Question

Predict the chemical formula for each of the following binary ionic compounds given the formula of sodium chloride, $\mathrm{NaCl}$ : (a) rubidium chloride (b) sodium bromide

Short answer

(a) Rubidium chloride: $\text{RbCl}$, (b) Sodium bromide: $\text{NaBr}$.

Step-by-step solution

Understand the Sodium Chloride Formula

Sodium chloride has the chemical formula $extNaCl$, where sodium (Na) is a metal with a +1 charge and chloride (Cl) is a non-metal with a -1 charge. This compound is neutral because the charges balance each other.

Determine the Charge of Rubidium and Chloride Ions

Rubidium (Rb), like sodium, is an alkali metal in group 1 of the periodic table, so it also has a +1 charge. Chloride (Cl) is in group 17 and has a -1 charge. Since both ions have charges that balance, they form a neutral compound. The formula for rubidium chloride is $extRbCl$.

Determine the Charge of Sodium and Bromide Ions

Sodium (Na) has a +1 charge, similar to rubidium, as it is also an alkali metal. Bromide (Br) is in the same group as chloride, and thus has a -1 charge. The charges of sodium and bromide also balance each other out, forming a neutral compound. The formula for sodium bromide is $extNaBr$.

Final Step: Write the Chemical Formulas

The chemical formula for rubidium chloride is $extRbCl$, and for sodium bromide, it is $extNaBr$. Each compound must have equal positive and negative charges to be neutral, which is satisfied in both cases with a 1:1 ratio of ions.

Key concepts

Chemical Formula

A chemical formula is like a recipe for a compound. It tells you which elements are in the compound and their ratios. When writing the chemical formula for a compound, we consider the charges of the ions involved. The charge of ions determines how many of each ion are needed to balance out the charges and create a neutral compound.
For binary ionic compounds, which consist of just two different types of atoms, ensuring charge balance is key. For example, in sodium chloride ($\text{NaCl}$), sodium (Na) has a +1 charge and chloride (Cl) has a -1 charge. These charges balance each other, resulting in the neutral compound $\text{NaCl}$. The subscript numbers in a chemical formula show the number of each type of atom in a molecule. If there is no number, it implies the presence of one atom. The beauty of a chemical formula lies in its simplicity and ability to convey complex information succinctly.

Sodium Chloride

Sodium chloride is perhaps the most well-known example of a binary ionic compound. It is what we commonly refer to as table salt. This compound is formed when sodium (Na), a silvery alkali metal, donates an electron to chlorine (Cl), a yellow-green gas, creating ions.

• Sodium (Na): Located in group 1 of the periodic table, it forms $+1$ charged ions by losing an electron.
• Chloride (Cl): Positioned in group 17, it forms $-1$ charged ions by gaining an electron.

When these ions combine in a 1:1 ratio, they create the stable, electrically neutral compound known as sodium chloride. The process of creating such compounds is called ionic bonding, which is a key concept in chemistry. The ions are held together by the electrostatic force between oppositely charged bodies, which is strong and results in a solid crystalline structure.

Alkali Metals

Alkali metals make up Group 1 of the periodic table and include elements like lithium (Li), sodium (Na), potassium (K), and rubidium (Rb). These metals are highly reactive and are known for their tendency to form positive ions, known as cations, by losing one electron.
Key characteristics of alkali metals include:

• Softness: These metals can often be cut with a knife.
• Low Density: They are less dense compared to other metals.
• High Reactivity: Particularly with water, forming hydroxides and hydrogen gas.

In ionic compounds such as sodium chloride ($\text{NaCl}$) or rubidium chloride ($\text{RbCl}$), the alkali metals readily give up their outermost electron to achieve a noble gas electron configuration. This loss of an electron results in a $+1$ ion. The vigor with which alkali metals lose their valence electron increases as you move down the group, making rubidium more reactive than sodium. This characteristic is important when predicting the formulas of compounds they form with halogens and other non-metals.