Question

Derive the correct formulas for the following ionic compounds by writing Lewis structures. (a) lithium sulfide; (b) sodium fluoride; (c) calcium iodide; (d) scandium chloride.

Short answer

The correct formula for (a) lithium sulfide is \( Li_2S \), (b) sodium fluoride is \( NaF \), (c) calcium iodide is \( CaI_2 \), and (d) scandium chloride is \( ScCl_3 \).

Step-by-step solution

Define the Lewis structure and Valence Electrons

For any compound, a Lewis structure can be created to illustrate how atoms are bonded and any lone pairs of electrons. To begin with, determine the number of valence electrons for each of the atoms. Lithium (Li) has 1 valence electron, Sulfur (S) has 6, Sodium (Na) has 1, Fluorine (F) has 7, Calcium (Ca) has 2, Iodine (I) has 7, Scandium (Sc) has 3 and Chlorine (Cl) has 7.

Lithium Sulfide - Li2S

Since Sulfur needs two more electrons to complete its octet, and Lithium has only one electron to give (it loses this electron to achieve stability), two Lithium atoms will be required to bond with one Sulfur atom to form lithium sulfide, creating a formula of \( Li_2S \). The Lewis structure would represent this.

Sodium Fluoride - NaF

Fluorine needs one more electron to complete its octet, and Sodium has that one electron to give, so one atom of each will bond together to form sodium fluoride, creating a formula of \( NaF \). The Lewis structure would represent this.

Calcium Iodide - CaI2

Iodine needs one more electron to complete its octet, but Calcium has two electrons to give (it loses its electrons to achieve stability), two Iodine atoms will be required to bond with one Calcium atom to make calcium iodide, with a formula of \( CaI_2 \). The Lewis structure would represent this.

Scandium Chloride - ScCl3

Chlorine needs one more electron to complete its octet, and since Scandium has three electrons to give (it loses its electrons to achieve stability), three Chlorine atoms will be necessary to bond with one Scandium atom to make scandium chloride, with a formula of \( ScCl_3 \). The Lewis structure would represent this.

Key concepts

Ionic Compounds

Ionic compounds are chemical compounds made up of ions held together by ionic bonds. They consist of positively charged ions, known as cations, and negatively charged ions, known as anions. An ionic bond is formed when there is a complete transfer of valence electrons between atoms. This transfer occurs often between metals and non-metals, where metals lose electrons and non-metals gain electrons.
The result is an electrostatic attraction that holds the ions together, creating a stable compound.
For example:

• Lithium sulfide ( Li_2S) is an ionic compound formed from two lithium ions, each with a positive charge, and one sulfide ion, with a double negative charge.
• Sodium fluoride ( NaF) results from the combination of a single sodium ion with a single fluoride ion.
• Calcium iodide ( CaI_2) involves one calcium ion bonding with two iodide ions.
• Scandium chloride ( ScCl_3) includes one scandium ion pairing with three chloride ions.

Understanding ionic compounds is essential in explaining the behavior of elements when they interact and form new substances.

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. These are the electrons involved in forming chemical bonds.
They are important because they determine how an atom will react with others. Each element's position on the periodic table can help us predict its number of valence electrons.
For instance:

• Lithium (Li) has 1 valence electron.
• Sodium (Na) also has 1 valence electron.
• Calcium (Ca) possesses 2 valence electrons.
• Scandium (Sc) has 3 valence electrons.
• Fluorine (F), Sulfur (S), Iodine (I), and Chlorine (Cl) each have 7 valence electrons.

Valence electrons play a crucial role in the creation of Lewis structures, as they highlight how atoms share or transfer electrons to form stable compounds.

Octet Rule

The octet rule is a guiding principle in chemistry that states that atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons. This rule is central to understanding how atoms interact to form compounds.
The rule is based on the stability observed in noble gases, which naturally have full valence shells.
Let's see how this works for our examples:

• Sulfur requires two additional electrons to complete its octet, so it bonds with two lithium atoms forming Li_2S.
• Fluorine needs one more electron, which it gains from sodium in NaF, thus achieving stability.
• Iodine lacks a single electron, so two iodine atoms bond with one calcium atom in CaI_2 to fulfill the rule.
• Chlorine is similar in needing one more electron, and it pairs with scandium, forming ScCl_3 as three chloride ions balance with scandium’s electrons.

By following the octet rule, atoms form stable configurations mimicking the electron configuration of noble gases.