Quiz 9

What is lattice energy?

Lattice energy is the heat energy released when an ionic crystalline compound is formed from gaseous ions. This occurs because, since the cations are positively charged and the anions are negatively charged, there is a lowering of potential energy according to Coulomb's law when the ions bond as a lattice. This is emitted as heat.

According to the Lewis model, what is a chemical bond?

A chemical bond is a sharing or transferring of electrons to reach a stable electron configuration for the bonding atoms. Transfer=ionic bond, shared=covalent.

What is a dipole moment?

A dipole moment occurs any time there is a separation of positive and negative charge, and it is used to quantify the polarity of a bond. The magnitude of a dipole moment created by separating two particles of equal but opposite charges of magnitude (q) by a distance (r) is given by (dipole moment)=qr

In a covalent Lewis structure, what is the difference between lone pair and bonding pair electrons?

A lone pair is a pair of electrons that is only associated with one atom, and not involved in any bonding. A bonding pair is a pair of electrons shared between two atoms.

How do a pure covalent bond, a polar covalent bond, and an ionic bond differ?

A pure (non polar) covalent bond occurs when two elements with identical electronegativities share a pair of electrons equally. A polar covalent bond occurs when two elements with an intermediate difference in electronegativity share a pair of electrons, such as between two different nonmetals. An ionic bond occurs when there is a large difference in electronegativity between two elements, such as often occurs between a metal and nonmetal, resulting in almost complete transfer of the electron from the metal to the nonmetal.

How does the ionic bonding model explain the relatively high melting points of ionic compounds?

A significant amount of heat must be present to overcome the omnidirectional coulombic forces holding together the lattice model of ionic bonds.

In what ways are double and triple covalent bonds different from single covalent bonds?

A single covalent bond occurs when one pair of electrons is shared between two atoms. A double bond occurs when two pairs are shared between two atoms, and these bonds are stronger and shorter than single bonds. In a triple bond, three pairs of electrons are shared between the two atoms, resulting in even shorter and stronger bonds.

How does lattice energy relate to ionic radii? To ion charge?

As the ionic radii increase moving down a group, the ions cannot get as close to each other, and do not release as much energy when the lattice forms. Therefore, lattice energy becomes less negative. As ionic charge increases, so does the amount of energy released.

Why are bonding theories important? Provide some examples of what bonding theories can predict.

Bonding theories explain how atoms bond together to form molecules. This includes explaining why some molecules are stable and others are not.

Describe the octet rule in the Lewis model.

Bonds are formed when an atoms reach a stable electron configuration, often containing 8 valence electrons to fill the outermost shell, thus the name "octet rule".

Why do chemical bonds form? What basic forces are involved in this bonding?

Chemical bonds form because they lower the potential energy between the charged particles that compose the atom. Attraction and repulsion of charged particles are involved in bonding.

How does the Lewis model for covalent bonding account for the relatively low melting and boiling points of molecular compounds (compared to ionic compounds)?

Covalent bonds are highly directional, and the attraction of two covalently bonded atoms is due to the sharing of one or more electron pairs. In this case, each bond links only one specific pair of atoms, in contrast to ionic bonds, which hold together in a lattice framework. The fundamental units of covalent compounds are the individual molecules, which interact weakly with each other, but strongly on a bonding level within each individual molecule. When a molecular compound melts or boils, only the interactions between molecules must be overcome, and the molecules themselves remain intact. This is as opposed to the melting and boiling of ionic compounds, which require enough heat energy to dissociate the strongly bonded individual ions.

How can Lewis structures be used to determine the formula of ionic compounds? Give an example.

Determine the number of valence electrons lost by the metal to form an octet, and the number of valence electrons gained by the non-metal to form an octet. Consider Na (Sodium) and S (Sulfur). The lewis structure shows that sodium must lose one electron to form an octet, while sulfur must gain 2. Therefore, it can be determined that the formula for the molecule would be Na(sub2)S

What is electronegativity? What are the periodic trends in electronegativity?

Electronegativity is the ability of an atom to attract electrons to itself in a polar chemical bond. Electronegativity generally increases moving across the table to the right, and decreases moving down a group. Fluorine is the most electronegative element.

How do you draw an ionic Lewis structure?

For an ionic bond, draw the lewis structure by moving electron dots from the metal to the nonmetal, allowing the charged ions to form a crystalline lattice of alternating cations and anions. Cation loses electrons to form an octet at the previous principal energy level, and the anion gains electrons to form an octet at its present principal energy level. The lewis structure of the anion is usually written within brackets with the charge in the upper right, outside the brackets. The positive and negative charges attract one another to result in a compound.

