Module 6/Quiz 5

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6.1

Ionic bonds are formed by the electrostatic attraction between positive and negative ions

The Lewis Dot Symbol is the symbol of the element with the valence electrons distributed around the symbol of the element.

Lewis Dot Symbols are written by first determining from the electron configuration the number of valence electrons in the atom or ion.

These electrons are distributed about the symbol of the element by placing one electron at a time until each side, i.e., north, south, east, and west, until each side has at least one electron, then continuing adding electrons by pairing the electrons until a maximum of eight electrons (an octet) have been distributed around the symbol of the atom.

If a sodium atom lost its valence electron to the valence electron shell of a chlorine atom, the result would be an ionic bond between the resulting sodium ion and chloride ion.

The bonded state should be more stable than the atoms forming the bonded state.

In going from the reactant to the product, the energy should decrease since the ionic bonded state is more stable than the atoms.

Since the bonding in sodium chloride is ionic, the energy of pairing a sodium ion with a chloride ion can be calculated from

The Crystal Lattice Energy is the energy required to separate the ionic solid into its component gaseous ions.

This energy is +789 kJ/mol (look at the discussion on the Born Haber Cycle); therefore, it would take considerable energy to separate solid sodium chloride into its component ions in the gaseous state.

Ionic Bonding:

occurs between metals and nonmetals

results, generally, in the formation of a solid

occurs between two elements if the ionization energy of the metal component is small and the electron affinity of the nonmetal component is a large negative value

The Born Haber Cycle includes a series of equations with enthalpies (heat energies) that lead to the formation of a solid ionic compound. These steps include, at least, the following enthalpies:

ionization: energy required to remove an electron from the valence shell of an atom to produce a cation

sublimation: energy required to convert an atom in the solid-state to the atom in the gaseous state and passing the liquid state

dissociation: energy required to separate a diatomic molecule in the gaseous state into a monatomic atom in the gaseous state

electron affinity: the energy needed to add an additional electron to the valence shell of an atom to produce an anion

the reverse of the crystal lattice energy: the crystal lattice energy is the energy required to break up a solid compound into its component ions in its gaseous states

Depending on the nature of the component elements of the solid ionic compound, other steps may be required to complete a specific Born Haber cycle.

When the steps of the Born Haber Cycle are added, the result is the formation of an ionic compound in the solid-state from its component elements in their natural states.

The enthalpy associated with the formation reaction is the energy required to form a solid compound from its component elements in its natural states

Step 1: energy of sublimation, ΔH sublimation

Step 2: ½ the energy of dissociation, ½ ΔH dissociation

Step 3: ionization energy, ΔH ionization potential

Step 4: electron affinity energy, ΔH electron affinity

Step 5: crystal lattice energy, ΔH crystal lattice energy

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6.2

Valence electrons can easily be removed from the configurations of Groups IA and IIA metals to give +1 and +2 charges respectively.

No charges will be found greater than the group number for the element.

Groups VIA and VIIA have large negative electron affinities; therefore, they prefer to gain electrons not to lose electrons.

Ionic Radii are:

obtained by x-ray diffraction studies

measurements of the size of the spherical region around the nucleus of ions within the probability of finding electrons

Covalent bonding involves the sharing of electrons.

The notation sigma,σ, represents a single bond, and σ21s represents a molecular orbital where two electrons with opposite spins reside.

In most cases, there is a tendency for atoms in molecules to have eight electrons in their valence shells. This is referred to as the octet rule, and the rationale for this observation is that eight electrons in a covalent bond about an atom in a molecule are isoelectronic with the outer shell stable configuration of all the noble gases except helium.

The stability of the covalent bond in hydrogen is isoelectronic with helium which has two electrons in the 1s orbital.

A polar covalent bond is a bond in which the bonding electrons are more closely associated with one atom more than the other.

Coordinate Covalent Bond is one in which two electrons are donated by one atom.

A double covalent bond is one in which two pairs of electrons are shared between two atoms.

A triple covalent bond is one in which three pairs of electrons are shared between two atoms.

Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons to itself.

Linus Pauling suggested electronegativity values from bond energies. However, to make this work, he had to arbitrarily assign an electronegativity value to the most electronegative element on the periodic chart. Obviously, the most electronegative value on the periodic chart is fluorine, F.

Pauling assigned a value of approximately 4.0 to F.

The electronegativity values of the other elements were calculated using 4.0 as the electronegativity of fluorine.

A Lewis electron-dot formula is a two-dimensional representation of electrons in a molecule. The single covalent bond can be represented by a straight line.

