Question

An element \(X\) has atomic number \(19 .\) What will be the formula of its oxide? (a) \(X_{2} \mathrm{O}\) (b) \(\mathrm{XO}\) (c) \(\mathrm{XO}_{2}\) (d) \(X_{2} \mathrm{O}_{3}\)

Short answer

The formula of the oxide will be (c) \(\mathrm{XO}_{2}\).

Step-by-step solution

Determining the Valency of X

Since the atomic number of element X is 19, it corresponds to potassium (K) which is in Group 1 of the periodic table. Elements in Group 1 have a valency of +1 because they have one electron in their outermost shell that they can lose to achieve a stable octet configuration.

Determining the Valency of Oxygen

Oxygen (O) is in Group 16 of the periodic table. It typically has a valency of -2 since it needs two electrons to fill its outermost shell and achieve a stable octet configuration.

Writing the formula using the Criss-Cross Method

The Criss-Cross method involves writing the valency of one element as the subscript for the other element. Since X has a valency of +1 and oxygen has a valency of -2, we cross over the valencies to get the formula XO2.

Key concepts

Valency

Valency is a fundamental concept in chemistry that refers to the combining power of an element, specifically, the number of hydrogen atoms it can combine with or displace to form a compound. In practice, valency determines how atoms of an element will bond with atoms of another element.

Let's delve into our exercise example: element X, which is potassium (K), has an atomic number of 19. Looking at the periodic table, potassium lies in Group 1, which tells us that it has one valence electron in its outer shell. The valency of an element in Group 1 is always +1 because these elements, including potassium, tend to lose one electron to achieve a stable electronic configuration similar to the nearest noble gas. For elements like oxygen (O), which falls under Group 16, they usually gain electrons to fill their valence shell, giving them a valency of -2. Knowing the valency is pivotal in predicting how elements will combine to form chemical compounds.

Criss-Cross Method

Once we understand valencies, we can apply the criss-cross method to figure out the chemical formula of a compound. This method is a straightforward technique to balance the charges of ions to ensure neutrality in the resulting compound. One can start by writing the symbol of each element, followed by its valency above and to the right of the symbol, like a superscript.

In the case of combining potassium (K) with a valency of +1 and oxygen (O) with a valency of -2, we write K⁺¹ and O^-2. You then 'criss-cross' the valencies to be the subscripts of the opposite element, which means the valency of potassium becomes the subscript for oxygen and vice versa. However, it's important to simplify the ratios to their smallest whole numbers. For K⁺¹ and O^-2, crossing the valencies would initially give us K₂O₁, which simplifies to K₂O. Therefore, the correct formula for the compound would reflect this stoichiometric balance.

Periodic Table Groups

The periodic table is a master key in chemistry, categorizing elements into groups and periods. The groups, often numbered from 1 to 18, run vertically and are sometimes referred to as families. Elements within the same group share similar chemical and physical properties, including their valency, as they have the same number of electrons in their outermost shell.

Using the periodic table groups can aid in predicting how elements will interact. For example, elements in Group 1, commonly known as alkali metals, have a single electron in their outer shell and exhibit a valency of +1, keen to lose that one electron to reach a stable configuration. In our exercise, understanding that potassium belongs to Group 1 plays a crucial role in determining the correct chemical formula for its oxide by defining its valency(O). Hence, the periodic table not only organizes elements but also guides us in predicting chemical reactions and compound formulations.