What is formal charge? How is formal charge calculated? How is it helpful?

Formal charge is a fictitious charge assigned to each atom in a lewis structure to help evaluate competing structures. This is the charge the atom would have if all bonding electrons were shared equally between the bonding atoms. To calculate, take the number of valence electrons in the atom and subtract the number of electrons that it "owns" in the lewis structure. An atom in a lewis structure "owns" all of its lone pair electrons and one half of its bonding pair electrons. This helps to distinguish between competing resonance or skeletal structures.

How does the Lewis model for covalent bonding account for why certain combinations of atoms are stable while others are not?

Generally, stable covalent bonds occur when each atom in the bond can satisfy the octet rule. Conversely, combinations that cannot satisfy this rule are not stable.

Do resonance structures always contribute equally to the overall structure of a molecule? Explain.

In some cases, resonance structures will not be equivalent. In some cases, one structure may be better than another, and in this case the hybrid will still be the average, but with one contributing more to that average than the other. Multiple nonequivalent resonance structures may be weighted differently in their contributions to the overall structure of a molecule.

How does the ionic bonding model explain the non-conductivity of ionic solids, and at the same time the conductivity of ionic solutions?

In the ionic bonding model, the only electron transfer that occurs is local, ie. the transfer of an electron from one cation to one anion. No free electrons conducting electricity are transferred in this model. However, this solution in water causes the dissociation of cations and anions, resulting in free ions that can move in response to electric forces, creating an electric current.

Explain percent ionic character of a bond. Do any bonds have 100% ionic character?

Percent ionic character of a bond is defined as the ratio of the bond's actual dipole moment to the dipole moment it would have if the electron were completely transferred, multiplied by 100. No bond is 100 percent ionic, but in general, bonds with >50percent ionic character are referred to as ionic bonds.

What are resonance structures? What is a resonance hybrid?

Resonance structures result when there are multiple possibilities for writing the lewis structure for the same molecule. A resonance hybrid is the weighted average of the resonance structures.

How do you determine how many dots to put around the Lewis symbol of an element?

The dots around the elements represent their valence electrons. The group the element's group on the periodic table can be used to determine the number of valence electrons it has.

Why is the formation of solid sodium chloride from solid sodium and gaseous chlorine exothermic, even though it takes more energy to form the Na + ion than the amount of energy released upon formation of Cl - ?

The formation of NaCl lattice from sodium cations and chlorine anions is highly exothermic, so much so that it compensates for the endothermic nature of the electron transfer process. Therefore the reaction is not exothermic because sodium wants to lose its electron and chlorine wants to gain it. Rather, it is exothermic because of the large amount of heat released when sodium and chlorine form a lattice structure.

What are the three basic types of chemical bonds? What happens to electrons in the bonding atoms in each type?

The three basic types of chemical bonds are covalent, metallic, and ionic. Covalent bonds are between nonmetals and involve shared electrons. Metallic bonds are between metals and involve pooled electrons. Ionic bonds are between metals and nonmetals and involve the transfer of electrons.

What is the Born-Haber cycle? List each step in the cycle and show how the cycle is used to calculate lattice energy.

This is a hypothetical cycle that illustrates the formation of an ionic compound from its constituent elements. The steps are designed so that the change in enthalpy for each one is known, except the final one which can be summed to get lattice energy. Steps for formation of NaCl: 1. Solid sodium forms gaseous sodium (enthalpy of sublimation of sodium) 2. Formation of a chlorine atom from a chlorine molecule (half of bond energy of chlorine) 3. Ionization of gaseous sodium (ionization energy of sodium) 4. Addition of electron to gaseous chlorine (electron affinity of chlorine) 5. Formation of the crystalline solid from gaseous ions (lattice energy) Hess's law can be used to get lattice energy

What is the basic procedure for writing a covalent Lewis structure?

To write a lewis structure for a covalent molecule: 1. Write the molecule's skeletal structure 2. Sum the valence electrons of each element in the molecule to find the total number of electrons for the lewis structure. 3. Distribute the electrons among the atoms, giving octets (duets for H) to as many atoms as possible. 4. If any atoms lack an octet, use a double or triple bond as necessary to give them an octet.

How do you determine the number of electrons in the Lewis structure of a molecule? A polyatomic ion?

Total number of electrons for the lewis structure of a molecule is given by the sum of valence electrons for all its constituent atoms The total number of electrons for the lewis structure of a polyatomic ion is given by summing the electrons and then subtracting one for each positive charge or adding one for each negative charge.