Chem quiz 9

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

Bonding theories are central to chemistry because they explain how atoms bond together to form molecules. Bonding theories explain why some combos of atoms are stable and others are not.

Describe the octet rule in the Lewis model.

Bonds are formed when atoms attain a stable electron configuration. Because the stable configuration usually has eight electrons in the outermost shell, this is known as the octet rule.

What is lattice energy?

Lattice energy is associated with forming a crystalline lattice of alternating cations and anions from the gaseous ions. Because the cations are positively charged and the anions are negatively charged, there is a lowering of potential-- as described by Coulomb's law-- when the ions come together to form a lattice. That energy is emitted as heat when the lattice forms.

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

Lewis theory shows that covalent bonds are highly directional. The attraction between two covalently bonded atoms is due to the sharing of one or more electron pairs. Thus, each bond links just one specific pair of atoms-- in contrast to ionic bonds, which are nondirectional and hold together the entire array of ions. The fundamental units of covalently bonded compounds are individual molecules. These molecules can interact with one another in a number of different ways; however, the interactions between molecules are generally much weaker than the bonding interactions within a molecule. When a molecular compound melts or boils, the molecules themselves remain intact. Only the weak interactions between the mole uses must be overcome. Consequently, molecular compounds tend to have lower melting and boiling points than ionic compounds.

What is a dipole moment?

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

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

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

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

A single bond occurs when one pair of electrons is shared between two atoms. A double bond results when two electron pairs are shared between the same two atoms. Double bonds are shorter and stronger than single bonds. A triple bond results when three electron pairs are shared between the same two atoms. Triple bonds are even shorter and stronger than double bonds.

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

As the ionic radii increase as you move down a group, the ions cannot get as close to each other and therefore do not release as much energy when the lattice forms. Thus, the lattice energy decreases (becomes less negative) as the radius increases. Because the magnitude of the potential energy of two interacting charges depends not only on the distance between the charges but also on the product of the charges, the lattice energies become more exothermic with increasing magnitude of ionic charge.

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

Chemical bonds form because they lower the potential energy between the charged particles that compose the atom. Bonds involve the attraction and repulsion of charged particles.

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

Electronegativity is the ability of an atom to attract electrons to itself in a chemical bond. This results in a polar bond. Electronegativity generally increases across a period in the periodic table. And electronegativity generally decreases down a column (group) in the periodic table. The most electronegative element is fluorine.

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 that helps us to distinguish among competing Lewis structures. The formal charge of an atom in a Lewis structure is the charge it would have if all bonding electrons were shared equally between the bonded atoms. Formal charge can be calculated simply by taking the number of valence electrons in the atom and subtracting the number of electrons that is "owns" in a Lewis structure. An atom in a Lewis structure "owns" all of its lone pair electrons and one half of its bonding electrons. Formal charge = number of valence electrons - (number of lone pair electrons + 1/2 number of bonding electrons) The concept of formal charge is useful because it helps us distinguish between competing skeletal structures or competing resonance structures.

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

Generally, combinations of atoms that can satisfy the octet rule on each atom are stable, while those combinations that do not satisfy the octet rule are not stable.

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

If two elements with identical electronegativities form a covalent bond, they share the electrons equally, and the bond is purely covalent or non polar. If there is an intermediate electronegativity difference between the two elements, such as between two different nonmetals, the bond is polar covalent. If there is a large electronegativity difference between the two elements in a bond, such as normally occurs between a metal and a nonmetal, the electron from the metal is almost completely transferred to the nonmetal, and the bond is ionic.

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

In Lewis theory, a chemical bond is the sharing or transferring of electrons to attain stable electron configurations for the bonding atoms. If electrons are transferred, the bond is an ionic bond. If the electrons are shared, the bond is a covalent bond.

How do you draw an ionic Lewis structure?

In Lewis theory, we represent ionic bonding by moving electron dots from the metal to the nonmetal and then allowing the resultant ions to form a crystalline lattice composed of alternating cations and anions. The cation loses its valence electron(s) and is left with an octet in the previous principal energy level; the anion gains electron(s) to form an octet. The Lewis structure of an anion is usually written within brackets with the charge in the upper right-hand corner, outside the brackets. The positive and negative charges attract one another, resulting in the compound.

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

In a Lewis structure, the valence electrons of main-group elements are represented as dots surrounding the symbol for the element. The valence electrons can be determined from the group they are in on the periodic table.

