Chapter 8.1 & 8.2 chem

Vocabulary

Covalent Bond: A chemical bond formed when electrons are shared between two electrons

Molecule: A neutral group of atoms joined together by covalent bonds

Diatomic Molecule: A molecule that contains two atoms

Molecular Compound: A compound composed of molecules

Molecular Formula: The chemical formula of a molecular compound

What is going on

Key Questions

What information does a molecular formula provide?
What representative units define molecular compounds and ionic compounds?

Molecules and Molecular Compounds

What information does a molecular formula provide?

In nature, only noble gas elements exist as uncombined atoms
Monatomic: consists of single atoms
NOTE: Not all elements are monatomic
Ionic compounds: Generally crystalline solids with a high melting point
Other compounds have different properties
- - Water: liquid at room temperature
  - Carbon Dioxide and Nitrous Oxide: Gases at room temperature
Attractions that hold together the atoms 02, H20, CO2, and N2O are not explained through ionic bonding
- - Do not involve the transfer of electrons

Sharing Electrons

Ionic bonds form when the combining atoms lose/gain electrons
You can also SHARE electrons
Atoms that are held together by sharing electrons are joined by a covalent bond.
In a covalent bond, a “tug of war” for electrons between atoms occur
A molecule is a neutral,l group of atoms that are joined together by a covalent bond
Oxygen gas is made up of oxygen molecules
Each oxygen molecule consists of two covalently bonded oxygen atoms

An oxygen molecule is an example of a diatomic molecule or a molecule that contains two atoms.

Found in nature and also form diatomic molecules: Hydrogen, Nitrogen, Halogens
NOTE: Molecules can be made up of atoms of different elements
A compound composed of molecules is called a molecular compound
Example: Water
Molecules in water are uniform
Each molecule is a tightly bound unit of two hydrogen atoms and one oxygen atom.

Representing Molecules

A molecular formula is a chemical formula of a molecular compound

A molecular formula shows how many atoms of each element a substance contains
Example: Molecular formula for water H2O
NOTE: That subscript ( a small-sized number on the bottom right of the symbol) shows the number of atoms of each element in the molecule
Molecular formulas also describe molecules consisting of atoms of one element.
Example: Oxygen: O2
Molecular formula DOES NOT show the structure of the molecule
It does not show the arrangement of the atoms in space/which atoms are covalently bonded to one another

Comparing Molecular and Ionic Compounds

What representative units define molecular compounds and ionic compounds?

Each type of compound contains atoms of different elements that are combined chemically.
Formulas describe different representative units.
The representative unit of a molecular compound is a molecule, and for an ionic compound, the representative unit is a formula unit.
A formula unit is the lowest whole number ratio of the ions in an ionic compound.
A molecule is made up of two or more atoms that act as a unit
No discrete units exist in an ionic compound
Consists of a continuous array of ions
Example: A molecule of sodium chloride or a molecule of magnesium chloride does NOT exist
They are collections of positively/negatively charged ions arranged in repeating three-dimensional patterns.
Molecular compounds usually have lower melting/boiling points than ionic compounds.
Most molecular compounds are gases/liquids at room temperature
Ionic Compounds are composed of atoms of two or more nonmetals
Example: One atom of carbon can combine with one atom of oxygen to produce one molecule of a compound called carbon monoxide
Carbon monoxide: a poisonous gas produced by burning gasoline in internal combustion engines or household gas appliances/furnaces
Molecular structure gives the atomic composition, bonding pattern, and shape.

8.2 The Nature of Covalent Bonding

What is the difference between the oxygen you breathe and the oxygen in ozone in the atmosphere?

Our atmosphere contains two different molecules that are both made of oxygen atoms.

The Octet Rule in Covalent Bonding

What is the result of electron sharing in covalent bonds?

In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.

In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.

For example, a single hydrogen atom has one electron. However, a pair of hydrogen atoms share electrons to form a covalent bond in a diatomic hydrogen molecule.
Each hydrogen atom thus attains the electron configuration of helium, a noble gas with two electrons.
Combinations of atoms of the nonmetals and metalloids in Groups 4A, 5A, 6A, and 7A of the periodic table are likely to form covalent bonds.
Combined atoms usually acquire eight or an octets by sharing electrons, so the octet rule applies.

Single Covalent Bond

The hydrogen atoms in a hydrogen molecule are held together mainly by the attraction of the shared electrons to the positive nuclei.
Two atoms held together by sharing one pair of electrons are joined by a single covalent bond.
Hydrogen gas consists of diatomic molecules whose atoms share only one pair of electrons, forming a single covalent bond.
An electron dot structure such as H: H represents the shared pair of electrons of the covalent bond by two dots.
The pair of shared electrons forming the covalent bond is often represented as a dash, as in H—H for hydrogen.
A structural formula represents the covalent bonds as dashes and shows the arrangement of covalently bonded atoms.
The halogens also form single covalent bonds in their diatomic molecules. Fluorine is one example.
By sharing electrons and forming a single covalent bond, two fluorine atoms each achieve the electron configuration of neon.
In the F2 molecule, each fluorine atom contributes one electron to complete the octet.
Notice that the two fluorine atoms share only one pair of valence electrons.
An unshared pair of valence electrons not shared between atoms is also known as a lone pair or a nonbinding pair.
In F2, each fluorine atom has three unshared pairs of electrons.
Methane contains four single covalent bonds.
The carbon atom has four valence electrons and needs four more valence electrons to attain a noble-gas configuration.
Each of the four hydrogen atoms contributes one electron to share with the carbon atom, forming four identical carbon-hydrogen bonds.
When carbon forms bonds with other atoms, it usually forms four bonds, as in methane. • You would not predict this pattern based on carbon’s electron configuration, shown below.