The steps that may be followed to write Lewis Dot Formulas include:

Select the central atom. Generally, this is the element in the structure with the least number of atoms. For example in MnO4-, the central atom is manganese, Mn.

Follow as much as possible the octet rule, but with the understanding that atoms with low-lying d orbitals can have exceptions to the octet rule.

Distribute the valence electrons about the atom in such a manner that when the electrons of the remaining atoms in the molecule are distributed about the central atom that the “octet” rule is generally followed.

Resonance is the phenomenon in which the actual structure is a hybrid between two or more structures.

Some elements in a compound can have exceptions to the octet rule. These elements possess low-lying d orbitals that are available to hybridize with other atomic orbitals to form molecular orbitals.

The formal charge is the hypothetical charge obtained by assuming that bonding electrons are equally shared between bonded atoms and that the electrons of each lone pair belong completely to one atom.

Calculating formal charges helps one to make decisions on selecting the appropriate Lewis structures because the fewer formal charges in a structure, the better the structure.

Formal charge, F, = V -(N/2)- P where V = number of valence electrons, N = total number of electrons involved in the covalent bond, and P = number of lone pair electrons.

Bond length is the distance between the nuclei in a bond. Bond lengths are determined experimentally using x-ray diffraction experiments or interpretation of molecular structures.

Bond order (BO) is the number of electron pairs in a bond. If the BO = 1, then the system has a single bond. If the BO = 2, then the system has a double bond. If the BO = 3, then the system has a triple bond.

The bond energy is the average enthalpy change that results when a bond in a molecule in the gaseous state is broken.

ΔHBE is always positive

BE is a measure of the strength of a bond.

If the electrons comprising a hypothetical covalent bond between atom A and atom B, i.e., A-B, are equally shared, then the resulting bond is said to be non-polar.

A: B

The electrons comprising the single covalent bond are equally shared and the bond is non-polar.

If the electronegativity of one atom, let’s say B is more electronegative than A in the A-B bond. Then the electrons comprising the A-B bond are not equally shared. Under these circumstances, the A-B bond is said to be a polar covalent bond where the electrons in the A-B are closer to the B atom than the A atom, and the A atom would have a partial positive charge and the B atom would have a partial negative charge.

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Finale

The elements with the highest ionization energies are generally those with the most negative electron affinities, which are located toward the upper right corner of the periodic table. Conversely, the elements with the lowest ionization energies are generally those with the least negative electron affinities and are located in the lower-left corner of the periodic table.

Because the tendency of an element to gain or lose electrons is so important in determining its chemistry, various methods have been developed to quantitatively describe this tendency. The most important method uses a measurement called electronegativity (represented by the Greek letter chi, χ,…), defined as the relative ability of an atom to attract electrons to itself in a chemical compound. Elements with high electronegativities tend to acquire electrons in chemical reactions and are found in the upper right corner of the periodic table. Elements with low electronegativities tend to lose electrons in chemical reactions and are found in the lower-left corner of the periodic table.

Unlike ionization energy or electron affinity, the electronegativity of an atom is not a simple, fixed property that can be directly measured in a single experiment. In fact, an atom’s electronegativity should depend to some extent on its chemical environment because the properties of an atom are influenced by its neighbors in a chemical compound. Nevertheless, when different methods for measuring the electronegativity of an atom are compared, they all tend to assign similar relative values to a given element. For example, all scales predict that fluorine has the highest electronegativity and cesium the lowest of the stable elements, which suggests that all the methods are measuring the same fundamental property.

Ionic bonds, which are characterized by a complete transfer of electrons from one atom to another, are said to have 100% ionic character. (Note that, in reality, there are no bonds that are 100% ionic in character because of the proximity of the bonded atoms. Even CsI (cesium iodide), with a relatively large distance of 3.37 Å, is still only about 75% ionic.

In contrast, the electrons in a nonpolar covalent bond are shared equally between two atoms. Therefore, nonpolar covalent bonds have 0% ionic character (100% covalent character).

Polar covalent bonds, which result from unequal sharing of electrons between the bonded atoms, are neither purely ionic nor purely covalent in character.

The dipole moment for a bond is a measure of the magnitude of polarity. Dipole moments can be used to calculate percent ionic character.

The dipole moment is symbolized by the Greek symbol and is equal to the charge, Q, of the atoms (measured in Coulombs, C) times the distance, r, of separation between the atoms (measured in meters, m):

Therefore, the units of dipole moment are Coulomb-meters, C⋅m. Another unit of dipole moment is the debye, D:

The percent ionic character is calculated by dividing the charge, Q, by the charge on a single electron,