Ionic Bonding, Lewis Structures, and Lattice Energy

Georges Seurat: A Sunday on La Grande Jatte—1884

Ionic Compounds

For an ionic compound to form:

• one element must lose one or more valence electrons to form a cation, and
• one element must gain one or more valence electrons to form an anion.

Here are the electron configurations of two atoms: K and Cl.

Notice that

• K can easily lose its valence electron (high energy valence shell, low effective charge)
• Cl can easily gain a valence electron (high effective charge, vacancy in valence shell).

When the electron transfers, what types of ions will be produced?

• Metals always lose electrons to empty their valence shell to form cations.
• Nonmetals always gain electrons to fill their valence shell to form anions.

Notice that both atoms now have a full valence shell.

• What noble gas has this same electron configuration?
• Atoms will lose or gain electrons to become isoelectric with noble gases and have a full octet (a full valence shell.)

The�Octet�Rule

“Atoms tend to gain, lose, or share electrons to attain a full valence shell.”

This is typically 8 valence electrons.

Notice both ions have 8 valence electrons now, thus fulfilling the octet rule.

Now that there is a positive ion and a negative ion, what will the ions do?

Now that there is a positive ion and a negative ion, they can form a bond through electrostatic attraction.

This attraction IS the ionic bond!

The chemical formula for this compound is KCl.

Ionic Crystal Lattices

• Because ions have charge, they will attract ions from all around to form an ionic crystal lattice (an ordered, repeating solid structure of ions in a fixed ratio to make a neutral compound).

Ionic Crystal Lattices

• Because ions have charge, they will attract ions from all around to form an ionic crystal lattice (an ordered, repeating solid structure of ions in a fixed ratio to make a neutral compound).
• The ratio of an ionic compound is called a formula unit (ex. KCl, MgCl₂).
• Ionic compounds are NOT composed of molecules.

Structures of a few ionic compounds

So to summarize:

Ionic bonds

• form between a metal (low electronegativity) and a nonmetal (high electronegativity) or polyatomic ion.
• electron(s) transfer from the metal to the non-metal to form a cation and an anion.
• the ions attract each other to form the ionic bond according to Coulomb’s Law (more on that later.)
• the ratio of the ions in the compound must create a neutral compound

Another way to represent valence electrons: Lewis Dot Structures

Instead of drawing electron configurations to represent electrons, Lewis structures can be used to represent the valence electrons.

​

Use dots around the symbol of the element to represent the valence electrons:

• one on each of four sides, and then pair them up.

Examples:

K

Cl

Al

S

Mg

Another way to represent valence electrons: Lewis Dot Structures

Instead of drawing electron configurations to represent electrons, Lewis structures can be used to represent the valence electrons and their movement.

​

Use dots around the symbol of the element to show the valence electrons.

​

Examples:

K

Cl

Al

S

Mg

lone pairs

Ionic Bonding and Lewis Structures

K

Cl

+

-

Ionic Bonding and Lewis Structures

K

Cl

+

-

What will the Lewis structure for magnesium fluoride look like?

Mg

F

Mg

F

F

What will the Lewis structure for magnesium fluoride look like?

Mg

F

F

What will the Lewis structure for magnesium fluoride look like?

-

-

2+

Coulomb’s Law

Lattice Energy: �the energy released in the formation of a crystal lattice of alternating cations and anions from gases ions.�M^a+(g) + X^b-(g) 🡪 M_bX_a(s)

Watch this video:

(click on the image)

Lattice Energy: �

Prediction

From what you learned, which pair of the following elements’ ions would you expect to highest lattice energy (strongest ionic bond? Why?

Prediction

From what you learned, which combination of the following elements’ ions would you expect to highest lattice energy (strongest ionic bond)? Why?

​

potassium, selenium, bromine, calcium, arsenic, and gallium

Prediction

From what you learned, which pair of the following elements’ ions would you expect to highest lattice energy (strongest ionic bond)? Why?

Why this trend?�

Why this difference?�

Why this difference?�

Why this difference?�

Remember?

Atomic/Ionic radius: The distance from the nucleus to the outermost electrons in an atom or ion.

How does the atomic radius change going down a group on the periodic table?

Prediction

Given our discussion from the previous slide, which pair of the following elements would you expect to have the highest lattice energy (strongest ionic bond)? Why?

potassium, fluorine, chlorine, sodium, lithium, bromine