Question

Predict the chemical formula of the ionic compound formed between the following pairs of elements: (a) Al and F, (b) K and $S,(\mathbf{c})\mathrm{Y}$ and $\mathrm{O},(\mathbf{d})\mathrm{Mg}$ and $\mathrm{N}.$

Short answer

The chemical formulas of the ionic compounds formed between the given pairs of elements are: (a) AlF₃, (b) K₂S, (c) Y₂O₃, and (d) Mg₃N₂.

Step-by-step solution

(a) Al and F

Al forms a +3 ion (Al³⁺) and F forms a -1 ion (F⁻). To make it a neutral compound: Al³⁺ + 3 F⁻ → AlF₃ So, the chemical formula of the ionic compound formed between aluminum (Al) and fluorine (F) is AlF₃.

(b) K and S

K forms a +1 ion (K⁺) and S forms a -2 ion (S²⁻). To make it a neutral compound: 2 K⁺ + S²⁻ → K₂S So, the chemical formula of the ionic compound formed between potassium (K) and sulfur (S) is K₂S.

(c) Y and O

Yttrium (Y) forms a +3 ion (Y³⁺) and oxygen (O) forms a -2 ion (O²⁻). To make it a neutral compound: 2 Y³⁺ + 3 O²⁻ → Y₂O₃ So, the chemical formula of the ionic compound formed between yttrium (Y) and oxygen (O) is Y₂O₃.

(d) Mg and N

Magnesium (Mg) forms a +2 ion (Mg²⁺) and nitrogen (N) forms a -3 ion (N³⁻). To make it a neutral compound: 3 Mg²⁺ + 2 N³⁻ → Mg₃N₂ So, the chemical formula of the ionic compound formed between magnesium (Mg) and nitrogen (N) is Mg₃N₂.

Key concepts

Ionic Bonding

Ionic bonding is a fundamental concept in chemistry that explains how atoms combine to form compounds. In ionic bonding, atoms transfer electrons to each other. This transfer allows atoms to achieve a complete valence shell, akin to the electron configuration of noble gases, which is typically stable.
In these bonds, typically a metal loses electrons to become a positively charged ion or cation, while a non-metal gains those electrons to become a negatively charged ion or anion. The electrostatic attraction between these oppositely charged ions holds them together, forming an ionic compound.
For example, when aluminum (Al) bonds with fluorine (F), aluminum loses three electrons to form a 3+ cation, and each fluorine atom gains one electron to become a 1- anion. The combination of these ions forms the ionic compound AlF₃. The transfer and sharing of electrons are the backbone of ionic bonding and the basis for predicting the chemical formulas of resulting compounds.

Chemical Formula Prediction

Predicting the chemical formula of an ionic compound involves matching the charges of the ions so that the compound is neutral overall. Neutral means that the total positive and negative charges are equal. The basic steps include identifying the charge of the ions formed by each atom and balancing these charges.
One way is to use the "criss-cross" method:

• Write the symbols of the ions formed by the elements, with their charges.
• Criss-cross the charges of the ions, ignoring the plus or minus sign, to become the subscript of the other ion.
• Simplify the subscripts, if necessary, to the smallest whole number ratio.

For instance, in the case of magnesium and nitrogen, the magnesium ion (Mg²⁺) and the nitrogen ion (N³⁻) will criss-cross their charges to become Mg₃N₂. This method ensures that the total positive and negative charges are equal, giving a neutral compound.

Charges of Ions

Understanding the charges of ions is crucial when forming ionic compounds. Alkali metals, like potassium (K), typically form +1 charges because they have one electron in their outermost shell, which they easily lose. Alkaline earth metals, such as magnesium (Mg), generally have a +2 charge because they lose two electrons.
On the flip side, non-metals usually gain electrons. For example, sulfur (S) gains two electrons to complete its outer shell, forming an S²⁻ ion. Similarly, oxygen (O) gains two electrons, forming an O²⁻ ion. Other elements, like fluorine (F), gain one electron to form a -1 charge.
The charge of the ion is a primary factor in determining the chemical formula of the resultant compound. Recognizing common ion charges helps in predicting how many ions will combine to produce a neutral compound, as exemplified in the formation of compounds like K₂S, where the potassium ions balance out the charge of the sulfur ion.