The electronic configuration of element Z is 1s ^2 2s ^2 2p ^6 3s ^2 3p ^1 . What is the formula of the…

1. Analyze the Question

We are given the electronic configuration of an element Z and asked to determine the chemical formula of the compound it forms when it reacts with the tetraoxosulphate(VI) ion ($SO_4^{2-}$).

2. Explain the Concept

• Determining Valency: The valency of an element is determined by the number of electrons in its outermost shell (valence shell). • If an element has 1, 2, or 3 electrons in its valence shell, it tends to lose them to become stable, forming a positive ion (cation) with a charge equal to the number of electrons lost.
• Writing Chemical Formulas: To write the formula of an ionic compound, we use the "criss-cross" method: • Write the symbols of the cation and the anion side-by-side. • Write their respective charges (valencies) as superscripts. • Criss-cross the numbers to become the subscripts of the opposite element.

3. Show the Solution

Step 1: Determine the valency of Z. The electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^1$.

• The outermost shell is the 3rd shell ($3s^2 3p^1$).
• Total valence electrons = $2 + 1 = 3$.
• Since Z has 3 valence electrons, it will lose them to achieve stability, forming an ion with a charge of +3 ($Z^{3+}$).

Step 2: Identify the charge of the anion. The tetraoxosulphate(VI) ion is $SO_4^{2-}$, which has a charge of -2.

Step 3: Apply the criss-cross method.

• Cation: $Z^{3+}$
• Anion: $(SO_4)^{2-}$

By criss-crossing the charges:

• The 2 from the sulphate ion becomes the subscript for Z.
• The 3 from the Z ion becomes the subscript for the sulphate group.

This gives us: $Z_2(SO_4)_3$

4. Identify the Answer

Comparing our result to the given options: A. $Z_3(SO_4)_2$ B. $ZSO_4$ C. $Z_2SO_4$ D. $Z_2(SO_4)_3$

The correct option is D.

5. Exam Tip

When dealing with polyatomic ions like $SO_4^{2-}$, $CO_3^{2-}$, or $PO_4^{3-}$, always remember to treat the entire group as a single unit. If the subscript you are criss-crossing is greater than 1, you must place the polyatomic ion in parentheses before adding the subscript (e.g., $(SO_4)_3$).