Question

Write a formula for the ionic compound that forms from each pair of elements. (a) aluminum and oxygen (b) beryllium and iodine (c) calcium and sulfur (d) calcium and iodine

Short answer

The formulas for the ionic compounds are: (a) Al_2O_3, (b) BeI_2, (c) CaS, (d) CaI_2.

Step-by-step solution

Identify the Charges of the Ions

Each element can form ions with characteristic charges based on their group in the periodic table. Aluminum (Al) typically forms a +3 ion (Al^{3+}), Oxygen (O) forms a -2 ion (O^{2-}). Beryllium (Be) forms a +2 ion (Be^{2+}), while Iodine (I) forms a -1 ion (I^{-}). Calcium (Ca) forms a +2 ion (Ca^{2+}), and Sulfur (S) forms a -2 ion (S^{2-}). Calcium (Ca) again forms a +2 ion when combined with Iodine (I), which forms a -1 ion.

Determine the Formula for Each Compound

To write the formula for the ionic compound, combine the ions in a ratio that results in an electrically neutral compound. (a) For aluminum and oxygen, we need two Al^{3+} ions for every three O^{2-} ions to balance the charges (making Al_2O_3). (b) For beryllium and iodine, one Be^{2+} ion will balance with two I^{-} ions to form BeI_2. (c) For calcium and sulfur, one Ca^{2+} ion will balance with one S^{2-} ion to form CaS. (d) For calcium and iodine, one Ca^{2+} ion will balance with two I^{-} ions to form CaI_2.

Key concepts

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in compounds. There are several types of chemical bonds, with ionic bonding being particularly important for understanding the formation of ionic compounds. Ionic bonds occur between atoms that have very different electronegativities, which is the ability of an atom to attract electrons. In an ionic bond, one atom donates electrons to another, leading to the formation of positively charged ions (cations) and negatively charged ions (anions). These ions are then held together by electrostatic forces, resulting in the formation of an ionic compound.

For instance, in the reaction between aluminum and oxygen, aluminum atoms lose electrons to become Al³⁺ cations, while oxygen atoms gain electrons to become O^2- anions. The opposite charges of these ions create a strong bond, forming an ionic compound.

Valence Electrons

Valence electrons play a crucial role in chemical bonding, as they are the electrons found in the outermost shell of an atom. These electrons are primarily responsible for an atom's ability to form bonds with other atoms. The number of valence electrons an element has can often predict how it will bond with other elements.

The step-by-step solution of the exercise showcases how to determine the number of valence electrons to predict the charge of ions: aluminum, for example, has three valence electrons in its outer shell, which it loses to form an Al³⁺ ion when bonding with oxygen. Oxygen, on the other hand, has six valence electrons and gains two more to achieve a full outer shell, thus forming an O^2- ion. Understanding valence electrons is essential for predicting how different atoms will combine to form compounds.

Periodic Table Charges

The periodic table is more than just a list of elements; it's a powerful tool for predicting the charges of ions based on an element's position within the table. Elements in the same group (column) typically have the same number of valence electrons and, as a result, form ions with similar charges. For instance, most elements in Group 1 form +1 cations, while most in Group 16 form -2 anions.

In the provided solutions, you can see that such trends are used to predict ionic charges: aluminum, in Group 13, commonly forms a +3 ion, while beryllium in Group 2 forms a +2 ion, and both sulfur and oxygen, elements in Group 16, form -2 anions. By using periodic table charges, students can efficiently solve problems involving the formation of ionic compounds.

Chemical Formula Writing

Writing chemical formulas for ionic compounds requires a systematic approach to ensure that the resulting compound is electrically neutral. One must balance the total positive charge of the cations with the total negative charge of the anions. This often involves using subscripts to indicate the ratio of each ion needed to balance the charges.

In the exercise solutions, for example, to balance the +3 charge of two aluminum ions with the -2 charge of three oxygen ions, we write Al₂O₃ for aluminum oxide. Similarly, for calcium iodide, a +2 charge from calcium is balanced with the -1 charge of two iodine ions, giving us the formula CaI₂. These formulas are not guesses but the result of a clear understanding of ionic charges and valence electrons. By mastering chemical formula writing, students can effectively determine the composition of any ionic compound.