Question

Predict the formula of a compound formed by aluminium and sulphur. (a) \(\mathrm{Al}_{2} \mathrm{~S}_{2}\) (b) \(\mathrm{Al}_{3} \mathrm{~S}_{2}\) (c) \(\mathrm{Al}_{2} \mathrm{~S}_{3}\) (d) AIS

Short answer

The formula of the compound formed by aluminium and sulphur is \(\mathrm{Al}_{2}\mathrm{S}_{3}\), which is choice (c).

Step-by-step solution

Identify the Charge of Individual Elements

The first step in predicting the formula of a compound is to determine the ionic charges of the constituent elements. Aluminium (Al) typically forms a +3 charge since it is in group 13 of the periodic table. Sulfur (S) typically forms a -2 charge since it is in group 16 of the periodic table.

Determine the Ratio of Ions for a Neutral Compound

To achieve electrical neutrality, the total positive charge must balance the total negative charge. In this case, since Aluminum has a +3 charge and Sulfur has a -2 charge, we need the lowest common multiple of 3 and 2 for the formula to be neutral. The lowest common multiple of 3 and 2 is 6, so we need 2 Aluminum atoms to get a total of +6 charge (2 x +3) and 3 Sulfur atoms to get a total of -6 charge (3 x -2).

Write the Formula of the Compound

Using the ratio determined in the previous step, we can write the formula of the compound. Since we need 2 Aluminum atoms and 3 Sulfur atoms to balance the charges, the formula for the compound formed by Aluminum and Sulfur is \(\mathrm{Al}_{2}\mathrm{S}_{3}\).

Key concepts

Ionic Charges

When atoms come together to form compounds, one of the essential considerations is the charge of each ion involved. An ion is an atom or molecule that has gained or lost one or more of its electrons, giving it a net positive or negative electrical charge. The ionic charge greatly influences how different atoms will combine to form a chemical compound.

For example, consider aluminum (Al), which forms a cation with a +3 charge. This means that it can donate three electrons to achieve a noble gas electron configuration. On the other hand, sulfur (S) forms an anion with a -2 charge, indicating that it tends to gain two electrons. To predict a stable formula for a compound, it's essential to consider these charges to ensure that the total number of positive charges equals the total number of negative charges, leading to an electrically neutral compound. This is also referred to as the rule of zero charge balance.

The idea is simple yet powerful: by knowing the charge on the ions, students can ‘unlock’ the correct stoichiometry for the compound they’re attempting to create. When the charges do not balance naturally, the principle of charge neutrality will lead to the correct ratio of cations to anions, ensuring that the total positive and negative charges in the compound are equal.

Chemical Compound Formation

The formation of a chemical compound is a process where two or more elements combine in a fixed ratio to create a new substance with distinct properties. In the example of aluminum and sulfur, we are witnessing the formation of an ionic compound, a type of chemical compound that consists of ions held together by electrostatic forces termed ionic bonds.

The process starts with the transfer of electrons from the metal (in this case, aluminum) to the non-metal (sulfur), resulting in the formation of positively charged metal cations and negatively charged non-metal anions. The electrostatic attraction between these oppositely charged ions leads to the formation of the compound. Following the rule that the total charge must be zero, the correct stoichiometry, or ratio of ions, becomes apparent through the application of the lowest common multiple. For aluminum and sulfur, this ratio ensures that the positive and negative charges cancel each other out, leading to a stable, neutral compound.

Common Pitfalls in Predicting Formulas

Students may struggle with conceptualizing the transfer of electrons and how it impacts the resulting formula. A hands-on approach to visualizing the transfer can aid understanding—envisioning electrons moving from one atom to the other until a balance is reached is a key step. Furthermore, mastering the ability to determine the lowest common multiple allows students to find the right ratio of atoms for compounds, such as the Al2S3 formed from aluminum and sulfur.

Periodic Table Groups

The periodic table is an organized chart of elements that provides a wealth of information regarding the properties of each element, including their tendencies to form certain charges when they become ions. Elements are arranged into groups (vertical columns) based on their similar properties, and these groups can aid in predicting ionic charges.

In the case of aluminum, it is located in group 13, which typically consists of elements with a +3 charge upon ionization. Meanwhile, sulfur is in group 16, where the elements tend to form ions with a -2 charge. By examining the group number, students can often anticipate the charge that an element will have when it forms an ion. This is critical for predicting the formulas of ionic compounds.

Group Trends and Valence Electrons

The group number of an element is often tied to the number of valence electrons, or electrons in the outermost shell, which are in play during compound formation. For instance, group 1 elements have one valence electron and often form +1 ions, whereas Group 17 elements have seven valence electrons and typically form -1 ions. Recognizing these patterns within the periodic table groups simplifies the process of determining how atoms will combine during chemical compound formation.