Question

Write the correct formulas for the compounds formed when the following react (a) calcium and chlorine, (b) magnesium and oxygen, (c) aluminum and oxygen, and (d) sodium and sulfur.

Short answer

The chemical formulas are: (a) CaCl_2, (b) MgO, (c) Al_2O_3, and (d) Na_2S.

Step-by-step solution

Determine Valencies

Identify the valencies or charges of the ions that each element forms. Calcium forms Ca^{2+} ions, chlorine forms Cl^{-} ions, magnesium forms Mg^{2+} ions, oxygen forms O^{2-} ions, aluminum forms Al^{3+} ions, sodium forms Na^{+} ions, and sulfur forms S^{2-} ions.

Write the Formulas for Calcium and Chlorine

Combine calcium (Ca^{2+}) with chlorine (Cl^{-}) in a ratio that balances the total charges. The correct ratio is 1:2, leading to the formula CaCl_2.

Write the Formulas for Magnesium and Oxygen

Combine magnesium (Mg^{2+}) with oxygen (O^{2-}) in a ratio that balances the total charges. As both ions have a charge of 2+ and 2-, the correct ratio is 1:1, giving us the formula MgO.

Write the Formulas for Aluminum and Oxygen

Combine aluminum (Al^{3+}) with oxygen (O^{2-}) in a ratio that satisfies the total charge balance. Using the least common multiple of the charges, which is 6, we get the ratio 2:3, giving us the formula Al_2O_3.

Write the Formulas for Sodium and Sulfur

Combine sodium (Na^{+}) with sulfur (S^{2-}) in a ratio that balances the total charges. The correct ratio is 2:1, leading to the formula Na_2S.

Key concepts

Valency and Chemical Bonding

Understanding valency is crucial in the world of chemistry, as it lays the foundation for chemical bonding, the force that holds atoms together in molecules. Valency refers to the combining capacity of an element's atom, representing its ability to form bonds with other atoms. An element's valency is determined by the number of electrons in its outermost shell or the electrons involved in bonding. These bonding electrons can either be gained, lost, or shared with another atom to attain a stable electronic configuration, often resembling the nearest noble gas.

For example, in calcium (Ca), which has a valency of 2+, it means calcium tends to lose two electrons to achieve stability. On the other hand, chlorine (Cl) has a valency of 1-, indicating it typically gains an electron. When these two atoms react, the calcium atom donates its two electrons to two different chlorine atoms, satisfying the octet rule for all involved, leading to the formation of a stable ionic bond.

It's important to picture valency as the 'atomic handshake' that governs which atoms can combine and in what proportions, essentially scripting the language of chemical compounds and their formulas.

Ionic Compound Formation

Ionic compound formation is like a dance between positively charged cations and negatively charged anions. An ionic bond is formed when electrons are transferred from one atom to another, resulting in the creation of ions with opposite charges that are attracted to each other. This transfer leads to a state of electrical neutrality, where the total positive charge of the cations equals the total negative charge of the anions.

Process of Formation

In the formation of an ionic compound, metals typically lose electrons to become positively charged ions, while non-metals gain electrons to become negatively charged ions. For instance, magnesium and oxygen react to form magnesium oxide (MgO), where magnesium loses two electrons, becoming Mg^2+, and each oxygen atom gains those electrons, becoming O^2-. They bond in a one-to-one ratio to attain a neutral charge in the compound MgO.

Following the octet rule, these ions seek to complete their outer shell of electrons, and the crisscross method can be used to determine the correct ratios in which different ions combine to maintain electrical neutrality in a compound. Proper understanding of the concepts of valency and the charge balance principle is essential for predicting the formation of these ionic compounds.

Chemical Formula Writing

Writing a chemical formula is akin to following a recipe, with precise ratios and ingredients leading to a perfect dish. The formula represents which elements are present in a compound and in what proportion. To write a chemical formula, follow these steps:

Identifying Charges

Start by identifying the charges or valencies of the constituent ions from the periodic table or known valencies. For example, aluminum (Al), forming Al^3+, and oxygen (O), forming O^2-, are the building blocks for aluminum oxide.

Crisscross Method

Use the crisscross method, where the charge of one ion becomes the subscript of the other. For aluminum oxide, crisscrossing the charges (3+ for Al and 2- for O) gives us Al_2O_3. It's essential to reduce subscripts to the smallest whole numbers that maintain the ratio of ions, ensuring the neutrality of the resulting compound, which is the hallmark of chemical formula writing.

Remember, chemical formulas provide a wealth of information, including the types of atoms, the exact number of each type of atom, and the arrangement of atoms within the compound, though the last aspect is often implied and not explicitly detailed in the formula itself.