump into each other.

• weak but make it possible for nonpolar molecules to form liquids and solids.


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Dipole-dipole interactions

Dipole-dipole interactions are

• Attractions between polar molecules.


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Hydrogen Bonds

Hydrogen bonds

• are the strongest intermolecular attraction.

• occur between hydrogen atoms bonded to F, O, or N, and a lone pair on F, O, or N.


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Comparison of Bonding andAttractive Forces


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Melting Points and Attractive Forces

Melting points of compounds

•are related to the strength of attractive forces between molecules or compounds.

•are lower due to weak forces such asdispersion forces.

•are higher due to stronger attractive forces such as hydrogen bonding.

•are highest in ionic compounds due to the strong attractive forces between ions in the compound.


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Melting Points and Attractive Forces


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Study Check

Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or London dispersion forces.

A. NCl3

B. H2O

London dispersion – nonpolar moleculesDipole-dipole – between 2 polar moleculesHydrogen bond – X-H --- X (X=N, O, or F)Ionic Bond – metal and nonmetal


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Study Check

Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or London dispersion forces.

C. Br-Br

D. KCl

E. NH3

London dispersion – nonpolar moleculesDipole-dipole – between 2 polar moleculesHydrogen bond – X-H --- X (X=N, O, or F)Ionic Bond – metal and nonmetal


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