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

In some cases, we can write resonance structures that are not equivalent. One possible resonance structure may be somewhat better than another. In such cases, the true structure may still be represented as an average of the resonance structures, but with the better resonance structure contributing more to the true structure. Multiple nonequivalent resonance structures may be weighted differently in their contributions to the true overall structure of a molecule.

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

In the ionic bonding model, electrons are transferred from the metal tot he nonmetal, but the transferred electrons remain localized on one atom. The model does not include any free electrons that might conduct electricity, and the ions themselves are fixed in place; therefore, our model accounts for the non conductivity of ionic solids. When the ionic solid dissolves in water, the cations and anions dissociate, forming free ions in solution. These ions can move in response to electrical forces, creating an electric current. Thus our model predicts that solutions of ionic compounds conduct electricity.

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

Percent ionic character is defined as the ratio of a bond's actual dipole moment to the dipole moment it would have if the electron were completely transferred from one atom to the other, multiplied by 100. A bond in which an electron is completely transferred from one atom to another would have 100 % ionic character. However, no bond is 100% ionic. Percent ionic character generally increases as the electronegativity difference increases. In general, bonds with greater than 50% ionic character are referred as ionic bonds.

What are resonance structures? What is a resonance hybrid?

Resonance structures result when you can write two or more Lewis structures for the same molecule. A resonance hybrid is then the weighted average of the resonance structures.

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

The Born-Haber cycle is a hypothetical series of steps that represents the formation of an ionic compound fro its constituent elements. The steps are chosen so that the changer in enthalpy of each step is known except for the last one, which is the lattice energy. In terms of the formation of NaCl, the steps are as follows: step 1: the formation of gaseous sodium from solid sodium (heat of sublimation of sodium) step 2: the formation of a chlorine atom from a chlorine molecule (bond energy of chlorine) step 3: the ionization of gaseous sodium (ionization energy of sodium) step 4: the addition of an electron to gaseous chlorine (the electron affinity of chlorine) step 5: the formation of the crystalline solid from the gaseous ions (the lattice energy) The overall reaction is the formation of NaCl (s), so we can use Hess's law to determine the lattice energy. Heat of formation=deltaH step 1 + delta H step 2 + delta H step 3 + delta H step 4 + delta H step 5 Heat of formation = heat of sublimation + 1/2bond energy + ionization energy + electron affinity + lattice energy because all of the terms are known except the lattice energy, we can calculate the lattice energy.

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 the crystalline NaCl lattice from sodium cations and chloride anions is highly exothermic and more than compensates for the endothermicity of the electron transfer process. In other words, the formation of ionic compounds is not exothermic because sodium "wants" to lose electrons and chlorine "wants" to gain them; rather, it is exothermic because of the large amount of heat released when sodium and chlorine ions coalesce to form a crystal lattice.

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

The steps for writing a Lewis structure are as follows: 1. Write the correct skeletal structure for the molecule 2. Calculate the total number of electrons for the Lewis structure by summing the valence electrons of each atom in the molecule. 3. Distribute the electrons among the atoms, giving octets (or duets for hydrogen) to as many atoms as possible. 4. If any atoms lack an octet, form double or triple bonds as necessary to give them octets.

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

The three types of bonds are ionic bonds, which occur between metals and nonmetals and are characterized by the transfer of electrons; covalent bonds, which occur between nonmetals and are characterized by the sharing of electrons; and metallic bonds, which occur between metals and are characterized by electrons being pooled.

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

The total number of electrons for a Lewis structure of a molecule is the sum of the valence electrons of each atom in the molecule. The total number of electrons for the Lewis structure of an ion is found by summing the number of valence electrons for each atom and then subtracting one electron for each positive charge or adding one electron for each negative charge.

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

To use Lewis theory to determine the formula of an ionic compound, determine the number of valence electrons lost by the metal to form an octet and the number of electrons gained by the nonmetals to form an octet. Consider the ionic compound formed between sodium and sulfur. The Lewis structures for sodium and sulfur are as follows: (see answers) Sodium must lose one electron to achieve an octet, while sulfur must gain two electrons to achieve an octet. So the compound requires two sodium atoms for each sulfur atom, giving the formula Na2S.

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

We modeled ionic solids as a lattice of individual ions held together by coulomb forces, which are equal in all directions, To melt the solid, these forces must be overcome, which requires a significant amount of heat. Therefore, the model accounts for the high melting point of ionic solids.

What is the magnitude of the dipole moment formed by separating a proton and an electron by 100 pm? 200 pm?

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