Draw Lewis dot structures for the following chemical formulas. 1) Fe2S3 2) CaH2 3) AlF3
The Correct Answer and Explanation is:
Lewis Dot Structures and Explanations
Let us consider each compound and draw the correct Lewis dot structure based on their ionic nature and electron arrangements.
1) Fe₂S₃ (Iron(III) sulfide)
Structure:
Fe (Iron) is a transition metal and does not follow the typical octet rule in Lewis structures. However, in Fe₂S₃, each Fe loses 3 electrons to form Fe³⁺, and each S gains 2 electrons to become S²⁻.
Dot Structure Representation:
cssCopyEditFe³⁺ Fe³⁺ [ :S: ]²⁻ [ :S: ]²⁻ [ :S: ]²⁻
.. .. ..
Each sulfur atom (S) has 6 valence electrons and gains 2 electrons to complete its octet (now having 8 electrons, shown as dots). Each iron atom loses 3 electrons (thus shown as Fe³⁺), and three sulfur atoms balance the charge from two Fe³⁺ ions. The total charge remains neutral.
2) CaH₂ (Calcium hydride)
Structure:
Calcium (Ca) is an alkaline earth metal and has two valence electrons. It donates both electrons, forming Ca²⁺. Each hydrogen gains one electron to become H⁻ (hydride ion).
Dot Structure Representation:
cssCopyEditCa²⁺ [ H: ]⁻ [ :H ]⁻
Hydrogen atoms are represented with two electrons, forming a complete shell (1s²). This is an ionic compound, and the Ca²⁺ cation is electrostatically attracted to two H⁻ anions.
3) AlF₃ (Aluminum fluoride)
Structure:
Aluminum (Al) has three valence electrons and donates one to each fluorine (F) atom to form Al³⁺. Each fluorine atom has 7 valence electrons and gains 1 electron to complete the octet, forming F⁻.
Dot Structure Representation:
cssCopyEditAl³⁺ [ :F: ]⁻ [ :F: ]⁻ [ :F: ]⁻
.. .. .. .. .. ..
Each fluorine ion has a full octet (8 valence electrons including the gained one), and aluminum is left as a 3+ ion. This results in a stable ionic structure.
Explanation (300 words)
Lewis dot structures are simplified diagrams that represent valence electrons around atoms. These are essential tools in visualizing the bonding in molecules and ionic compounds. In ionic compounds like Fe₂S₃, CaH₂, and AlF₃, atoms achieve full outer shells by transferring electrons rather than sharing them.
In Fe₂S₃, iron is a transition metal with a variable oxidation state. In this compound, it forms Fe³⁺ ions, meaning each iron atom loses three electrons. Sulfur, a nonmetal, gains electrons to become S²⁻. Since the total positive charge from two Fe³⁺ ions is +6, three S²⁻ ions are needed to balance the charges, making the compound electrically neutral.
Calcium hydride (CaH₂) demonstrates a classic metal-to-nonmetal ionic bond. Calcium loses two electrons (forming Ca²⁺), while each hydrogen atom gains one electron (forming two H⁻ hydride ions). The resulting structure shows strong electrostatic attraction between the Ca²⁺ and two H⁻ ions.
In aluminum fluoride (AlF₃), aluminum donates its three valence electrons to three fluorine atoms, each of which needs one more electron to complete an octet. This forms Al³⁺ and three F⁻ ions. The ionic bonds result from electrostatic forces between oppositely charged ions.
Although Lewis structures are more detailed for covalent compounds, they still help illustrate the transfer of electrons in ionic compounds. The dot representations highlight which atoms lose or gain electrons and help us understand charge balance and the resulting stability of compounds.
