Question

Consider an ion with the symbol \(\mathrm{X}^{2+}\) formed from a representative element. a. What is the group number of the element? b. What is the electron-dot symbol of the element? c. If \(\mathrm{X}\) is in Period 3 , what is the element? d. What is the formula of the compound formed from \(\mathrm{X}\) and the nitride ion?

Short answer

a. Group 2 b. \(\text{X} \cdot \cdot \) c. Magnesium d. \(\text{Mg}_3\text{N}_2\)

Step-by-step solution

- Determine the Group Number

To find the group number of the element, recognize that \(\text{X}^{2+}\) has lost 2 electrons. The group number corresponds to the number of valence electrons before ionization. For a 2+ ion, the element originally had 2 more electrons. Hence, it is in Group 2 of the periodic table.

- Electron-Dot Symbol

The electron-dot symbol, also known as the Lewis symbol, depicts valence electrons around the element's symbol. Group 2 elements have 2 valence electrons. Therefore, the electron-dot symbol is \(\text{X} \cdot \cdot \), with two dots representing the valence electrons.

- Identify the Element

Since \(\text{X}\) is in Period 3 and Group 2, the element is Magnesium (Mg).

- Compound Formula with Nitride Ion

The nitride ion has a charge of \(\text{N}^{3-}\). To form a neutral compound, combine \(\text{Mg}^{2+}\) with \(\text{N}^{3-}\). The formula of the compound is \(\text{Mg}_3\text{N}_2\) because 3 \(\text{Mg}^{2+}\) ions (total charge +6) balance 2 \(\text{N}^{3-}\) ions (total charge -6).

Key concepts

Group Number

The group number in the periodic table helps us understand the behavior of elements, especially their valence electrons. Valence electrons are the outermost electrons and determine how an element will react chemically.
For instance, in the exercise, we identified the element \(\text{X}\) as having a 2+ charge. This means it has lost 2 electrons. If an element loses 2 electrons to form a 2+ ion, it originally had 2 more electrons. This places it in Group 2, also known as the alkaline earth metals. These elements are known to be quite reactive, especially with non-metals like nitrogen.

Electron-Dot Symbol

The electron-dot symbol, or Lewis symbol, is a simple way to represent the valence electrons of an element. Valence electrons are crucial in forming chemical bonds.
For Group 2 elements, there are 2 valence electrons. In our case, the electron-dot symbol for element X would show two dots around the symbol of the element (X). This symbol is helpful when predicting how the element will bond with other elements. For example, \(\text{Mg}\), when in its neutral state, is represented as \(\text{Mg} \cdot \cdot \). These two dots indicate the presence of two valence electrons, which can participate in chemical bonding.

Periodic Table

The periodic table is a powerful tool in chemistry. It organizes elements based on their atomic number (number of protons in the nucleus) and groups them into columns based on similar chemical properties.
Each element has a unique position on the table. Elements in the same group (vertical column) have similar properties and the same number of valence electrons. For example, Group 2 elements all have two valence electrons, making them likely to form 2+ ions. The periodic table also shows periodic trends like electronegativity, atomic radius, and ionization energy, which help predict how elements will interact with each other. For our exercise, knowing that X is in Period 3 and Group 2, we identified it as Magnesium (Mg).

Compound Formation

When elements form compounds, they engage in chemical bonding. This often involves the transfer or sharing of valence electrons to achieve a full outer electron shell, a stable electron configuration.
In our example, we looked at Magnesium (Mg) and its interaction with the nitride ion (\text{N}^3-). Magnesium forms a 2+ ion by losing two electrons, while nitrogen forms a 3- ion by gaining three electrons. To balance the charges and form a neutral compound, we combined three \text{Mg}^2+ ions (total charge +6) with two \text{N}^3- ions (total charge -6). Hence, the chemical formula for the compound is \( \text{Mg}_3\text{N}_2 \). This balance of charges is essential in understanding how compounds are formed and ensures the stability of the resulting compound.