Drawing an Electron Dot Structure

Hydrochloric acid (HCl (aq)) is prepared by dissolving gaseous hydrogen chloride (HCl (g)) in water. Hydrogen chloride is a diatomic molecule with a single covalent bond. Draw the electron dot structure for HCl.

Double and triple covalent bonds

Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two or three pairs of electrons.
A double covalent bond is a bond that involves two shared pairs of electrons.
Similarly, a bond formed by sharing three pairs of electrons is a triple covalent bond.
The carbon dioxide (CO2) molecule contains two oxygens, each of which shares two electrons with carbon to form two carbon-oxygen double bonds.
Nitrogen (N2), a significant component of Earth’s atmosphere, contains triple bonds.
A single nitrogen atom has five valence electrons; each nitrogen atom in the molecule must share three electrons to have the electron configuration of neon.
You might think that an oxygen atom with six valence electrons would form a double bond by sharing two of its electrons with another oxygen atom.
All the electrons within the molecule would be paired in such an arrangement.
Experimental evidence, however, indicates that two of the electrons in O2 are still unpaired.
Thus, the oxygen molecule (O2) bonding does not obey the octet rule.

The “octet” in the octet rule refers to eight of what?

Each atom joined by a covalent bond usually acquires eight electrons in its valence shell. Most noble gases have eight valence electrons.

Coordinate Covalent Bonds

How are coordinate covalent bonds different from other covalent bonds?

Carbon monoxide (CO) is a type of covalent bonding different from that seen in water, ammonia, methane, and carbon dioxide.
A type of bonding called coordinate covalent bonding allows both carbon (which needs to gain four electrons) and oxygen (which needs to gain two electrons) to achieve noble-gas electron configurations.
A covalent bond in which one atom contributes both bonding electrons is a coordinate covalent bond.

Do all atoms joined in covalent bonds donate electrons to the bond?

No. In coordinate covalent bonds, the shared electron pair comes from one of the bonding atoms.

Exceptions to the Octet Rule

What are some exceptions to the octet rule?

The octet rule cannot be satisfied in molecules whose total number of valence electrons is odd. There are also molecules in which an atom has less or more than a complete octet of valence electrons.
Two plausible electron dot structures can be drawn for the NO2 molecule, which has seventeen valence electrons.
It is impossible to draw an electron dot structure for NO2 that satisfies the octet rule for all atoms, yet NO2 does exist as a stable molecule.
Some molecules with an even number of valence electrons, such as some boron compounds, also fail to follow the octet rule.
A few atoms, especially phosphorus and sulfur, expand the octet to ten or twelve electrons.
Sulfur hexafluoride (SF6) is an example.

Are molecules that do not obey the octet rule necessarily unstable?

No. There are molecules like NO2 that do not obey the octet rule but are stable naturally occurring molecules.

Bonding Dissociation Energies

How is the strength of a covalent bond related to its bond dissociation energy?
A large quantity of heat is released when hydrogen atoms combine to form hydrogen molecules.
This heat release suggests that the product is more stable than the reactants.
The covalent bond in the hydrogen molecule (H2) is so strong that it would take 435 kJ of energy to break apart all of the bonds in 1 mole (about 2 grams) of H2.
The energy required to break the bond between two covalently bonded atoms is known as the bond dissociation energy.
The units for this energy are often given in kJ/mol, the energy needed to break one mole of bonds.

True or False: A solid covalent bond has a low bond dissociation energy.

False. A significant bond dissociation energy corresponds to a strong covalent bond.

Resonance

How are resonance structures used?

The ozone molecule has two possible electron dot structures.
Notice that the structure on the left can be converted to the one on the right by shifting electron pairs without changing the positions of the oxygen atoms.
Because earlier chemists imagined that the electron pairs rapidly flip back and forth, or resonate, between the different electron dot structures, they used double-headed arrows to indicate that two or more structures are in resonance
Double covalent bonds are usually shorter than single bonds, so the bond lengths in ozone were believed to be unequal.
However, Experimental measurements show that the two ozone bonds are the same length.
The actual bonding is a hybrid, or mixture, of the extremes represented by the resonance forms.
The two-electron dot structures for ozone are examples of what is still referred to as resonance structures.
Resonance structures occur when drawing two or more valid electron dot structures with the same number of electron pairs for a molecule or ion is possible.

Chemists use resonance structures to envision the bonding in molecules that a single structural formula cannot adequately describe.

Although no back-and-forth changes occur, double-headed arrows connect resonance structures.

Do resonance structures accurately represent actual bonding?

No. Resonance structures are a way to envision the bonding in specific molecules. The actual bonding is a hybrid, or mixture, of the extremes represented by the resonance forms.
single covalent bond: a bond formed when two atoms share a pair of electrons
structural formula: a chemical formula that shows the arrangement of atoms in a molecule or a polyatomic ion; each dash between a pair of atoms indicates a pair of shared electrons
unshared pair: a pair of valence electrons that is not shared between atoms
double covalent bond: a bond in which two atoms share two pairs of electrons
triple covalent bond: a covalent bond in which two atoms share three pairs of electrons
coordinate covalent bond: a covalent bond in which one atom contributes both bonding electrons
polyatomic ion: a tightly bound group of atoms that behaves as a unit and has a positive or negative charge
bond dissociation energy: the energy required to break the bond between two covalently bonded atoms; this value is usually expressed in kJ per mol of substance
resonance structure: one of the two or more equally valid electron dot structures of a molecule or polyatomic ion